BTRFS(5) BTRFS BTRFS(5)
NAME
btrfs - topics about the BTRFS filesystem (mount options, supported file
attributes and other)
DESCRIPTION
This document describes topics related to BTRFS that are not specific to
the tools. Currently covers:
1. mount options
2. filesystem features
3. checksum algorithms
4. compression
5. sysfs interface
6. filesystem exclusive operations
7. filesystem limits
8. bootloader support
9. file attributes
10. zoned mode
11. control device
12. filesystems with multiple block group profiles
13. seeding device
14. RAID56 status and recommended practices
15. glossary
16. storage model, hardware considerations
MOUNT OPTIONS
BTRFS SPECIFIC MOUNT OPTIONS
This section describes mount options specific to BTRFS. For the generic
mount options please refer to ]8;;https://man7.org/linux/man-pages/man8/mount.8.html\mount(8)]8;;\ manual page and also see the sec-
tion with BTRFS specifics below. The options are sorted alphabetically
(discarding the no prefix).
NOTE:
Most mount options apply to the whole filesystem and only options in
the first mounted subvolume will take effect. This is due to lack of
implementation and may change in the future. This means that (for ex-
ample) you can't set per-subvolume nodatacow, nodatasum, or compress
using mount options. This should eventually be fixed, but it has
proved to be difficult to implement correctly within the Linux VFS
framework.
Mount options are processed in order, only the last occurrence of an op-
tion takes effect and may disable other options due to constraints (see
e.g. nodatacow and compress). The output of mount command shows which
options have been applied.
acl, noacl
(default: on)
Enable/disable support for POSIX Access Control Lists (ACLs).
See the ]8;;https://man7.org/linux/man-pages/man5/acl.5.html\acl(5)]8;;\ manual page for more information about ACLs.
The support for ACL is build-time configurable
(BTRFS_FS_POSIX_ACL) and mount fails if acl is requested but the
feature is not compiled in.
autodefrag, noautodefrag
(since: 3.0, default: off)
Enable automatic file defragmentation. When enabled, small ran-
dom writes into files (in a range of tens of kilobytes, currently
it's 64KiB) are detected and queued up for the defragmentation
process. May not be well suited for large database workloads.
The read latency may increase due to reading the adjacent blocks
that make up the range for defragmentation, successive write will
merge the blocks in the new location.
WARNING:
Defragmenting with Linux kernel versions < 3.9 or ≥ 3.14-rc2
as well as with Linux stable kernel versions ≥ 3.10.31, ≥
3.12.12 or ≥ 3.13.4 will break up the reflinks of COW data
(for example files copied with cp --reflink, snapshots or
de-duplicated data). This may cause considerable increase of
space usage depending on the broken up reflinks.
barrier, nobarrier
(default: on)
Ensure that all IO write operations make it through the device
cache and are stored permanently when the filesystem is at its
consistency checkpoint. This typically means that a flush command
is sent to the device that will synchronize all pending data and
ordinary metadata blocks, then writes the superblock and issues
another flush.
The write flushes incur a slight hit and also prevent the IO
block scheduler to reorder requests in a more effective way. Dis-
abling barriers gets rid of that penalty but will most certainly
lead to a corrupted filesystem in case of a crash or power loss.
The ordinary metadata blocks could be yet unwritten at the time
the new superblock is stored permanently, expecting that the
block pointers to metadata were stored permanently before.
On a device with a volatile battery-backed write-back cache, the
nobarrier option will not lead to filesystem corruption as the
pending blocks are supposed to make it to the permanent storage.
clear_cache
Force clearing and rebuilding of the free space cache if some-
thing has gone wrong.
For free space cache v1, this only clears (and, unless no-
space_cache is used, rebuilds) the free space cache for block
groups that are modified while the filesystem is mounted with
that option. To actually clear an entire free space cache v1, see
btrfs check --clear-space-cache v1.
For free space cache v2, this clears the entire free space cache.
To do so without requiring to mounting the filesystem, see btrfs
check --clear-space-cache v2.
See also: space_cache.
commit=<seconds>
(since: 3.12, default: 30)
Set the interval of periodic transaction commit when data are
synchronized to permanent storage. Higher interval values lead to
larger amount of unwritten data to accumulate in memory, which
has obvious consequences when the system crashes. The upper
bound is not forced, but a warning is printed if it's more than
300 seconds (5 minutes). Use with care.
The periodic commit is not the only mechanism to do the transac-
tion commit, this can also happen by explicit sync or indirectly
by other commands that affect the global filesystem state or in-
ternal kernel mechanisms that flush based on various thresholds
or policies (e.g. cgroups).
compress, compress=<type[:level]>, compress-force, com-
press-force=<type[:level]>
(default: off, level support since: 5.1)
Control BTRFS file data compression. Type may be specified as
zlib, lzo, zstd or no (for no compression, used for remounting).
If no type is specified, zlib is used. If compress-force is
specified, then compression will always be attempted, but the
data may end up uncompressed if the compression would make them
larger.
Both zlib and zstd (since version 5.1) expose the compression
level as a tunable knob with higher levels trading speed and mem-
ory (zstd) for higher compression ratios. This can be set by ap-
pending a colon and the desired level. ZLIB accepts the range
[1, 9] and ZSTD accepts [1, 15]. If no level is set, both cur-
rently use a default level of 3. The value 0 is an alias for the
default level.
Otherwise some simple heuristics are applied to detect an incom-
pressible file. If the first blocks written to a file are not
compressible, the whole file is permanently marked to skip com-
pression. As this is too simple, the compress-force is a
workaround that will compress most of the files at the cost of
some wasted CPU cycles on failed attempts. Since kernel 4.15, a
set of heuristic algorithms have been improved by using frequency
sampling, repeated pattern detection and Shannon entropy calcula-
tion to avoid that.
NOTE:
If compression is enabled, nodatacow and nodatasum are dis-
abled.
datacow, nodatacow
(default: on)
Enable data copy-on-write for newly created files. Nodatacow im-
plies nodatasum, and disables compression. All files created un-
der nodatacow are also set the NOCOW file attribute (see ]8;;https://man7.org/linux/man-pages/man1/chattr.1.html\-
chattr(1)]8;;\).
NOTE:
If nodatacow or nodatasum are enabled, compression is dis-
abled.
Updates in-place improve performance for workloads that do fre-
quent overwrites, at the cost of potential partial writes, in
case the write is interrupted (system crash, device failure).
datasum, nodatasum
(default: on)
Enable data checksumming for newly created files. Datasum im-
plies datacow, i.e. the normal mode of operation. All files cre-
ated under nodatasum inherit the "no checksums" property, however
there's no corresponding file attribute (see ]8;;https://man7.org/linux/man-pages/man1/chattr.1.html\chattr(1)]8;;\).
NOTE:
If nodatacow or nodatasum are enabled, compression is dis-
abled.
There is a slight performance gain when checksums are turned off,
the corresponding metadata blocks holding the checksums do not
need to updated. The cost of checksumming of the blocks in mem-
ory is much lower than the IO, modern CPUs feature hardware sup-
port of the checksumming algorithm.
degraded
(default: off)
Allow mounts with fewer devices than the RAID profile constraints
require. A read-write mount (or remount) may fail when there are
too many devices missing, for example if a stripe member is com-
pletely missing from RAID0.
Since 4.14, the constraint checks have been improved and are ver-
ified on the chunk level, not at the device level. This allows
degraded mounts of filesystems with mixed RAID profiles for data
and metadata, even if the device number constraints would not be
satisfied for some of the profiles.
Example: metadata -- raid1, data -- single, devices -- /dev/sda,
/dev/sdb
Suppose the data are completely stored on sda, then missing sdb
will not prevent the mount, even if 1 missing device would nor-
mally prevent (any) single profile to mount. In case some of the
data chunks are stored on sdb, then the constraint of single/data
is not satisfied and the filesystem cannot be mounted.
device=<devicepath>
Specify a path to a device that will be scanned for BTRFS
filesystem during mount. This is usually done automatically by a
device manager (like udev) or using the btrfs device scan command
(e.g. run from the initial ramdisk). In cases where this is not
possible the device mount option can help.
NOTE:
Booting e.g. a RAID1 system may fail even if all filesystem's
device paths are provided as the actual device nodes may not
be discovered by the system at that point.
discard, discard=sync, discard=async, nodiscard
(default: async when devices support it since 6.2, async support
since: 5.6)
Enable discarding of freed file blocks. This is useful for
SSD/NVMe devices, thinly provisioned LUNs, or virtual machine im-
ages; however, every storage layer must support discard for it to
work.
In the synchronous mode (sync or without option value), lack of
asynchronous queued TRIM on the backing device TRIM can severely
degrade performance, because a synchronous TRIM operation will be
attempted instead. Queued TRIM requires SATA devices with
chipsets revision newer than 3.1 and devices.
The asynchronous mode (async) gathers extents in larger chunks
before sending them to the devices for TRIM. The overhead and
performance impact should be negligible compared to the previous
mode and it's supposed to be the preferred mode if needed.
If it is not necessary to immediately discard freed blocks, then
the fstrim tool can be used to discard all free blocks in a
batch. Scheduling a TRIM during a period of low system activity
will prevent latent interference with the performance of other
operations. Also, a device may ignore the TRIM command if the
range is too small, so running a batch discard has a greater
probability of actually discarding the blocks.
enospc_debug, noenospc_debug
(default: off)
Enable verbose output for some ENOSPC conditions. It's safe to
use but can be noisy if the system reaches near-full state.
fatal_errors=<action>
(since: 3.4, default: bug)
Action to take when encountering a fatal error.
bug BUG() on a fatal error, the system will stay in the
crashed state and may be still partially usable, but re-
boot is required for full operation
panic panic() on a fatal error, depending on other system con-
figuration, this may be followed by a reboot. Please refer
to the documentation of kernel boot parameters, e.g.
panic, oops or crashkernel.
flushoncommit, noflushoncommit
(default: off)
This option forces any data dirtied by a write in a prior trans-
action to commit as part of the current commit, effectively a
full filesystem sync.
This makes the committed state a fully consistent view of the
file system from the application's perspective (i.e. it includes
all completed file system operations). This was previously the
behavior only when a snapshot was created.
When off, the filesystem is consistent but buffered writes may
last more than one transaction commit.
fragment=<type>
(depends on compile-time option CONFIG_BTRFS_DEBUG, since: 4.4,
default: off)
A debugging helper to intentionally fragment given type of block
groups. The type can be data, metadata or all. This mount option
should not be used outside of debugging environments and is not
recognized if the kernel config option CONFIG_BTRFS_DEBUG is not
enabled.
nologreplay
(default: off, even read-only)
The tree-log contains pending updates to the filesystem until the
full commit. The log is replayed on next mount, this can be dis-
abled by this option. See also treelog. Note that nologreplay
is the same as norecovery.
WARNING:
Currently, the tree log is replayed even with a read-only
mount! To disable that behaviour, mount also with nologreplay.
max_inline=<bytes>
(default: min(2048, page size) )
Specify the maximum amount of space, that can be inlined in a
metadata b-tree leaf. The value is specified in bytes, option-
ally with a K suffix (case insensitive). In practice, this value
is limited by the filesystem block size (named sectorsize at mkfs
time), and memory page size of the system. In case of sectorsize
limit, there's some space unavailable due to b-tree leaf headers.
For example, a 4KiB sectorsize, maximum size of inline data is
about 3900 bytes.
Inlining can be completely turned off by specifying 0. This will
increase data block slack if file sizes are much smaller than
block size but will reduce metadata consumption in return.
NOTE:
The default value has changed to 2048 in kernel 4.6.
metadata_ratio=<value>
(default: 0, internal logic)
Specifies that 1 metadata chunk should be allocated after every
value data chunks. Default behaviour depends on internal logic,
some percent of unused metadata space is attempted to be main-
tained but is not always possible if there's not enough space
left for chunk allocation. The option could be useful to override
the internal logic in favor of the metadata allocation if the ex-
pected workload is supposed to be metadata intense (snapshots,
reflinks, xattrs, inlined files).
norecovery
(since: 4.5, default: off)
Do not attempt any data recovery at mount time. This will disable
logreplay and avoids other write operations. Note that this op-
tion is the same as nologreplay.
NOTE:
The opposite option recovery used to have different meaning
but was changed for consistency with other filesystems, where
norecovery is used for skipping log replay. BTRFS does the
same and in general will try to avoid any write operations.
rescan_uuid_tree
(since: 3.12, default: off)
Force check and rebuild procedure of the UUID tree. This should
not normally be needed. Alternatively the tree can be cleared
from userspace by command btrfs rescue clear-uuid-tree and then
it will be automatically rebuilt in kernel (the mount option is
not needed in that case).
rescue (since: 5.9)
Modes allowing mount with damaged filesystem structures, all re-
quires the filesystem to be mounted read-only and doesn't allow
remount to read-write. This is supposed to provide unified and
more fine grained tuning of errors that affect filesystem opera-
tion.
• usebackuproot (since 5.9)
Try to use backup root slots inside super block. Replaces
standalone option usebackuproot
• nologreplay (since 5.9)
Do not replay any dirty logs. Replaces standalone option nolo-
greplay
• ignorebadroots, ibadroots (since: 5.11)
Ignore bad tree roots, greatly improve the chance for data sal-
vage.
• ignoredatacsums, idatacsums (since: 5.11)
Ignore data checksum verification.
• ignoremetacsums, imetacsums (since 6.12)
Ignore metadata checksum verification, useful for interrupted
checksum conversion.
• all (since: 5.9)
Enable all supported rescue options.
skip_balance
(since: 3.3, default: off)
Skip automatic resume of an interrupted balance operation. The
operation can later be resumed with btrfs balance resume, or the
paused state can be removed with btrfs balance cancel. The de-
fault behaviour is to resume an interrupted balance immediately
after the filesystem is mounted.
space_cache, space_cache=<version>, nospace_cache
(nospace_cache since: 3.2, space_cache=v1 and space_cache=v2
since 4.5, default: space_cache=v2)
Options to control the free space cache. The free space cache
greatly improves performance when reading block group free space
into memory. However, managing the space cache consumes some re-
sources, including a small amount of disk space.
There are two implementations of the free space cache. The origi-
nal one, referred to as v1, used to be a safe default but has
been superseded by v2. The v1 space cache can be disabled at
mount time with nospace_cache without clearing.
On very large filesystems (many terabytes) and certain workloads,
the performance of the v1 space cache may degrade drastically.
The v2 implementation, which adds a new b-tree called the free
space tree, addresses this issue. Once enabled, the v2 space
cache will always be used and cannot be disabled unless it is
cleared. Use clear_cache,space_cache=v1 or clear_cache,no-
space_cache to do so. If v2 is enabled, and v1 space cache will
be cleared (at the first mount) and kernels without v2 support
will only be able to mount the filesystem in read-only mode. On
an unmounted filesystem the caches (both versions) can be cleared
by "btrfs check --clear-space-cache".
The btrfs-check(8) and :doc:`mkfs.btrfs commands have full v2
free space cache support since v4.19.
If a version is not explicitly specified, the default implementa-
tion will be chosen, which is v2.
ssd, ssd_spread, nossd, nossd_spread
(default: SSD autodetected)
Options to control SSD allocation schemes. By default, BTRFS
will enable or disable SSD optimizations depending on status of a
device with respect to rotational or non-rotational type. This is
determined by the contents of /sys/block/DEV/queue/rotational).
If it is 0, the ssd option is turned on. The option nossd will
disable the autodetection.
The optimizations make use of the absence of the seek penalty
that's inherent for the rotational devices. The blocks can be
typically written faster and are not offloaded to separate
threads.
NOTE:
Since 4.14, the block layout optimizations have been dropped.
This used to help with first generations of SSD devices. Their
FTL (flash translation layer) was not effective and the opti-
mization was supposed to improve the wear by better aligning
blocks. This is no longer true with modern SSD devices and the
optimization had no real benefit. Furthermore it caused in-
creased fragmentation. The layout tuning has been kept intact
for the option ssd_spread.
The ssd_spread mount option attempts to allocate into bigger and
aligned chunks of unused space, and may perform better on low-end
SSDs. ssd_spread implies ssd, enabling all other SSD heuristics
as well. The option nossd will disable all SSD options while
nossd_spread only disables ssd_spread.
subvol=<path>
Mount subvolume from path rather than the toplevel subvolume. The
path is always treated as relative to the toplevel subvolume.
This mount option overrides the default subvolume set for the
given filesystem.
subvolid=<subvolid>
Mount subvolume specified by a subvolid number rather than the
toplevel subvolume. You can use btrfs subvolume list of btrfs
subvolume show to see subvolume ID numbers. This mount option
overrides the default subvolume set for the given filesystem.
NOTE:
If both subvolid and subvol are specified, they must point at
the same subvolume, otherwise the mount will fail.
thread_pool=<number>
(default: min(NRCPUS + 2, 8) )
The number of worker threads to start. NRCPUS is number of
on-line CPUs detected at the time of mount. Small number leads to
less parallelism in processing data and metadata, higher numbers
could lead to a performance hit due to increased locking con-
tention, process scheduling, cache-line bouncing or costly data
transfers between local CPU memories.
treelog, notreelog
(default: on)
Enable the tree logging used for fsync and O_SYNC writes. The
tree log stores changes without the need of a full filesystem
sync. The log operations are flushed at sync and transaction com-
mit. If the system crashes between two such syncs, the pending
tree log operations are replayed during mount.
WARNING:
Currently, the tree log is replayed even with a read-only
mount! To disable that behaviour, also mount with nologreplay.
The tree log could contain new files/directories, these would not
exist on a mounted filesystem if the log is not replayed.
usebackuproot
(since: 4.6, default: off)
Enable autorecovery attempts if a bad tree root is found at mount
time. Currently this scans a backup list of several previous
tree roots and tries to use the first readable. This can be used
with read-only mounts as well.
NOTE:
This option has replaced recovery which has been deprecated.
user_subvol_rm_allowed
(default: off)
Allow subvolumes to be deleted by their respective owner. Other-
wise, only the root user can do that.
NOTE:
Historically, any user could create a snapshot even if he was
not owner of the source subvolume, the subvolume deletion has
been restricted for that reason. The subvolume creation has
been restricted but this mount option is still required. This
is a usability issue. Since 4.18, the ]8;;https://man7.org/linux/man-pages/man2/rmdir.2.html\rmdir(2)]8;;\ syscall can
delete an empty subvolume just like an ordinary directory.
Whether this is possible can be detected at runtime, see
rmdir_subvol feature in FILESYSTEM FEATURES.
DEPRECATED MOUNT OPTIONS
List of mount options that have been removed, kept for backward compati-
bility.
recovery
(since: 3.2, default: off, deprecated since: 4.5)
NOTE:
This option has been replaced by usebackuproot and should not
be used but will work on 4.5+ kernels.
inode_cache, noinode_cache
(removed in: 5.11, since: 3.0, default: off)
NOTE:
The functionality has been removed in 5.11, any stale data
created by previous use of the inode_cache option can be re-
moved by btrfs rescue clear-ino-cache.
check_int, check_int_data, check_int_print_mask=<value>
(removed in: 6.7, since: 3.0, default: off)
These debugging options control the behavior of the integrity
checking module (the BTRFS_FS_CHECK_INTEGRITY config option re-
quired). The main goal is to verify that all blocks from a given
transaction period are properly linked.
check_int enables the integrity checker module, which examines
all block write requests to ensure on-disk consistency, at a
large memory and CPU cost.
check_int_data includes extent data in the integrity checks, and
implies the check_int option.
check_int_print_mask takes a bit mask of BTRFSIC_PRINT_MASK_*
values as defined in fs/btrfs/check-integrity.c, to control the
integrity checker module behavior.
See comments at the top of fs/btrfs/check-integrity.c for more
information.
NOTES ON GENERIC MOUNT OPTIONS
Some of the general mount options from ]8;;https://man7.org/linux/man-pages/man8/mount.8.html\mount(8)]8;;\ that affect BTRFS and
are worth mentioning.
context
The context refers to the SELinux contexts and policy definitions
passed as mount options. This works properly since version v6.8
(because the mount option parser of BTRFS was ported to new API
that also understood the options).
noatime
under read intensive work-loads, specifying noatime significantly
improves performance because no new access time information needs
to be written. Without this option, the default is relatime,
which only reduces the number of inode atime updates in compari-
son to the traditional strictatime. The worst case for atime up-
dates under relatime occurs when many files are read whose atime
is older than 24 h and which are freshly snapshotted. In that
case the atime is updated and COW happens - for each file - in
bulk. See also ]8;;https://lwn.net/Articles/499293/\https://lwn.net/Articles/499293/]8;;\ - Atime and
btrfs: a bad combination? (LWN, 2012-05-31).
Note that noatime may break applications that rely on atime upti-
mes like the venerable Mutt (unless you use maildir mailboxes).
FILESYSTEM FEATURES
The basic set of filesystem features gets extended over time. The back-
ward compatibility is maintained and the features are optional, need to
be explicitly asked for so accidental use will not create incompatibili-
ties.
There are several classes and the respective tools to manage the fea-
tures:
at mkfs time only
This is namely for core structures, like the b-tree nodesize or
checksum algorithm, see mkfs.btrfs(8) for more details.
after mkfs, on an unmounted filesystem
Features that may optimize internal structures or add new struc-
tures to support new functionality, see btrfstune(8). The command
btrfs inspect-internal dump-super /dev/sdx will dump a su-
perblock, you can map the value of incompat_flags to the features
listed below
after mkfs, on a mounted filesystem
The features of a filesystem (with a given UUID) are listed in
/sys/fs/btrfs/UUID/features/, one file per feature. The status is
stored inside the file. The value 1 is for enabled and active,
while 0 means the feature was enabled at mount time but turned
off afterwards.
Whether a particular feature can be turned on a mounted filesys-
tem can be found in the directory /sys/fs/btrfs/features/, one
file per feature. The value 1 means the feature can be enabled.
List of features (see also mkfs.btrfs(8) section FILESYSTEM FEATURES):
big_metadata
(since: 3.4)
the filesystem uses nodesize for metadata blocks, this can be
bigger than the page size
block_group_tree
(since: 6.1)
block group item representation using a dedicated b-tree, this
can greatly reduce mount time for large filesystems
compress_lzo
(since: 2.6.38)
the lzo compression has been used on the filesystem, either as a
mount option or via btrfs filesystem defrag.
compress_zstd
(since: 4.14)
the zstd compression has been used on the filesystem, either as a
mount option or via btrfs filesystem defrag.
default_subvol
(since: 2.6.34)
the default subvolume has been set on the filesystem
extended_iref
(since: 3.7)
increased hardlink limit per file in a directory to 65536, older
kernels supported a varying number of hardlinks depending on the
sum of all file name sizes that can be stored into one metadata
block
free_space_tree
(since: 4.5)
free space representation using a dedicated b-tree, successor of
v1 space cache
metadata_uuid
(since: 5.0)
the main filesystem UUID is the metadata_uuid, which stores the
new UUID only in the superblock while all metadata blocks still
have the UUID set at mkfs time, see btrfstune(8) for more
mixed_backref
(since: 2.6.31)
the last major disk format change, improved backreferences, now
default
mixed_groups
(since: 2.6.37)
mixed data and metadata block groups, i.e. the data and metadata
are not separated and occupy the same block groups, this mode is
suitable for small volumes as there are no constraints how the
remaining space should be used (compared to the split mode, where
empty metadata space cannot be used for data and vice versa)
on the other hand, the final layout is quite unpredictable and
possibly highly fragmented, which means worse performance
no_holes
(since: 3.14)
improved representation of file extents where holes are not ex-
plicitly stored as an extent, saves a few percent of metadata if
sparse files are used
raid1c34
(since: 5.5)
extended RAID1 mode with copies on 3 or 4 devices respectively
raid_stripe_tree
(since: 6.7)
a separate tree for tracking file extents on RAID profiles
RAID56 (since: 3.9)
the filesystem contains or contained a RAID56 profile of block
groups
rmdir_subvol
(since: 4.18)
indicate that ]8;;https://man7.org/linux/man-pages/man2/rmdir.2.html\rmdir(2)]8;;\ syscall can delete an empty subvolume just
like an ordinary directory. Note that this feature only depends
on the kernel version.
skinny_metadata
(since: 3.10)
reduced-size metadata for extent references, saves a few percent
of metadata
send_stream_version
(since: 5.10)
number of the highest supported send stream version
simple_quota
(since: 6.7)
simplified quota accounting
supported_checksums
(since: 5.5)
list of checksum algorithms supported by the kernel module, the
respective modules or built-in implementing the algorithms need
to be present to mount the filesystem, see section CHECKSUM ALGO-
RITHMS.
supported_sectorsizes
(since: 5.13)
list of values that are accepted as sector sizes (mkfs.btrfs
--sectorsize) by the running kernel
supported_rescue_options
(since: 5.11)
list of values for the mount option rescue that are supported by
the running kernel, see btrfs(5)
zoned (since: 5.12)
zoned mode is allocation/write friendly to host-managed zoned de-
vices, allocation space is partitioned into fixed-size zones that
must be updated sequentially, see section ZONED MODE
SWAPFILE SUPPORT
A swapfile, when active, is a file-backed swap area. It is supported
since kernel 5.0. Use ]8;;https://man7.org/linux/man-pages/man8/swapon.8.html\swapon(8)]8;;\ to activate it, until then (respec-
tively again after deactivating it with ]8;;https://man7.org/linux/man-pages/man8/swapoff.8.html\swapoff(8)]8;;\) it's just a normal
file (with NODATACOW set), for which the special restrictions for active
swapfiles don't apply.
There are some limitations of the implementation in BTRFS and Linux swap
subsystem:
• filesystem - must be only single device
• filesystem - must have only single data profile
• subvolume - cannot be snapshotted if it contains any active swapfiles
• swapfile - must be preallocated (i.e. no holes)
• swapfile - must be NODATACOW (i.e. also NODATASUM, no compression)
The limitations come namely from the COW-based design and mapping layer
of blocks that allows the advanced features like relocation and
multi-device filesystems. However, the swap subsystem expects simpler
mapping and no background changes of the file block location once
they've been assigned to swap. The constraints mentioned above (single
device and single profile) are related to the swapfile itself, i.e. the
extents and their placement. It is possible to create swapfile on
multi-device filesystem as long as the extents are on one device but
this cannot be affected by user and depends on free space fragmentation
and available unused space for new chunks.
With active swapfiles, the following whole-filesystem operations will
skip swapfile extents or may fail:
• balance - block groups with extents of any active swapfiles are
skipped and reported, the rest will be processed normally
• resize grow - unaffected
• resize shrink - works as long as the extents of any active swapfiles
are outside of the shrunk range
• device add - if the new devices do not interfere with any already ac-
tive swapfiles this operation will work, though no new swapfile can be
activated afterwards
• device delete - if the device has been added as above, it can be also
deleted
• device replace - ditto
When there are no active swapfiles and a whole-filesystem exclusive op-
eration is running (e.g. balance, device delete, shrink), the swapfiles
cannot be temporarily activated. The operation must finish first.
To create and activate a swapfile run the following commands:
# truncate -s 0 swapfile
# chattr +C swapfile
# fallocate -l 2G swapfile
# chmod 0600 swapfile
# mkswap swapfile
# swapon swapfile
Since version 6.1 it's possible to create the swapfile in a single com-
mand (except the activation):
# btrfs filesystem mkswapfile --size 2G swapfile
# swapon swapfile
Please note that the UUID returned by the mkswap utility identifies the
swap "filesystem" and because it's stored in a file, it's not generally
visible and usable as an identifier unlike if it was on a block device.
Once activated the file will appear in /proc/swaps:
# cat /proc/swaps
Filename Type Size Used Priority
/path/swapfile file 2097152 0 -2
The swapfile can be created as one-time operation or, once properly cre-
ated, activated on each boot by the swapon -a command (usually started
by the service manager). Add the following entry to /etc/fstab, assuming
the filesystem that provides the /path has been already mounted at this
point. Additional mount options relevant for the swapfile can be set
too (like priority, not the BTRFS mount options).
/path/swapfile none swap defaults 0 0
From now on the subvolume with the active swapfile cannot be snapshotted
until the swapfile is deactivated again by swapoff. Then the swapfile is
a regular file and the subvolume can be snapshotted again, though this
would prevent another activation any swapfile that has been snapshotted.
New swapfiles (not snapshotted) can be created and activated.
Otherwise, an inactive swapfile does not affect the containing subvol-
ume. Activation creates a temporary in-memory status and prevents some
file operations, but is not stored permanently.
HIBERNATION
A swapfile can be used for hibernation but it's not straightforward. Be-
fore hibernation a resume offset must be written to file /sys/power/re-
sume_offset or the kernel command line parameter resume_offset must be
set.
The value is the physical offset on the device. Note that this is not
the same value that filefrag prints as physical offset!
Btrfs filesystem uses mapping between logical and physical addresses but
here the physical can still map to one or more device-specific physical
block addresses. It's the device-specific physical offset that is suit-
able as resume offset.
Since version 6.1 there's a command btrfs inspect-internal map-swapfile
that will print the device physical offset and the adjusted value for
/sys/power/resume_offset. Note that the value is divided by page size,
i.e. it's not the offset itself.
# btrfs filesystem mkswapfile swapfile
# btrfs inspect-internal map-swapfile swapfile
Physical start: 811511726080
Resume offset: 198122980
For scripting and convenience the option -r will print just the offset:
# btrfs inspect-internal map-swapfile -r swapfile
198122980
The command map-swapfile also verifies all the requirements, i.e. no
holes, single device, etc.
TROUBLESHOOTING
If the swapfile activation fails please verify that you followed all the
steps above or check the system log (e.g. dmesg or journalctl) for more
information.
Notably, the swapon utility exits with a message that does not say what
failed:
# swapon /path/swapfile
swapon: /path/swapfile: swapon failed: Invalid argument
The specific reason is likely to be printed to the system log by the
btrfs module:
# journalctl -t kernel | grep swapfile
kernel: BTRFS warning (device sda): swapfile must have single data profile
CHECKSUM ALGORITHMS
Data and metadata are checksummed by default. The checksum is calculated
before writing and verified after reading the blocks from devices. The
whole metadata block has an inline checksum stored in the b-tree node
header. Each data block has a detached checksum stored in the checksum
tree.
NOTE:
Since a data checksum is calculated just before submitting to the
block device, btrfs has a strong requirement that the corresponding
data block must not be modified until the writeback is finished.
This requirement is met for a buffered write as btrfs has the full
control on its page cache, but a direct write (O_DIRECT) bypasses
page cache, and btrfs can not control the direct IO buffer (as it can
be in user space memory). Thus it's possible that a user space pro-
gram modifies its direct write buffer before the buffer is fully
written back, and this can lead to a data checksum mismatch.
To avoid this, kernel starting with version 6.14 will force a direct
write to fall back to buffered, if the inode requires a data check-
sum. This will bring a small performance penalty. If you require
true zero-copy direct writes, then set the NODATASUM flag for the in-
ode and make sure the direct IO buffer is fully aligned to block
size.
There are several checksum algorithms supported. The default and back-
ward compatible algorithm is crc32c. Since kernel 5.5 there are three
more with different characteristics and trade-offs regarding speed and
strength. The following list may help you to decide which one to select.
CRC32C (32 bits digest)
Default, best backward compatibility. Very fast, modern CPUs have
instruction-level support, not collision-resistant but still good
error detection capabilities.
XXHASH (64 bits digest)
Can be used as CRC32C successor. Very fast, optimized for modern
CPUs utilizing instruction pipelining, good collision resistance
and error detection.
SHA256 (256 bits digest)
Cryptographic-strength hash. Relatively slow but with possible
CPU instruction acceleration or specialized hardware cards. FIPS
certified and in wide use.
BLAKE2b (256 bits digest)
Cryptographic-strength hash. Relatively fast, with possible CPU
acceleration using SIMD extensions. Not standardized but based on
BLAKE which was a SHA3 finalist, in wide use. The algorithm used
is BLAKE2b-256 that's optimized for 64-bit platforms.
The digest size affects overall size of data block checksums stored in
the filesystem. The metadata blocks have a fixed area up to 256 bits
(32 bytes), so there's no increase. Each data block has a separate
checksum stored, with additional overhead of the b-tree leaves.
Approximate relative performance of the algorithms, measured against
CRC32C using implementations on a 11th gen 3.6GHz intel CPU:
┌─────────┬─────────────┬───────┬──────────────────┐
│ Digest │ Cycles/4KiB │ Ratio │ Implementation │
├─────────┼─────────────┼───────┼──────────────────┤
│ CRC32C │ 470 │ 1.00 │ CPU instruction, │
│ │ │ │ PCL combination │
├─────────┼─────────────┼───────┼──────────────────┤
│ XXHASH │ 870 │ 1.9 │ reference impl. │
├─────────┼─────────────┼───────┼──────────────────┤
│ SHA256 │ 7600 │ 16 │ libgcrypt │
├─────────┼─────────────┼───────┼──────────────────┤
│ SHA256 │ 8500 │ 18 │ openssl │
├─────────┼─────────────┼───────┼──────────────────┤
│ SHA256 │ 8700 │ 18 │ botan │
├─────────┼─────────────┼───────┼──────────────────┤
│ SHA256 │ 32000 │ 68 │ builtin, CPU in- │
│ │ │ │ struction │
├─────────┼─────────────┼───────┼──────────────────┤
│ SHA256 │ 37000 │ 78 │ libsodium │
├─────────┼─────────────┼───────┼──────────────────┤
│ SHA256 │ 78000 │ 166 │ builtin, refer- │
│ │ │ │ ence impl. │
├─────────┼─────────────┼───────┼──────────────────┤
│ BLAKE2b │ 10000 │ 21 │ builtin/AVX2 │
├─────────┼─────────────┼───────┼──────────────────┤
│ BLAKE2b │ 10900 │ 23 │ libgcrypt │
├─────────┼─────────────┼───────┼──────────────────┤
│ BLAKE2b │ 13500 │ 29 │ builtin/SSE41 │
├─────────┼─────────────┼───────┼──────────────────┤
│ BLAKE2b │ 13700 │ 29 │ libsodium │
├─────────┼─────────────┼───────┼──────────────────┤
│ BLAKE2b │ 14100 │ 30 │ openssl │
├─────────┼─────────────┼───────┼──────────────────┤
│ BLAKE2b │ 14500 │ 31 │ kcapi │
├─────────┼─────────────┼───────┼──────────────────┤
│ BLAKE2b │ 14500 │ 34 │ builtin, refer- │
│ │ │ │ ence impl. │
└─────────┴─────────────┴───────┴──────────────────┘
Many kernels are configured with SHA256 as built-in and not as a module.
The accelerated versions are however provided by the modules and must be
loaded explicitly (modprobe sha256) before mounting the filesystem to
make use of them. You can check in /sys/fs/btrfs/FSID/checksum which one
is used. If you see sha256-generic, then you may want to unmount and
mount the filesystem again. Changing that on a mounted filesystem is not
possible. Check the file /proc/crypto, when the implementation is
built-in, you'd find:
name : sha256
driver : sha256-generic
module : kernel
priority : 100
...
While accelerated implementation is e.g.:
name : sha256
driver : sha256-avx2
module : sha256_ssse3
priority : 170
...
COMPRESSION
Btrfs supports transparent file compression. There are three algorithms
available: ZLIB, LZO and ZSTD (since v4.14), with various levels. The
compression happens on the level of file extents and the algorithm is
selected by file property, mount option or by a defrag command. You can
have a single btrfs mount point that has some files that are uncom-
pressed, some that are compressed with LZO, some with ZLIB, for instance
(though you may not want it that way, it is supported).
Once the compression is set, all newly written data will be compressed,
i.e. existing data are untouched. Data are split into smaller chunks
(128KiB) before compression to make random rewrites possible without a
high performance hit. Due to the increased number of extents the meta-
data consumption is higher. The chunks are compressed in parallel.
The algorithms can be characterized as follows regarding the speed/ratio
trade-offs:
ZLIB
• slower, higher compression ratio
• levels: 1 to 9, mapped directly, default level is 3
• good backward compatibility
LZO
• faster compression and decompression than ZLIB, worse compres-
sion ratio, designed to be fast
• no levels
• good backward compatibility
ZSTD
• compression comparable to ZLIB with higher compression/decom-
pression speeds and different ratio
• levels: -15..15, mapped directly, default is 3
• support since 4.14
• levels 1..15 supported since 5.1
• levels -15..-1 supported since 6.15
The differences depend on the actual data set and cannot be expressed by
a single number or recommendation. Higher levels consume more CPU time
and may not bring a significant improvement, lower levels are close to
real time.
HOW TO ENABLE COMPRESSION
Typically the compression can be enabled on the whole filesystem, speci-
fied for the mount point. Note that the compression mount options are
shared among all mounts of the same filesystem, either bind mounts or
subvolume mounts. Please refer to btrfs(5) section MOUNT OPTIONS.
$ mount -o compress=zstd /dev/sdx /mnt
This will enable the zstd algorithm on the default level (which is 3).
The level can be specified manually too like zstd:3. Higher levels com-
press better at the cost of time. This in turn may cause increased write
latency, low levels are suitable for real-time compression and on rea-
sonably fast CPU don't cause noticeable performance drops.
$ btrfs filesystem defrag -czstd file
The command above will start defragmentation of the whole file and apply
the compression, regardless of the mount option. (Note: specifying level
is not yet implemented). The compression algorithm is not persistent and
applies only to the defragmentation command, for any other writes other
compression settings apply.
Persistent settings on a per-file basis can be set in two ways:
$ chattr +c file
$ btrfs property set file compression zstd
The first command is using legacy interface of file attributes inherited
from ext2 filesystem and is not flexible, so by default the zlib com-
pression is set. The other command sets a property on the file with the
given algorithm. (Note: setting level that way is not yet implemented.)
COMPRESSION LEVELS
The level support of ZLIB has been added in v4.14, LZO does not support
levels (the kernel implementation provides only one), ZSTD level support
has been added in v5.1 and the negative levels in v6.15.
There are 9 levels of ZLIB supported (1 to 9), mapping 1:1 from the
mount option to the algorithm defined level. The default is level 3,
which provides the reasonably good compression ratio and is still rea-
sonably fast. The difference in compression gain of levels 7, 8 and 9 is
comparable but the higher levels take longer.
The ZSTD support includes levels -15..15, a subset of full range of what
ZSTD provides. Levels -15..-1 are real-time with worse compression ra-
tio, levels 1..3 are near real-time with good compression, 4..8 are
slower with improved compression and 9..15 try even harder though the
resulting size may not be significantly improved. Higher levels also re-
quire more memory and as they need more CPU the system performance is
affected.
Level 0 always maps to the default. The compression level does not af-
fect compatibility.
INCOMPRESSIBLE DATA
Files with already compressed data or with data that won't compress well
with the CPU and memory constraints of the kernel implementations are
using a simple decision logic. If the first portion of data being com-
pressed is not smaller than the original, the compression of the file is
disabled -- unless the filesystem is mounted with compress-force. In
that case compression will always be attempted on the file only to be
later discarded. This is not optimal and subject to optimizations and
further development.
If a file is identified as incompressible, a flag is set (NOCOMPRESS)
and it's sticky. On that file compression won't be performed unless
forced. The flag can be also set by chattr +m (since e2fsprogs 1.46.2)
or by properties with value no or none. Empty value will reset it to the
default that's currently applicable on the mounted filesystem.
There are two ways to detect incompressible data:
• actual compression attempt - data are compressed, if the result is not
smaller, it's discarded, so this depends on the algorithm and level
• pre-compression heuristics - a quick statistical evaluation on the
data is performed and based on the result either compression is per-
formed or skipped, the NOCOMPRESS bit is not set just by the heuris-
tic, only if the compression algorithm does not make an improvement
$ lsattr file
---------------------m file
Using the forcing compression is not recommended, the heuristics are
supposed to decide that and compression algorithms internally detect in-
compressible data too.
PRE-COMPRESSION HEURISTICS
The heuristics aim to do a few quick statistical tests on the compressed
data in order to avoid probably costly compression that would turn out
to be inefficient. Compression algorithms could have internal detection
of incompressible data too but this leads to more overhead as the com-
pression is done in another thread and has to write the data anyway. The
heuristic is read-only and can utilize cached memory.
The tests performed based on the following: data sampling, long repeated
pattern detection, byte frequency, Shannon entropy.
COMPATIBILITY
Compression is done using the COW mechanism so it's incompatible with
nodatacow. Direct IO read works on compressed files but will fall back
to buffered writes and leads to no compression even if force compression
is set. Currently nodatasum and compression don't work together.
The compression algorithms have been added over time so the version com-
patibility should be also considered, together with other tools that may
access the compressed data like bootloaders.
SYSFS INTERFACE
Btrfs has a sysfs interface to provide extra knobs.
The top level path is /sys/fs/btrfs/, and the main directory layout is
the following:
┌──────────────────────────────┬──────────────────────┬─────────┐
│ Relative Path │ Description │ Version │
├──────────────────────────────┼──────────────────────┼─────────┤
│ features/ │ All supported fea- │ 3.14 │
│ │ tures │ │
├──────────────────────────────┼──────────────────────┼─────────┤
│ <UUID>/ │ Mounted fs UUID │ 3.14 │
├──────────────────────────────┼──────────────────────┼─────────┤
│ <UUID>/allocation/ │ Space allocation │ 3.14 │
│ │ info │ │
├──────────────────────────────┼──────────────────────┼─────────┤
│ <UUID>/bdi/ │ Backing device info │ 5.9 │
│ │ (writeback) │ │
├──────────────────────────────┼──────────────────────┼─────────┤
│ <UUID>/devices/<DE- │ Symlink to each │ 5.6 │
│ VID>/ │ block device sysfs │ │
├──────────────────────────────┼──────────────────────┼─────────┤
│ <UUID>/devinfo/<DE- │ Btrfs specific info │ 5.6 │
│ VID>/ │ for each device │ │
├──────────────────────────────┼──────────────────────┼─────────┤
│ <UUID>/discard/ │ Discard stats and │ 6.1 │
│ │ tunables │ │
├──────────────────────────────┼──────────────────────┼─────────┤
│ <UUID>/features/ │ Features of the │ 3.14 │
│ │ filesystem │ │
├──────────────────────────────┼──────────────────────┼─────────┤
│ <UUID>/qgroups/ │ Global qgroup info │ 5.9 │
├──────────────────────────────┼──────────────────────┼─────────┤
│ <UUID>/qgroups/<LEVEL>_<ID>/ │ Info for each qgroup │ 5.9 │
└──────────────────────────────┴──────────────────────┴─────────┘
For /sys/fs/btrfs/features/ directory, each file means a supported fea-
ture of the current kernel. Most files have value 0. Otherwise it de-
pends on the file, value 1 typically means the feature can be turned on
a mounted filesystem.
For /sys/fs/btrfs/<UUID>/features/ directory, each file means an enabled
feature on the mounted filesystem.
The features share the same name in section FILESYSTEM FEATURES.
UUID
Files in /sys/fs/btrfs/<UUID>/ directory are:
bg_reclaim_threshold
(RW, since: 5.19)
Used space percentage of total device space to start auto block
group claim. Mostly for zoned devices.
checksum
(RO, since: 5.5)
The checksum used for the mounted filesystem. This includes both
the checksum type (see section CHECKSUM ALGORITHMS) and the im-
plemented driver (mostly shows if it's hardware accelerated).
clone_alignment
(RO, since: 3.16)
The bytes alignment for clone and dedupe ioctls.
commit_stats
(RW, since: 6.0)
The performance statistics for btrfs transaction commit since the
first mount. Mostly for debugging purposes.
Writing into this file will reset the maximum commit duration
(max_commit_ms) to 0. The file looks like:
commits 70649
last_commit_ms 2
max_commit_ms 131
total_commit_ms 170840
• commits - number of transaction commits since the first mount
• last_commit_ms - duration in milliseconds of the last commit
• max_commit_ms - maximum time a transaction commit took since
first mount or last reset
• total_commit_ms - sum of all transaction commit times
exclusive_operation
(RO, since: 5.10)
Shows the running exclusive operation. Check section FILESYSTEM
EXCLUSIVE OPERATIONS for details.
generation
(RO, since: 5.11)
Show the generation of the mounted filesystem.
label (RW, since: 3.14)
Show the current label of the mounted filesystem.
metadata_uuid
(RO, since: 5.0)
Shows the metadata UUID of the mounted filesystem. Check meta-
data_uuid feature for more details.
nodesize
(RO, since: 3.14)
Show the nodesize of the mounted filesystem.
quota_override
(RW, since: 4.13)
Shows the current quota override status. 0 means no quota over-
ride. 1 means quota override, quota can ignore the existing
limit settings.
read_policy
(RW, since: 5.11)
Shows the current balance policy for reads. Currently only pid
(balance using the process id (pid) value) is supported. More
balancing policies are available in experimental build, namely
round-robin.
sectorsize
(RO, since: 3.14)
Shows the sectorsize of the mounted filesystem.
temp_fsid
(RO, since 6.7)
Indicate that this filesystem got assigned a temporary FSID at
mount time, making possible to mount devices with the same FSID.
UUID/allocations
Files and directories in /sys/fs/btrfs/<UUID>/allocations directory are:
global_rsv_reserved
(RO, since: 3.14)
The used bytes of the global reservation.
global_rsv_size
(RO, since: 3.14)
The total size of the global reservation.
data/, metadata/ and system/ directories
(RO, since: 5.14)
Space info accounting for the 3 block group types.
UUID/allocations/{data,metadata,system}
Files in /sys/fs/btrfs/<UUID>/allocations/data,metadata,system directory
are:
bg_reclaim_threshold
(RW, since: 5.19)
Reclaimable space percentage of block group's size (excluding
permanently unusable space) to reclaim the block group. Can be
used on regular or zoned devices.
bytes_*
(RO)
Values of the corresponding data structures for the given block
group type and profile that are used internally and may change
rapidly depending on the load.
Complete list: bytes_may_use, bytes_pinned, bytes_readonly,
bytes_reserved, bytes_used, bytes_zone_unusable
chunk_size
(RW, since: 6.0)
Shows the chunk size. Can be changed for data and metadata (inde-
pendently) and cannot be set for system block group type. Cannot
be set for zoned devices as it depends on the fixed device zone
size. Upper bound is 10% of the filesystem size, the value must
be multiple of 256MiB and greater than 0.
size_classes
(RO, since: 6.3)
Numbers of block groups of a given classes based on heuristics
that measure extent length, age and fragmentation.
none 136
small 374
medium 282
large 93
UUID/bdi
Symlink to the sysfs directory of the backing device info (BDI), which
is related to writeback process and infrastructure.
UUID/devices
Files in /sys/fs/btrfs/<UUID>/devices directory are symlinks named after
device nodes (e.g. sda, dm-0) and pointing to their sysfs directory.
UUID/devinfo
The directory contains subdirectories named after device ids (numeric
values). Each subdirectory has information about the device of the given
devid.
UUID/devinfo/DEVID
Files in /sys/fs/btrfs/<UUID>/devinfo/<DEVID> directory are:
error_stats:
(RO, since: 5.14)
Shows device stats of this device, same as btrfs device stats (-
btrfs-device(8)).
write_errs 0
read_errs 0
flush_errs 0
corruption_errs 0
generation_errs 0
fsid: (RO, since: 5.17)
Shows the fsid which the device belongs to. It can be different
than the UUID if it's a seed device.
in_fs_metadata
(RO, since: 5.6)
Shows whether we have found the device. Should always be 1, as
if this turns to 0, the DEVID directory would get removed auto-
matically.
missing
(RO, since: 5.6)
Shows whether the device is considered missing by the kernel mod-
ule.
replace_target
(RO, since: 5.6)
Shows whether the device is the replace target. If no device re-
place is running, this value is 0.
scrub_speed_max
(RW, since: 5.14)
Shows the scrub speed limit for this device. The unit is Bytes/s.
0 means no limit. The value can be set but is not persistent.
writeable
(RO, since: 5.6)
Show if the device is writeable.
UUID/qgroups
Files in /sys/fs/btrfs/<UUID>/qgroups/ directory are:
enabled
(RO, since: 6.1)
Shows if qgroup is enabled. Also, if qgroup is disabled, the
qgroups directory will be removed automatically.
inconsistent
(RO, since: 6.1)
Shows if the qgroup numbers are inconsistent. If 1, it's recom-
mended to do a qgroup rescan.
drop_subtree_threshold
(RW, since: 6.1)
Shows the subtree drop threshold to automatically mark qgroup in-
consistent.
When dropping large subvolumes with qgroup enabled, there would
be a huge load for qgroup accounting. If we have a subtree whose
level is larger than or equal to this value, we will not trigger
qgroup account at all, but mark qgroup inconsistent to avoid the
huge workload.
Default value is 3, which means that trees of low height will be
accounted properly as this is sufficiently fast. The value was 8
until 6.13 where no subtree drop can trigger qgroup rescan making
it less useful.
Lower value can reduce qgroup workload, at the cost of extra
qgroup rescan to re-calculate the numbers.
UUID/qgroups/LEVEL_ID
Files in each /sys/fs/btrfs/<UUID>/qgroups/<LEVEL>_<ID>/ directory are:
exclusive
(RO, since: 5.9)
Shows the exclusively owned bytes of the qgroup.
limit_flags
(RO, since: 5.9)
Shows the numeric value of the limit flags. If 0, means no limit
implied.
max_exclusive
(RO, since: 5.9)
Shows the limits on exclusively owned bytes.
max_referenced
(RO, since: 5.9)
Shows the limits on referenced bytes.
referenced
(RO, since: 5.9)
Shows the referenced bytes of the qgroup.
rsv_data
(RO, since: 5.9)
Shows the reserved bytes for data.
rsv_meta_pertrans
(RO, since: 5.9)
Shows the reserved bytes for per transaction metadata.
rsv_meta_prealloc
(RO, since: 5.9)
Shows the reserved bytes for preallocated metadata.
UUID/discard
Files in /sys/fs/btrfs/<UUID>/discard/ directory are:
discardable_bytes
(RO, since: 6.1)
Shows amount of bytes that can be discarded in the async discard
and nodiscard mode.
discardable_extents
(RO, since: 6.1)
Shows number of extents to be discarded in the async discard and
nodiscard mode.
discard_bitmap_bytes
(RO, since: 6.1)
Shows amount of discarded bytes from data tracked as bitmaps.
discard_extent_bytes
(RO, since: 6.1)
Shows amount of discarded extents from data tracked as bitmaps.
discard_bytes_saved
(RO, since: 6.1)
Shows the amount of bytes that were reallocated without being
discarded.
kbps_limit
(RW, since: 6.1)
Tunable limit of kilobytes per second issued as discard IO in the
async discard mode.
iops_limit
(RW, since: 6.1)
Tunable limit of number of discard IO operations to be issued in
the async discard mode.
max_discard_size
(RW, since: 6.1)
Tunable limit for size of one IO discard request.
FILESYSTEM EXCLUSIVE OPERATIONS
There are several operations that affect the whole filesystem and cannot
be run in parallel. Attempt to start one while another is running will
fail (see exceptions below).
Since kernel 5.10 the currently running operation can be obtained from
/sys/fs/UUID/exclusive_operation with following values and operations:
• balance
• balance paused (since 5.17)
• device add
• device delete
• device replace
• resize
• swapfile activate
• none
Enqueuing is supported for several btrfs subcommands so they can be
started at once and then serialized.
There's an exception when a paused balance allows to start a device add
operation as they don't really collide and this can be used to add more
space for the balance to finish.
FILESYSTEM LIMITS
maximum file name length
255
This limit is imposed by Linux VFS, the structures of BTRFS could
store larger file names.
maximum symlink target length
depends on the nodesize value, for 4KiB it's 3949 bytes, for
larger nodesize it's 4095 due to the system limit PATH_MAX
The symlink target may not be a valid path, i.e. the path name
components can exceed the limits (NAME_MAX), there's no content
validation at ]8;;https://man7.org/linux/man-pages/man3/symlink.3.html\symlink(3)]8;;\ creation.
maximum number of inodes
264 but depends on the available metadata space as the inodes are
created dynamically
Each subvolume is an independent namespace of inodes and thus
their numbers, so the limit is per subvolume, not for the whole
filesystem.
inode numbers
minimum number: 256 (for subvolumes), regular files and directo-
ries: 257, maximum number: (264 - 256)
The inode numbers that can be assigned to user created files are
from the whole 64bit space except first 256 and last 256 in that
range that are reserved for internal b-tree identifiers.
maximum file length
inherent limit of BTRFS is 264 (16 EiB) but the practical limit
of Linux VFS is 263 (8 EiB)
maximum number of subvolumes
the subvolume ids can go up to 248 but the number of actual sub-
volumes depends on the available metadata space
The space consumed by all subvolume metadata includes bookkeeping
of shared extents can be large (MiB, GiB). The range is not the
full 64bit range because of qgroups that use the upper 16 bits
for another purposes.
maximum number of hardlinks of a file in a directory
65536 when the extref feature is turned on during mkfs (default),
roughly 100 otherwise and depends on file name length that fits
into one metadata node
minimum filesystem size
the minimal size of each device depends on the mixed-bg feature,
without that (the default) it's about 109MiB, with mixed-bg it's
is 16MiB
BOOTLOADER SUPPORT
GRUB2 (]8;;https://www.gnu.org/software/grub\https://www.gnu.org/software/grub]8;;\) has the most advanced support
of booting from BTRFS with respect to features.
U-Boot (]8;;https://www.denx.de/wiki/U-Boot/\https://www.denx.de/wiki/U-Boot/]8;;\) has decent support for booting
but not all BTRFS features are implemented, check the documentation.
In general, the first 1MiB on each device is unused with the exception
of primary superblock that is on the offset 64KiB and spans 4KiB. The
rest can be freely used by bootloaders or for other system information.
Note that booting from a filesystem on zoned device is not supported.
FILE ATTRIBUTES
The btrfs filesystem supports setting file attributes or flags. Note
there are old and new interfaces, with confusing names. The following
list should clarify that:
• attributes: ]8;;https://man7.org/linux/man-pages/man1/chattr.1.html\chattr(1)]8;;\ or ]8;;https://man7.org/linux/man-pages/man1/lsattr.1.html\lsattr(1)]8;;\ utilities (the ioctls are
FS_IOC_GETFLAGS and FS_IOC_SETFLAGS), due to the ioctl names the at-
tributes are also called flags
• xflags: to distinguish from the previous, it's extended flags, with
tunable bits similar to the attributes but extensible and new bits
will be added in the future (the ioctls are FS_IOC_FSGETXATTR and
FS_IOC_FSSETXATTR but they are not related to extended attributes that
are also called xattrs), there's no standard tool to change the bits,
there's support in ]8;;https://man7.org/linux/man-pages/man8/xfs_io.8.html\xfs_io(8)]8;;\ as command xfs_io -c chattr
Attributes
a append only, new writes are always written at the end of the file
A no atime updates
c compress data, all data written after this attribute is set will
be compressed. Please note that compression is also affected by
the mount options or the parent directory attributes.
When set on a directory, all newly created files will inherit
this attribute. This attribute cannot be set with 'm' at the
same time.
C no copy-on-write, file data modifications are done in-place
When set on a directory, all newly created files will inherit
this attribute.
NOTE:
Due to implementation limitations, this flag can be set/unset
only on empty files.
d no dump, makes sense with 3rd party tools like ]8;;https://man7.org/linux/man-pages/man8/dump.8.html\dump(8)]8;;\, on BTRFS
the attribute can be set/unset but no other special handling is
done
D synchronous directory updates, for more details search ]8;;https://man7.org/linux/man-pages/man2/open.2.html\open(2)]8;;\
for O_SYNC and O_DSYNC
i immutable, no file data and metadata changes allowed even to the
root user as long as this attribute is set (obviously the excep-
tion is unsetting the attribute)
m no compression, permanently turn off compression on the given
file. Any compression mount options will not affect this file. (]8;;https://man7.org/linux/man-pages/man1/chattr.1.html\-
chattr(1)]8;;\ support added in 1.46.2)
When set on a directory, all newly created files will inherit
this attribute. This attribute cannot be set with c at the same
time.
S synchronous updates, for more details search ]8;;https://man7.org/linux/man-pages/man2/open.2.html\open(2)]8;;\ for O_SYNC
and O_DSYNC
No other attributes are supported. For the complete list please refer
to the ]8;;https://man7.org/linux/man-pages/man1/chattr.1.html\chattr(1)]8;;\ manual page.
XFLAGS
There's an overlap of letters assigned to the bits with the attributes,
this list refers to what ]8;;https://man7.org/linux/man-pages/man8/xfs_io.8.html\xfs_io(8)]8;;\ provides:
i immutable, same as the attribute
a append only, same as the attribute
s synchronous updates, same as the attribute S
A no atime updates, same as the attribute
d no dump, same as the attribute
ZONED MODE
Since version 5.12 btrfs supports so called zoned mode. This is a spe-
cial on-disk format and allocation/write strategy that's friendly to
zoned devices. In short, a device is partitioned into fixed-size zones
and each zone can be updated by append-only manner, or reset. As btrfs
has no fixed data structures, except the super blocks, the zoned mode
only requires block placement that follows the device constraints. You
can learn about the whole architecture at ]8;;https://zonedstorage.io\https://zonedstorage.io]8;;\ .
The devices are also called SMR/ZBC/ZNS, in host-managed mode. Note that
there are devices that appear as non-zoned but actually are, this is
drive-managed and using zoned mode won't help.
The zone size depends on the device, typical sizes are 256MiB or 1GiB.
In general it must be a power of two. Emulated zoned devices like
null_blk allow to set various zone sizes.
Requirements, limitations
• all devices must have the same zone size
• maximum zone size is 8GiB
• minimum zone size is 4MiB
• mixing zoned and non-zoned devices is possible, the zone writes are
emulated, but this is namely for testing
• the super block is handled in a special way and is at different loca-
tions than on a non-zoned filesystem:
• primary: 0B (and the next two zones)
• secondary: 512GiB (and the next two zones)
• tertiary: 4TiB (4096GiB, and the next two zones)
Incompatible features
The main constraint of the zoned devices is lack of in-place update of
the data. This is inherently incompatible with some features:
• NODATACOW - overwrite in-place, cannot create such files
• fallocate - preallocating space for in-place first write
• mixed-bg - unordered writes to data and metadata, fixing that means
using separate data and metadata block groups
• booting - the zone at offset 0 contains superblock, resetting the zone
would destroy the bootloader data
Initial support lacks some features but they're planned:
• only single (data, metadata) and DUP (metadata) profile is supported
• fstrim - due to dependency on free space cache v1
Super block
As said above, super block is handled in a special way. In order to be
crash safe, at least one zone in a known location must contain a valid
superblock. This is implemented as a ring buffer in two consecutive
zones, starting from known offsets 0B, 512GiB and 4TiB.
The values are different than on non-zoned devices. Each new super block
is appended to the end of the zone, once it's filled, the zone is reset
and writes continue to the next one. Looking up the latest super block
needs to read offsets of both zones and determine the last written ver-
sion.
The amount of space reserved for super block depends on the zone size.
The secondary and tertiary copies are at distant offsets as the capacity
of the devices is expected to be large, tens of terabytes. Maximum zone
size supported is 8GiB, which would mean that e.g. offset 0-16GiB would
be reserved just for the super block on a hypothetical device of that
zone size. This is wasteful but required to guarantee crash safety.
Zone reclaim, garbage collection
As the zones are append-only, overwriting data or COW changes in meta-
data make parts of the zones used but not connected to the filesystem
structures. This makes the space unusable and grows over time. Once the
ratio hits a (configurable) threshold a background reclaim process is
started and relocates the remaining blocks in use to a new zone. The old
one is reset and can be used again.
This process may take some time depending on other background work or
amount of new data written. It is possible to hit an intermittent
ENOSPC. Some devices also limit number of active zones.
Devices
Real hardware
The WD Ultrastar series 600 advertises HM-SMR, i.e. the host-managed
zoned mode. There are two more: DA (device managed, no zoned information
exported to the system), HA (host aware, can be used as regular disk but
zoned writes improve performance). There are not many devices available
at the moment, the information about exact zoned mode is hard to find,
check data sheets or community sources gathering information from real
devices.
Note: zoned mode won't work with DM-SMR disks.
• Ultrastar® DC ZN540 NVMe ZNS SSD (]8;;https://documents.westerndigital.com/content/dam/doc-library/en_us/assets/public/western-digital/collateral/product-brief/product-brief-ultrastar-dc-zn540.pdf\product brief]8;;\)
Emulated: null_blk
The driver null_blk provides memory backed device and is suitable for
testing. There are some quirks setting up the devices. The module must
be loaded with nr_devices=0 or the numbering of device nodes will be
offset. The configfs must be mounted at /sys/kernel/config and the ad-
ministration of the null_blk devices is done in /sys/kernel/con-
fig/nullb. The device nodes are named like /dev/nullb0 and are numbered
sequentially. NOTE: the device name may be different than the named di-
rectory in sysfs!
Setup:
modprobe configfs
modprobe null_blk nr_devices=0
Create a device mydev, assuming no other previously created devices,
size is 2048MiB, zone size 256MiB. There are more tunable parameters,
this is a minimal example taking defaults:
cd /sys/kernel/config/nullb/
mkdir mydev
cd mydev
echo 2048 > size
echo 1 > zoned
echo 1 > memory_backed
echo 256 > zone_size
echo 1 > power
This will create a device /dev/nullb0 and the value of file index will
match the ending number of the device node.
Remove the device:
rmdir /sys/kernel/config/nullb/mydev
Then continue with mkfs.btrfs /dev/nullb0, the zoned mode is auto-de-
tected.
For convenience, there's a script wrapping the basic null_blk management
operations ]8;;https://github.com/kdave/nullb.git\https://github.com/kdave/nullb.git]8;;\, the above commands be-
come:
nullb setup
nullb create -s 2g -z 256
mkfs.btrfs /dev/nullb0
...
nullb rm nullb0
Emulated: TCMU runner
TCMU is a framework to emulate SCSI devices in userspace, providing var-
ious backends for the storage, with zoned support as well. A file-backed
zoned device can provide more options for larger storage and zone size.
Please follow the instructions at ]8;;https://zonedstorage.io/projects/tcmu-runner/\-
https://zonedstorage.io/projects/tcmu-runner/]8;;\ .
Compatibility, incompatibility
• the feature sets an incompat bit and requires new kernel to access the
filesystem (for both read and write)
• superblock needs to be handled in a special way, there are still 3
copies but at different offsets (0, 512GiB, 4TiB) and the 2 consecu-
tive zones are a ring buffer of the superblocks, finding the latest
one needs reading it from the write pointer or do a full scan of the
zones
• mixing zoned and non zoned devices is possible (zones are emulated)
but is recommended only for testing
• mixing zoned devices with different zone sizes is not possible
• zone sizes must be power of two, zone sizes of real devices are e.g.
256MiB or 1GiB, larger size is expected, maximum zone size supported
by btrfs is 8GiB
Status, stability, reporting bugs
The zoned mode has been released in 5.12 and there are still some rough
edges and corner cases one can hit during testing. Please report bugs to
]8;;https://github.com/naota/linux/issues/\https://github.com/naota/linux/issues/]8;;\ .
References
• ]8;;https://zonedstorage.io\https://zonedstorage.io]8;;\
• ]8;;https://zonedstorage.io/projects/libzbc/\https://zonedstorage.io/projects/libzbc/]8;;\ -- libzbc is library and
set of tools to directly manipulate devices with ZBC/ZAC support
• ]8;;https://zonedstorage.io/projects/libzbd/\https://zonedstorage.io/projects/libzbd/]8;;\ -- libzbd uses the kernel
provided zoned block device interface based on the ioctl() system
calls
• ]8;;https://hddscan.com/blog/2020/hdd-wd-smr.html\https://hddscan.com/blog/2020/hdd-wd-smr.html]8;;\ -- some details about
exact device types
• ]8;;https://lwn.net/Articles/853308/\https://lwn.net/Articles/853308/]8;;\ -- Btrfs on zoned block devices
• ]8;;https://www.usenix.org/conference/vault20/presentation/bjorling\https://www.usenix.org/conference/vault20/presentation/bjorling]8;;\ --
Zone Append: A New Way of Writing to Zoned Storage
CONTROL DEVICE
There's a character special device /dev/btrfs-control with major and mi-
nor numbers 10 and 234 (the device can be found under the misc cate-
gory).
$ ls -l /dev/btrfs-control
crw------- 1 root root 10, 234 Jan 1 12:00 /dev/btrfs-control
The device accepts some ioctl calls that can perform following actions
on the filesystem module:
• scan devices for btrfs filesystem (i.e. to let multi-device filesys-
tems mount automatically) and register them with the kernel module
• similar to scan, but also wait until the device scanning process is
finished for a given filesystem
• get the supported features (can be also found under /sys/fs/btrfs/fea-
tures)
The device is created when btrfs is initialized, either as a module or a
built-in functionality and makes sense only in connection with that.
Running e.g. mkfs without the module loaded will not register the device
and will probably warn about that.
In rare cases when the module is loaded but the device is not present
(most likely accidentally deleted), it's possible to recreate it by
# mknod --mode=600 /dev/btrfs-control c 10 234
or (since 5.11) by a convenience command
# btrfs rescue create-control-device
The control device is not strictly required but the device scanning will
not work and a workaround would need to be used to mount a multi-device
filesystem. The mount option device can trigger the device scanning
during mount, see also btrfs device scan.
FILESYSTEM WITH MULTIPLE PROFILES
It is possible that a btrfs filesystem contains multiple block group
profiles of the same type. This could happen when a profile conversion
using balance filters is interrupted (see btrfs-balance(8)). Some btrfs
commands perform a test to detect this kind of condition and print a
warning like this:
WARNING: Multiple block group profiles detected, see 'man btrfs(5)'.
WARNING: Data: single, raid1
WARNING: Metadata: single, raid1
The corresponding output of btrfs filesystem df might look like:
WARNING: Multiple block group profiles detected, see 'man btrfs(5)'.
WARNING: Data: single, raid1
WARNING: Metadata: single, raid1
Data, RAID1: total=832.00MiB, used=0.00B
Data, single: total=1.63GiB, used=0.00B
System, single: total=4.00MiB, used=16.00KiB
Metadata, single: total=8.00MiB, used=112.00KiB
Metadata, RAID1: total=64.00MiB, used=32.00KiB
GlobalReserve, single: total=16.25MiB, used=0.00B
There's more than one line for type Data and Metadata, while the pro-
files are single and RAID1.
This state of the filesystem OK but most likely needs the user/adminis-
trator to take an action and finish the interrupted tasks. This cannot
be easily done automatically, also the user knows the expected final
profiles.
In the example above, the filesystem started as a single device and sin-
gle block group profile. Then another device was added, followed by bal-
ance with convert=raid1 but for some reason hasn't finished. Restarting
the balance with convert=raid1 will continue and end up with filesystem
with all block group profiles RAID1.
NOTE:
If you're familiar with balance filters, you can use con-
vert=raid1,profiles=single,soft, which will take only the unconverted
single profiles and convert them to raid1. This may speed up the con-
version as it would not try to rewrite the already convert raid1 pro-
files.
Having just one profile is desired as this also clearly defines the pro-
file of newly allocated block groups, otherwise this depends on internal
allocation policy. When there are multiple profiles present, the order
of selection is RAID56, RAID10, RAID1, RAID0 as long as the device num-
ber constraints are satisfied.
Commands that print the warning were chosen so they're brought to user
attention when the filesystem state is being changed in that regard.
This is: device add, device delete, balance cancel, balance pause. Com-
mands that report space usage: filesystem df, device usage. The command
filesystem usage provides a line in the overall summary:
Multiple profiles: yes (data, metadata)
SEEDING DEVICE
The COW mechanism and multiple devices under one hood enable an inter-
esting concept, called a seeding device: extending a read-only filesys-
tem on a device with another device that captures all writes. For exam-
ple imagine an immutable golden image of an operating system enhanced
with another device that allows to use the data from the golden image
and normal operation. This idea originated on CD-ROMs with base OS and
allowing to use them for live systems, but this became obsolete. There
are technologies providing similar functionality, like ]8;;https://en.wikipedia.org/wiki/Union_mount\unionmount]8;;\, ]8;;https://en.wikipedia.org/wiki/OverlayFS\-
overlayfs]8;;\ or ]8;;https://en.wikipedia.org/wiki/Qcow#qcow2\qcow2]8;;\ image snapshot.
The seeding device starts as a normal filesystem, once the contents is
ready, btrfstune -S 1 is used to flag it as a seeding device. Mounting
such device will not allow any writes, except adding a new device by
btrfs device add. Then the filesystem can be remounted as read-write.
Given that the filesystem on the seeding device is always recognized as
read-only, it can be used to seed multiple filesystems from one device
at the same time. The UUID that is normally attached to a device is au-
tomatically changed to a random UUID on each mount.
Once the seeding device is mounted, it needs the writable device. After
adding it, unmounting and mounting with umount /path; mount
/dev/writable /path or remounting read-write with remount -o remount,rw
makes the filesystem at /path ready for use.
NOTE:
There is a known bug with using remount to make the mount writeable:
remount will leave the filesystem in a state where it is unable to
clean deleted snapshots, so it will leak space until it is unmounted
and mounted properly.
Furthermore, deleting the seeding device from the filesystem can turn it
into a normal filesystem, provided that the writable device can also
contain all the data from the seeding device.
The seeding device flag can be cleared again by btrfstune -f -S 0, e.g.
allowing to update with newer data but please note that this will inval-
idate all existing filesystems that use this particular seeding device.
This works for some use cases, not for others, and the forcing flag to
the command is mandatory to avoid accidental mistakes.
Example how to create and use one seeding device:
# mkfs.btrfs /dev/sda
# mount /dev/sda /mnt/mnt1
... fill mnt1 with data
# umount /mnt/mnt1
# btrfstune -S 1 /dev/sda
# mount /dev/sda /mnt/mnt1
# btrfs device add /dev/sdb /mnt/mnt1
# umount /mnt/mnt1
# mount /dev/sdb /mnt/mnt1
... /mnt/mnt1 is now writable
Now /mnt/mnt1 can be used normally. The device /dev/sda can be mounted
again with a another writable device:
# mount /dev/sda /mnt/mnt2
# btrfs device add /dev/sdc /mnt/mnt2
# umount /mnt/mnt2
# mount /dev/sdc /mnt/mnt2
... /mnt/mnt2 is now writable
The writable device (file:/dev/sdb) can be decoupled from the seeding
device and used independently:
# btrfs device delete /dev/sda /mnt/mnt1
As the contents originated in the seeding device, it's possible to turn
/dev/sdb to a seeding device again and repeat the whole process.
A few things to note:
• it's recommended to use only single device for the seeding device, it
works for multiple devices but the single profile must be used in or-
der to make the seeding device deletion work
• block group profiles single and dup support the use cases above
• the label is copied from the seeding device and can be changed by
btrfs filesystem label
• each new mount of the seeding device gets a new random UUID
• umount /path; mount /dev/writable /path can be replaced with mount -o
remount,rw /path but it won't reclaim space of deleted subvolumes un-
til the seeding device is mounted read-write again before making it
seeding again
Chained seeding devices
Though it's not recommended and is rather an obscure and untested use
case, chaining seeding devices is possible. In the first example, the
writable device /dev/sdb can be turned onto another seeding device
again, depending on the unchanged seeding device /dev/sda. Then using
/dev/sdb as the primary seeding device it can be extended with another
writable device, say /dev/sdd, and it continues as before as a simple
tree structure on devices.
# mkfs.btrfs /dev/sda
# mount /dev/sda /mnt/mnt1
... fill mnt1 with data
# umount /mnt/mnt1
# btrfstune -S 1 /dev/sda
# mount /dev/sda /mnt/mnt1
# btrfs device add /dev/sdb /mnt/mnt1
# mount -o remount,rw /mnt/mnt1
... /mnt/mnt1 is now writable
# umount /mnt/mnt1
# btrfstune -S 1 /dev/sdb
# mount /dev/sdb /mnt/mnt1
# btrfs device add /dev/sdc /mnt
# mount -o remount,rw /mnt/mnt1
... /mnt/mnt1 is now writable
# umount /mnt/mnt1
As a result we have:
• sda is a single seeding device, with its initial contents
• sdb is a seeding device but requires sda, the contents are from the
time when sdb is made seeding, i.e. contents of sda with any later
changes
• sdc last writable, can be made a seeding one the same way as was sdb,
preserving its contents and depending on sda and sdb
As long as the seeding devices are unmodified and available, they can be
used to start another branch.
RAID56 STATUS AND RECOMMENDED PRACTICES
The RAID56 feature provides striping and parity over several devices,
same as the traditional RAID5/6. There are some implementation and de-
sign deficiencies that make it unreliable for some corner cases and the
feature should not be used in production, only for evaluation or test-
ing. The power failure safety for metadata with RAID56 is not 100%.
Metadata
Do not use raid5 nor raid6 for metadata. Use raid1 or raid1c3 respec-
tively.
The substitute profiles provide the same guarantees against loss of 1 or
2 devices, and in some respect can be an improvement. Recovering from
one missing device will only need to access the remaining 1st or 2nd
copy, that in general may be stored on some other devices due to the way
RAID1 works on btrfs, unlike on a striped profile (similar to raid0)
that would need all devices all the time.
The space allocation pattern and consumption is different (e.g. on N de-
vices): for raid5 as an example, a 1GiB chunk is reserved on each de-
vice, while with raid1 there's each 1GiB chunk stored on 2 devices. The
consumption of each 1GiB of used metadata is then N * 1GiB for vs 2 *
1GiB. Using raid1 is also more convenient for balancing/converting to
other profile due to lower requirement on the available chunk space.
Missing/incomplete support
When RAID56 is on the same filesystem with different raid profiles, the
space reporting is inaccurate, e.g. df, btrfs filesystem df or btrfs
filesystem usage. When there's only a one profile per block group type
(e.g. RAID5 for data) the reporting is accurate.
When scrub is started on a RAID56 filesystem, it's started on all de-
vices that degrade the performance. The workaround is to start it on
each device separately. Due to that the device stats may not match the
actual state and some errors might get reported multiple times.
The write hole problem. An unclean shutdown could leave a partially
written stripe in a state where the some stripe ranges and the parity
are from the old writes and some are new. The information which is which
is not tracked. Write journal is not implemented. Alternatively a full
read-modify-write would make sure that a full stripe is always written,
avoiding the write hole completely, but performance in that case turned
out to be too bad for use.
The striping happens on all available devices (at the time the chunks
were allocated), so in case a new device is added it may not be utilized
immediately and would require a rebalance. A fixed configured stripe
width is not implemented.
GLOSSARY
Terms in italics also appear in this glossary.
allocator
Usually allocator means the block allocator, i.e. the logic in-
side the filesystem which decides where to place newly allocated
blocks in order to maintain several constraints (like data local-
ity, low fragmentation).
In btrfs, allocator may also refer to chunk allocator, i.e. the
logic behind placing chunks on devices.
balance
An operation that can be done to a btrfs filesystem, for example
through btrfs balance /path. A balance passes all data in the
filesystem through the allocator again. It is primarily intended
to rebalance the data in the filesystem across the devices when a
device is added or removed. A balance will regenerate missing
copies for the redundant RAID levels, if a device has failed. As
of Linux kernel 3.3, a balance operation can be made selective
about which parts of the filesystem are rewritten using filters.
barrier
An instruction to the underlying hardware to ensure that every-
thing before the barrier is physically written to permanent stor-
age before anything after it. Used in btrfs's copy on write ap-
proach to ensure filesystem consistency.
block A single physically and logically contiguous piece of storage on
a device, of size e.g. 4K. In some contexts also referred to as
sector, though the term block is preferred.
block group
The unit of allocation of space in btrfs. A block group is laid
out on the disk by the btrfs allocator, and will consist of one
or more chunks, each stored on a different device. The number of
chunks used in a block group will depend on its RAID level.
B-tree The fundamental storage data structure used in btrfs. Except for
the superblocks, all of btrfs metadata is stored in one of sev-
eral B-trees on disk. B-trees store key/item pairs. While the
same code is used to implement all of the B-trees, there are a
few different categories of B-tree. The name btrfs refers to its
use of B-trees.
btrfsck, fsck, btrfs-check
Tool in btrfs-progs that checks an unmounted filesystem (offline)
and reports on any errors in the filesystem structures it finds.
By default the tool runs in read-only mode as fixing errors is
potentially dangerous. See also scrub.
btrfs-progs
User mode tools to manage btrfs-specific features. Maintained at
]8;;http://github.com/kdave/btrfs-progs.git\http://github.com/kdave/btrfs-progs.git]8;;\ . The main frontend to
btrfs features is the standalone tool btrfs, although other tools
such as mkfs.btrfs and btrfstune are also part of btrfs-progs.
chunk A part of a block group. Chunks are either 1 GiB in size (for
data) or 256 MiB (for metadata), depending on the overall
filesystem size.
chunk tree
A layer that keeps information about mapping between physical and
logical block addresses. It's stored within the system group.
cleaner
Usually referred to in context of deleted subvolumes. It's a
background process that removes the actual data once a subvolume
has been deleted. Cleaning can involve lots of IO and CPU activ-
ity depending on the fragmentation and amount of shared data with
other subvolumes.
The cleaner kernel thread also processes defragmentation trig-
gered by the autodefrag mount option, removing of empty blocks
groups and some other finalization tasks.
copy-on-write, COW
Also known as COW. The method that btrfs uses for modifying data.
Instead of directly overwriting data in place, btrfs takes a copy
of the data, alters it, and then writes the modified data back to
a different (unused) location on the disk. It then updates the
metadata to reflect the new location of the data. In order to up-
date the metadata, the affected metadata blocks are also treated
in the same way. In COW filesystems, files tend to fragment as
they are modified. Copy-on-write is also used in the implementa-
tion of snapshots and reflink copies. A copy-on-write filesystem
is, in theory, always consistent, provided the underlying hard-
ware supports barriers.
default subvolume
The subvolume in a btrfs filesystem which is mounted when mount-
ing the filesystem without using the subvol= mount option.
device A Linux block device, e.g. a whole disk, partition, LVM logical
volume, loopback device, or network block device. A btrfs
filesystem can reside on one or more devices.
df A standard Unix tool for reporting the amount of space used and
free in a filesystem. The standard tool does not give accurate
results, but the btrfs command from btrfs-progs has an implemen-
tation of df which shows space available in more detail. See the
[[FAQ#Why_does_df_show_incorrect_free_space_for_my_RAID_vol-
ume.3F|FAQ]] for a more detailed explanation of btrfs free space
accounting.
DUP A form of "RAID" which stores two copies of each piece of data on
the same device. This is similar to RAID1, and protects against
block-level errors on the device, but does not provide any guar-
antees if the entire device fails. By default, btrfs uses DUP
profile for metadata on single device filesystem.s
ENOSPC Error code returned by the OS to a user program when the filesys-
tem cannot allocate enough data to fulfill the user request. In
most filesystems, it indicates there is no free space available
in the filesystem. Due to the additional space requirements from
btrfs's COW behaviour, btrfs can sometimes return ENOSPC when
there is apparently (in terms of df) a large amount of space
free. This is effectively a bug in btrfs, and (if it is repeat-
able), using the mount option enospc_debug may give a report that
will help the btrfs developers. See the [[FAQ#if_your_de-
vice_is_large_.28.3E16GiB.29|FAQ entry]] on free space.
extent Contiguous sequence of bytes on disk that holds file data. It's a
compact representation that tracks the start and length of the
byte range, so the logic behind allocating blocks (delayed allo-
cation) strives for maximizing the length before writing the ex-
tents to the devices.
extent buffer
An abstraction of a b-tree metadata block storing item keys and
item data. The underlying related structures are physical device
block and a CPU memory page.
fallocate
Command line tool in util-linux, and a syscall, that reserves
space in the filesystem for a file, without actually writing any
file data to the filesystem. First data write will turn the pre-
allocated extents into regular ones. See ]8;;https://man7.org/linux/man-pages/man1/fallocate.1.html\fallocate(1)]8;;\ and ]8;;https://man7.org/linux/man-pages/man2/fallocate.2.html\-
fallocate(2)]8;;\ manual pages for more details.
filefrag
A tool to show the number of extents in a file, and hence the
amount of fragmentation in the file. It is usually part of the
e2fsprogs package on most Linux distributions. While initially
developed for the ext2 filesystem, it works on Btrfs as well. It
uses the FIEMAP ioctl.
free space cache
Also known as "space cache v1". A separate cache tracking free
space as btrfs only tracks the allocated space. The free space is
by definition any hole between allocated ranges. Finding the free
ranges can be I/O intensive so the cache stores a condensed rep-
resentation of it. It is updated every transaction commit.
The v1 free space cache has been superseded by free space tree.
free space tree
Successor of free space cache, also known as "space cache v2" and
now default. The free space is tracked in a better way and using
COW unlike a custom mechanism of v1.
fsync On Unix and Unix-like operating systems (of which Linux is the
latter), the ]8;;https://man7.org/linux/man-pages/man2/fsync.2.html\fsync(2)]8;;\ system call causes all buffered file de-
scriptor related data changes to be flushed to the underlying
block device. When a file is modified on a modern operating sys-
tem the changes are generally not written to the disk immediately
but rather those changes are buffered in memory for performance
reasons, calling ]8;;https://man7.org/linux/man-pages/man2/fsync.2.html\fsync(2)]8;;\ causes any in-memory changes to be
written to disk.
generation
An internal counter which updates for each transaction. When a
metadata block is written (using copy on write), current genera-
tion is stored in the block, so that blocks which are too new
(and hence possibly inconsistent) can be identified.
key A fixed sized tuple used to identify and sort items in a B-tree.
The key is broken up into 3 parts: objectid, type, and offset.
The type field indicates how each of the other two fields should
be used, and what to expect to find in the item.
item A variable sized structure stored in B-tree leaves. Items hold
different types of data depending on key type.
log tree
A b-tree that temporarily tracks ongoing metadata updates until a
full transaction commit is done. It's a performance optimization
of fsync. The log tracked in the tree are replayed if the
filesystem is not unmounted cleanly.
metadata
Data about data. In btrfs, this includes all of the internal data
structures of the filesystem, including directory structures,
filenames, file permissions, checksums, and the location of each
file's extents. All btrfs metadata is stored in B-trees.
mkfs.btrfs
The tool (from btrfs-progs) to create a btrfs filesystem.
offline
A filesystem which is not mounted is offline. Some tools (e.g.
btrfsck) will only work on offline filesystems. Compare online.
online A filesystem which is mounted is online. Most btrfs tools will
only work on online filesystems. Compare offline.
orphan A file that's still in use (opened by a running process) but all
directory entries of that file have been removed.
RAID A class of different methods for writing some additional redun-
dant data across multiple devices so that if one device fails,
the missing data can be reconstructed from the remaining ones.
See RAID0, RAID1, RAID5, RAID6, RAID10, DUP and single. Tradi-
tional RAID methods operate across multiple devices of equal
size, whereas btrfs' RAID implementation works inside block
groups.
RAID0 A form of RAID which provides no guarantees of error recovery,
but stripes a single copy of data across multiple devices for
performance purposes. The stripe size is fixed to 64KB for now.
RAID1, RAID1C3, RAID1C4
A form of RAID which stores two/three/four complete copies of
each piece of data. Each copy is stored on a different device.
btrfs requires a minimum of two devices to use RAID-1 or
three/four respectively. This is the default block group profile
for btrfs's metadata on more than one device.
RAID5 A form of RAID which stripes a single copy of data across multi-
ple devices, including one device's worth of additional parity
data. Can be used to recover from a single device failure.
RAID6 A form of RAID which stripes a single copy of data across multi-
ple devices, including two device's worth of additional parity
data. Can be used to recover from the failure of two devices.
RAID10 A form of RAID which stores two complete copies of each piece of
data, and also stripes each copy across multiple devices for per-
formance.
reflink
Commonly used as a reference to a shallow copy of file extents
that share the extents until the first change. Reflinked files
(e.g. by the cp) are different files but point to the same ex-
tents, any change will be detected and new copy of the data cre-
ated, keeping the files independent. Related to that is extent
range cloning, that works on a range of a file.
relocation
The process of moving block groups within the filesystem while
maintaining full filesystem integrity and consistency. This func-
tionality is underlying balance and device removing features.
scrub An online filesystem checking tool. Reads all the data and meta-
data on the filesystem, verifies checksums and eventually uses
redundant copies from RAID or DUP repair any corrupt data/meta-
data.
seed device
A readonly device can be used as a filesystem seed or template
(e.g. a CD-ROM containing an OS image). Read/write devices can be
added to store modifications (using copy on write), changes to
the writable devices are persistent across reboots. The original
device remains unchanged and can be removed at any time (after
Btrfs has been instructed to copy over all missing blocks). Mul-
tiple read/write file systems can be built from the same seed.
single A block group profile storing a single copy of each piece of
data.
snapshot
A subvolume which is a copy on write copy of another subvolume.
The two subvolumes share all of their common (unmodified) data,
which means that snapshots can be used to keep the historical
state of a filesystem very cheaply. After the snapshot is made,
the original subvolume and the snapshot are of equal status: the
original does not "own" the snapshot, and either one can be
deleted without affecting the other one.
subvolume
A tree of files and directories inside a btrfs that can be
mounted as if it were an independent filesystem. A subvolume is
created by taking a reference on the root of another subvolume.
Each btrfs filesystem has at least one subvolume, the top-level
subvolume, which contains everything else in the filesystem. Ad-
ditional subvolumes can be created and deleted with the btrfs<
tool. All subvolumes share the same pool of free space in the
filesystem. See also default subvolume.
super block
A special metadata block that is a main access point of the
filesystem structures. It's size is fixed and there are fixed lo-
cations on the devices used for detecting and opening the
filesystem. Updating the superblock defines one transaction. The
super blocks contains filesystem identification (UUID), checksum
type, block pointers to fundamental trees, features and creation
parameters.
system array
A technical term for super block metadata describing how to as-
semble a filesystem from multiple device, storing information
about chunks and devices that are required to be scanned/regis-
tered at the time the mount happens. Scanning is done by command
btrfs device scan, alternatively all the required devices can be
specified by a mount option device=/path.
top-level subvolume
The subvolume at the very top of the filesystem. This is the only
subvolume present in a newly-created btrfs filesystem, and inter-
nally has ID 5, otherwise could be referenced as 0 (e.g. within
the set-default subcommand of btrfs).
transaction
A consistent set of changes. To avoid generating very large
amounts of disk activity, btrfs caches changes in RAM for up to
30 seconds (sometimes more often if the filesystem is running
short on space or doing a lot of fsync*s), and then writes (com-
mits) these changes out to disk in one go (using *copy on write
behaviour). This period of caching is called a transaction. Only
one transaction is active on the filesystem at any one time.
transid
An alternative term for generation.
writeback
Writeback in the context of the Linux kernel can be defined as
the process of writing "dirty" memory from the page cache to the
disk, when certain conditions are met (timeout, number of dirty
pages over a ratio).
STORAGE MODEL, HARDWARE CONSIDERATIONS
Storage model
A storage model is a model that captures key physical aspects of data
structure in a data store. A filesystem is the logical structure orga-
nizing data on top of the storage device.
The filesystem assumes several features or limitations of the storage
device and utilizes them or applies measures to guarantee reliability.
BTRFS in particular is based on a COW (copy on write) mode of writing,
i.e. not updating data in place but rather writing a new copy to a dif-
ferent location and then atomically switching the pointers.
In an ideal world, the device does what it promises. The filesystem as-
sumes that this may not be true so additional mechanisms are applied to
either detect misbehaving hardware or get valid data by other means. The
devices may (and do) apply their own detection and repair mechanisms but
we won't assume any.
The following assumptions about storage devices are considered (sorted
by importance, numbers are for further reference):
1. atomicity of reads and writes of blocks/sectors (the smallest unit of
data the device presents to the upper layers)
2. there's a flush command that instructs the device to forcibly order
writes before and after the command; alternatively there's a barrier
command that facilitates the ordering but may not flush the data
3. data sent to write to a given device offset will be written without
further changes to the data and to the offset
4. writes can be reordered by the device, unless explicitly serialized
by the flush command
5. reads and writes can be freely reordered and interleaved
The consistency model of BTRFS builds on these assumptions. The logical
data updates are grouped, into a generation, written on the device, se-
rialized by the flush command and then the super block is written ending
the generation. All logical links among metadata comprising a consis-
tent view of the data may not cross the generation boundary.
When things go wrong
No or partial atomicity of block reads/writes (1)
• Problem: a partial block contents is written (torn write), e.g. due to
a power glitch or other electronics failure during the read/write
• Detection: checksum mismatch on read
• Repair: use another copy or rebuild from multiple blocks using some
encoding scheme
The flush command does not flush (2)
This is perhaps the most serious problem and impossible to mitigate by
filesystem without limitations and design restrictions. What could hap-
pen in the worst case is that writes from one generation bleed to an-
other one, while still letting the filesystem consider the generations
isolated. Crash at any point would leave data on the device in an incon-
sistent state without any hint what exactly got written, what is missing
and leading to stale metadata link information.
Devices usually honor the flush command, but for performance reasons may
do internal caching, where the flushed data are not yet persistently
stored. A power failure could lead to a similar scenario as above, al-
though it's less likely that later writes would be written before the
cached ones. This is beyond what a filesystem can take into account. De-
vices or controllers are usually equipped with batteries or capacitors
to write the cache contents even after power is cut. (Battery backed
write cache)
Data get silently changed on write (3)
Such thing should not happen frequently, but still can happen spuriously
due the complex internal workings of devices or physical effects of the
storage media itself.
• Problem: while the data are written atomically, the contents get
changed
• Detection: checksum mismatch on read
• Repair: use another copy or rebuild from multiple blocks using some
encoding scheme
Data get silently written to another offset (3)
This would be another serious problem as the filesystem has no informa-
tion when it happens. For that reason the measures have to be done ahead
of time. This problem is also commonly called ghost write.
The metadata blocks have the checksum embedded in the blocks, so a cor-
rect atomic write would not corrupt the checksum. It's likely that after
reading such block the data inside would not be consistent with the
rest. To rule that out there's embedded block number in the metadata
block. It's the logical block number because this is what the logical
structure expects and verifies.
The following is based on information publicly available, user feedback,
community discussions or bug report analyses. It's not complete and fur-
ther research is encouraged when in doubt.
Main memory
The data structures and raw data blocks are temporarily stored in com-
puter memory before they get written to the device. It is critical that
memory is reliable because even simple bit flips can have vast conse-
quences and lead to damaged structures, not only in the filesystem but
in the whole operating system.
Based on experience in the community, memory bit flips are more common
than one would think. When it happens, it's reported by the tree-checker
or by a checksum mismatch after reading blocks. There are some very ob-
vious instances of bit flips that happen, e.g. in an ordered sequence of
keys in metadata blocks. We can easily infer from the other data what
values get damaged and how. However, fixing that is not straightforward
and would require cross-referencing data from the entire filesystem to
see the scope.
If available, ECC memory should lower the chances of bit flips, but this
type of memory is not available in all cases. A memory test should be
performed in case there's a visible bit flip pattern, though this may
not detect a faulty memory module because the actual load of the system
could be the factor making the problems appear. In recent years attacks
on how the memory modules operate have been demonstrated (rowhammer)
achieving specific bits to be flipped. While these were targeted, this
shows that a series of reads or writes can affect unrelated parts of
memory.
Block group profiles with redundancy (like RAID1) will not protect
against memory errors as the blocks are first stored in memory before
they are written to the devices from the same source.
A filesystem mounted read-only will not affect the underlying block de-
vice in almost 100% (with highly unlikely exceptions). The exception is
a tree-log that needs to be replayed during mount (and before the
read-only mount takes place), working memory is needed for that and that
can be affected by bit flips. There's a theoretical case where bit flip
changes the filesystem status from read-only to read-write.
Further reading:
• ]8;;https://en.wikipedia.org/wiki/Row_hammer\https://en.wikipedia.org/wiki/Row_hammer]8;;\
• memory overclocking, XMP, potential risks
What to do:
• run memtest, note that sometimes memory errors happen only when the
system is under heavy load that the default memtest cannot trigger
• memory errors may appear as filesystem going read-only due to "pre
write" check, that verify meta data before they get written but fail
some basic consistency checks
• newly built systems should be tested before being put to production
use, ideally start a IO/CPU load that will be run on such system
later; namely systems that will utilize overclocking or special per-
formance features
Direct memory access (DMA)
Another class of errors is related to DMA (direct memory access) per-
formed by device drivers. While this could be considered a software er-
ror, the data transfers that happen without CPU assistance may acciden-
tally corrupt other pages. Storage devices utilize DMA for performance
reasons, the filesystem structures and data pages are passed back and
forth, making errors possible in case page life time is not properly
tracked.
There are lots of quirks (device-specific workarounds) in Linux kernel
drivers (regarding not only DMA) that are added when found. The quirks
may avoid specific errors or disable some features to avoid worse prob-
lems.
What to do:
• use up-to-date kernel (recent releases or maintained long term support
versions)
• as this may be caused by faulty drivers, keep the systems up-to-date
Rotational disks (HDD)
Rotational HDDs typically fail at the level of individual sectors or
small clusters. Read failures are caught on the levels below the
filesystem and are returned to the user as EIO - Input/output error.
Reading the blocks repeatedly may return the data eventually, but this
is better done by specialized tools and filesystem takes the result of
the lower layers. Rewriting the sectors may trigger internal remapping
but this inevitably leads to data loss.
Disk firmware is technically software but from the filesystem perspec-
tive is part of the hardware. IO requests are processed, and caching or
various other optimizations are performed, which may lead to bugs under
high load or unexpected physical conditions or unsupported use cases.
Disks are connected by cables with two ends, both of which can cause
problems when not attached properly. Data transfers are protected by
checksums and the lower layers try hard to transfer the data correctly
or not at all. The errors from badly-connecting cables may manifest as
large amount of failed read or write requests, or as short error bursts
depending on physical conditions.
What to do:
• check smartctl for potential issues
Solid state drives (SSD)
The mechanism of information storage is different from HDDs and this af-
fects the failure mode as well. The data are stored in cells grouped in
large blocks with limited number of resets and other write constraints.
The firmware tries to avoid unnecessary resets and performs optimiza-
tions to maximize the storage media lifetime. The known techniques are
deduplication (blocks with same fingerprint/hash are mapped to same
physical block), compression or internal remapping and garbage collec-
tion of used memory cells. Due to the additional processing there are
measures to verify the data e.g. by ECC codes.
The observations of failing SSDs show that the whole electronic fails at
once or affects a lot of data (e.g. stored on one chip). Recovering such
data may need specialized equipment and reading data repeatedly does not
help as it's possible with HDDs.
There are several technologies of the memory cells with different char-
acteristics and price. The lifetime is directly affected by the type and
frequency of data written. Writing "too much" distinct data (e.g. en-
crypted) may render the internal deduplication ineffective and lead to a
lot of rewrites and increased wear of the memory cells.
There are several technologies and manufacturers so it's hard to de-
scribe them but there are some that exhibit similar behaviour:
• expensive SSD will use more durable memory cells and is optimized for
reliability and high load
• cheap SSD is projected for a lower load ("desktop user") and is opti-
mized for cost, it may employ the optimizations and/or extended error
reporting partially or not at all
It's not possible to reliably determine the expected lifetime of an SSD
due to lack of information about how it works or due to lack of reliable
stats provided by the device.
Metadata writes tend to be the biggest component of lifetime writes to a
SSD, so there is some value in reducing them. Depending on the device
class (high end/low end) the features like DUP block group profiles may
affect the reliability in both ways:
• high end are typically more reliable and using single for data and
metadata could be suitable to reduce device wear
• low end could lack ability to identify errors so an additional redun-
dancy at the filesystem level (checksums, DUP) could help
Only users who consume 50 to 100% of the SSD's actual lifetime writes
need to be concerned by the write amplification of btrfs DUP metadata.
Most users will be far below 50% of the actual lifetime, or will write
the drive to death and discover how many writes 100% of the actual life-
time was. SSD firmware often adds its own write multipliers that can be
arbitrary and unpredictable and dependent on application behavior, and
these will typically have far greater effect on SSD lifespan than DUP
metadata. It's more or less impossible to predict when a SSD will run
out of lifetime writes to within a factor of two, so it's hard to jus-
tify wear reduction as a benefit.
Further reading:
• ]8;;https://www.snia.org/educational-library/ssd-and-deduplication-end-spinning-disk-2012\-
https://www.snia.org/educational-library/ssd-and-deduplica-
tion-end-spinning-disk-2012]8;;\
• ]8;;https://www.snia.org/educational-library/realities-solid-state-storage-2013-2013\-
https://www.snia.org/educational-library/realities-solid-state-stor-
age-2013-2013]8;;\
• ]8;;https://www.snia.org/educational-library/ssd-performance-primer-2013\https://www.snia.org/educational-library/ssd-performance-primer-2013]8;;\
• ]8;;https://www.snia.org/educational-library/how-controllers-maximize-ssd-life-2013\-
https://www.snia.org/educational-library/how-controllers-maxi-
mize-ssd-life-2013]8;;\
What to do:
• run smartctl or self-tests to look for potential issues
• keep the firmware up-to-date
NVM express, non-volatile memory (NVMe)
NVMe is a type of persistent memory usually connected over a system bus
(PCIe) or similar interface and the speeds are an order of magnitude
faster than SSD. It is also a non-rotating type of storage, and is not
typically connected by a cable. It's not a SCSI type device either but
rather a complete specification for logical device interface.
In a way the errors could be compared to a combination of SSD class and
regular memory. Errors may exhibit as random bit flips or IO failures.
There are tools to access the internal log (nvme log and nvme-cli) for a
more detailed analysis.
There are separate error detection and correction steps performed e.g.
on the bus level and in most cases never making in to the filesystem
level. Once this happens it could mean there's some systematic error
like overheating or bad physical connection of the device. You may want
to run self-tests (using smartctl).
• ]8;;https://en.wikipedia.org/wiki/NVM_Express\https://en.wikipedia.org/wiki/NVM_Express]8;;\
• ]8;;https://www.smartmontools.org/wiki/NVMe_Support\https://www.smartmontools.org/wiki/NVMe_Support]8;;\
Drive firmware
Firmware is technically still software but embedded into the hardware.
As all software has bugs, so does firmware. Storage devices can update
the firmware and fix known bugs. In some cases the it's possible to
avoid certain bugs by quirks (device-specific workarounds) in Linux ker-
nel.
A faulty firmware can cause wide range of corruptions from small and lo-
calized to large affecting lots of data. Self-repair capabilities may
not be sufficient.
What to do:
• check for firmware updates in case there are known problems, note that
updating firmware can be risky on itself
• use up-to-date kernel (recent releases or maintained long term support
versions)
SD flash cards
There are a lot of devices with low power consumption and thus using
storage media based on low power consumption too, typically flash memory
stored on a chip enclosed in a detachable card package. An improperly
inserted card may be damaged by electrical spikes when the device is
turned on or off. The chips storing data in turn may be damaged perma-
nently. All types of flash memory have a limited number of rewrites, so
the data are internally translated by FTL (flash translation layer).
This is implemented in firmware (technically a software) and prone to
bugs that manifest as hardware errors.
Adding redundancy like using DUP profiles for both data and metadata can
help in some cases but a full backup might be the best option once prob-
lems appear and replacing the card could be required as well.
Hardware as the main source of filesystem corruptions
If you use unreliable hardware and don't know about that, don't blame
the filesystem when it tells you.
SEE ALSO
]8;;https://man7.org/linux/man-pages/man5/acl.5.html\acl(5)]8;;\, btrfs(8), ]8;;https://man7.org/linux/man-pages/man1/chattr.1.html\chattr(1)]8;;\, ]8;;https://man7.org/linux/man-pages/man8/fstrim.8.html\fstrim(8)]8;;\, ]8;;https://man7.org/linux/man-pages/man2/ioctl.2.html\ioctl(2)]8;;\, btrfs-ioctl(2),
mkfs.btrfs(8), ]8;;https://man7.org/linux/man-pages/man8/mount.8.html\mount(8)]8;;\, ]8;;https://man7.org/linux/man-pages/man8/swapon.8.html\swapon(8)]8;;\
6.14 Apr 17, 2025 BTRFS(5)
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