btrfs-balance(8) - Man pages

BTRFS-BALANCE(8) BTRFS BTRFS-BALANCE(8)

NAME
btrfs-balance - balance block groups on a btrfs filesystem

SYNOPSIS
btrfs balance <subcommand> <args>

DESCRIPTION
The primary purpose of the balance feature is to spread block groups
across all devices so they match constraints defined by the respective
profiles. See mkfs.btrfs(8) section PROFILES for more details. The
scope of the balancing process can be further tuned by use of filters
that can select the block groups to process. Balance works only on a
mounted filesystem. Extent sharing is preserved and reflinks are not
broken. Files are not defragmented nor recompressed, file extents are
preserved but the physical location on devices will change.

The balance operation is cancellable by the user. The on-disk state of
the filesystem is always consistent so an unexpected interruption (e.g.
system crash, reboot) does not corrupt the filesystem. The progress of
the balance operation is temporarily stored as an internal state and
will be resumed upon mount, unless the mount option skip_balance is
specified.

WARNING:
Running balance without filters will take a lot of time as it basi-
cally move data/metadata from the whole filesystem and needs to up-
date all block pointers.

The filters can be used to perform following actions:

• convert block group profiles (filter convert)

• make block group usage more compact (filter usage)

• perform actions only on a given device (filters devid, drange)

The filters can be applied to a combination of block group types (data,
metadata, system). Note that changing only the system type needs the
force option. Otherwise system gets automatically converted whenever
metadata profile is converted.

When metadata redundancy is reduced (e.g. from RAID1 to single) the
force option is also required and it is noted in system log.

NOTE:
The balance operation needs enough work space, i.e. space that is
completely unused in the filesystem, otherwise this may lead to
ENOSPC reports. See the section ENOSPC for more details.

COMPATIBILITY
NOTE:
The balance subcommand also exists under the btrfs filesystem name-
space. This still works for backward compatibility but is deprecated
and should not be used any more.

NOTE:
A short syntax btrfs balance <path> works due to backward compatibil-
ity but is deprecated and should not be used any more. Use btrfs bal-
ance start command instead.

PERFORMANCE IMPLICATIONS
Balancing operations are very IO intensive and can also be quite CPU in-
tensive, impacting other ongoing filesystem operations. Typically large
amounts of data are copied from one location to another, with corre-
sponding metadata updates.

Depending upon the block group layout, it can also be seek heavy. Per-
formance on rotational devices is noticeably worse compared to SSDs or
fast arrays.

SUBCOMMAND
cancel <path>
cancels a running or paused balance, the command will block and
wait until the current block group being processed completes

Since kernel 5.7 the response time of the cancellation is signif-
icantly improved, on older kernels it might take a long time un-
til currently processed chunk is completely finished.

pause <path>
pause running balance operation, this will store the state of the
balance progress and used filters to the filesystem

resume <path>
resume interrupted balance, the balance status must be stored on
the filesystem from previous run, e.g. after it was paused or
forcibly interrupted and mounted again with skip_balance

start [options] <path>
start the balance operation according to the specified filters,
without any filters the data and metadata from the whole filesys-
tem are moved. The process runs in the foreground.

NOTE:
The balance command without filters will basically move every-
thing in the filesystem to a new physical location on devices
(i.e. it does not affect the logical properties of file ex-
tents like offsets within files and extent sharing). The run
time is potentially very long, depending on the filesystem
size. To prevent starting a full balance by accident, the user
is warned and has a few seconds to cancel the operation before
it starts. The warning and delay can be skipped with
--full-balance option.

Please note that the filters must be written together with the
-d, -m and -s options, because they're optional and bare -d and
-m also work and mean no filters.

NOTE:
When the target profile for conversion filter is raid5 or
raid6, there's a safety timeout of 10 seconds to warn users
about the status of the feature

Options

-d[<filters>]
act on data block groups, see section FILTERS for details
about filters

-m[<filters>]
act on metadata chunks, see FILTERS for details about fil-
ters

-s[<filters>]
act on system chunks (requires -f), see FILTERS for de-
tails about filters.

-f force a reduction of metadata integrity, e.g. when going
from raid1 to single, or skip safety timeout when the tar-
get conversion profile is raid5 or raid6

--background|--bg
run the balance operation asynchronously in the back-
ground, uses ]8;;https://man7.org/linux/man-pages/man2/fork.2.html\fork(2)]8;;\ to start the process that calls the
kernel ioctl

--enqueue
wait if there's another exclusive operation running, oth-
erwise continue

-v (deprecated) alias for global '-v' option

status [-v] <path>
Show status of running or paused balance.

Options

-v (deprecated) alias for global -v option

FILTERS
From kernel 3.3 onwards, BTRFS balance can limit its action to a subset
of the whole filesystem, and can be used to change the replication con-
figuration (e.g. convert data from single to RAID1).

Balance can be limited to a block group profile with the following op-
tions:

• -d for data block groups

• -m for metadata block groups (also implicitly applies to -s)

• -s for system block groups

The options have an optional parameter which means that the parameter
must start right after the option without a space (this is mandatory
getopt syntax), like -dusage=10. Options for all block group types can
be specified in one command.

A filter has the following structure: filter[=params][,filter=...]

To combine multiple filters use ,, without spaces. Example: -dcon-
vert=raid1,soft

BTRFS can have different profiles on a single device or the same profile
on multiple device.

The main reason why you want to have different profiles for data and
metadata is to provide additional protection of the filesystem's meta-
data when devices fail, since a single sector of unrecoverable metadata
will break the filesystem, while a single sector of lost data can be
trivially recovered by deleting the broken file.

Before changing profiles, make sure there is enough unallocated space on
existing drives to create new metadata block groups (for filesystems
over 50GiB, this is 1GB * (number_of_devices + 2)).

Default profiles on BTRFS are:

• data: single

•

metadata:

• single devices: dup

• multiple devices: raid1

The available filter types are:

Filter types
profiles=<profiles>
Balances only block groups with the given profiles. Parameters
are a list of profile names separated by | (pipe).

usage=<percent>, usage=<range>
Balances only block groups with usage under the given percentage.
The value of 0 is allowed and will clean up completely unused
block groups, this should not require any new work space allo-
cated. You may want to use usage=0 in case balance is returning
ENOSPC and your filesystem is not too full.

The argument may be a single value or a range. The single value N
means at most N percent used, equivalent to ..N range syntax.
Kernels prior to 4.4 accept only the single value format. The
minimum range boundary is inclusive, maximum is exclusive.

devid=<id>
Balances only block groups which have at least one chunk on the
given device. To list devices with ids use btrfs filesystem show.

drange=<range>
Balance only block groups which overlap with the given byte range
on any device. Use in conjunction with devid to filter on a spe-
cific device. The parameter is a range specified as start..end.

vrange=<range>
Balance only block groups which overlap with the given byte range
in the filesystem's internal virtual address space. This is the
address space that most reports from btrfs in the kernel log use.
The parameter is a range specified as start..end.

convert=<profile>
Convert each selected block group to the given profile name iden-
tified by parameters.

NOTE:
Starting with kernel 4.5, the data chunks can be converted
to/from the DUP profile on a single device.

Starting with kernel 4.6, all profiles can be converted
to/from DUP on multi-device filesystems.

WARNING:
Bad or missing device are not detected immediately during run-
time and this depends on some later event like failed write or
failed transaction commit. If there's a known failing device,
or a device deleted by /sys/block/<dev>/device/delete inter-
face, the device will be still accessed and written to.

In such case, one should not convert to a profile with lower
redundancy (e.g. from RAID1 to SINGLE), as attempts to create
new chunks on the new devices will cause various problems.

The proper action is to use btrfs replace or btrfs device re-
move to handle the failing/missing device first. Then convert
will work with all devices correctly.

limit=<number>, limit=<range>
Process only given number of chunks, after all filters are ap-
plied. This can be used to specifically target a chunk in connec-
tion with other filters (drange, vrange) or just simply limit the
amount of work done by a single balance run.

The argument may be a single value or a range. The single value N
means at most N chunks, equivalent to ..N range syntax. Kernels
prior to 4.4 accept only the single value format. The range min-
imum and maximum are inclusive.

stripes=<range>
Balance only block groups which have the given number of stripes.
The parameter is a range specified as start..end. Makes sense for
block group profiles that utilize striping, i.e. RAID0/10/5/6.
The range minimum and maximum are inclusive.

soft Takes no parameters. Only has meaning when converting between
profiles, or When doing convert from one profile to another and
soft mode is on, chunks that already have the target profile are
left untouched. This is useful e.g. when half of the filesystem
was converted earlier but got cancelled.

The soft mode switch is (like every other filter) per-type. For
example, this means that we can convert metadata chunks the
"hard" way while converting data chunks selectively with soft
switch.

Profile names, used in profiles and convert are one of:

• raid0

• raid1

• raid1c3

• raid1c4

• raid10

• raid5

• raid6

• dup

• single

The mixed data/metadata profiles can be converted in the same way, but
conversion between mixed and non-mixed is not implemented. For the con-
straints of the profiles please refer to mkfs.btrfs(8) section PROFILES.

ENOSPC
The way balance operates, it usually needs to temporarily create a new
block group and move the old data there, before the old block group can
be removed. For that it needs the work space, otherwise it fails for
ENOSPC reasons. This is not the same ENOSPC as if the free space is ex-
hausted. This refers to the space on the level of block groups, which
are bigger parts of the filesystem that contain many file extents.

The free work space can be calculated from the output of the btrfs
filesystem show command:

Label: 'BTRFS' uuid: 8a9d72cd-ead3-469d-b371-9c7203276265
Total devices 2 FS bytes used 77.03GiB
devid 1 size 53.90GiB used 51.90GiB path /dev/sdc2
devid 2 size 53.90GiB used 51.90GiB path /dev/sde1

size - used = free work space

53.90GiB - 51.90GiB = 2.00GiB

An example of a filter that does not require workspace is usage=0. This
will scan through all unused block groups of a given type and will re-
claim the space. After that it might be possible to run other filters.

CONVERSIONS ON MULTIPLE DEVICES

Conversion to profiles based on striping (RAID0, RAID5/6) require the
work space on each device. An interrupted balance may leave partially
filled block groups that consume the work space.

EXAMPLES
A more comprehensive example when going from one to multiple devices,
and back, can be found in section TYPICAL USECASES of btrfs-device(8).

MAKING BLOCK GROUP LAYOUT MORE COMPACT
The layout of block groups is not normally visible; most tools report
only summarized numbers of free or used space, but there are still some
hints provided.

Let's use the following real life example and start with the output:

$ btrfs filesystem df /path
Data, single: total=75.81GiB, used=64.44GiB
System, RAID1: total=32.00MiB, used=20.00KiB
Metadata, RAID1: total=15.87GiB, used=8.84GiB
GlobalReserve, single: total=512.00MiB, used=0.00B

Roughly calculating for data, 75G - 64G = 11G, the used/total ratio is
about 85%. How can we can interpret that:

• chunks are filled by 85% on average, i.e. the usage filter with any-
thing smaller than 85 will likely not affect anything

• in a more realistic scenario, the space is distributed unevenly, we
can assume there are completely used chunks and the remaining are par-
tially filled

Compacting the layout could be used on both. In the former case it would
spread data of a given chunk to the others and removing it. Here we can
estimate that roughly 850 MiB of data have to be moved (85% of a 1 GiB
chunk).

In the latter case, targeting the partially used chunks will have to
move less data and thus will be faster. A typical filter command would
look like:

# btrfs balance start -dusage=50 /path
Done, had to relocate 2 out of 97 chunks

$ btrfs filesystem df /path
Data, single: total=74.03GiB, used=64.43GiB
System, RAID1: total=32.00MiB, used=20.00KiB
Metadata, RAID1: total=15.87GiB, used=8.84GiB
GlobalReserve, single: total=512.00MiB, used=0.00B

As you can see, the total amount of data is decreased by just 1 GiB,
which is an expected result. Let's see what will happen when we increase
the estimated usage filter.

# btrfs balance start -dusage=85 /path
Done, had to relocate 13 out of 95 chunks

$ btrfs filesystem df /path
Data, single: total=68.03GiB, used=64.43GiB
System, RAID1: total=32.00MiB, used=20.00KiB
Metadata, RAID1: total=15.87GiB, used=8.85GiB
GlobalReserve, single: total=512.00MiB, used=0.00B

Now the used/total ratio is about 94% and we moved about 74G - 68G = 6G
of data to the remaining block groups, i.e. the 6GiB are now free of
filesystem structures, and can be reused for new data or metadata block
groups.

We can do a similar exercise with the metadata block groups, but this
should not typically be necessary, unless the used/total ratio is really
off. Here the ratio is roughly 50% but the difference as an absolute
number is "a few gigabytes", which can be considered normal for a work-
load with snapshots or reflinks updated frequently.

# btrfs balance start -musage=50 /path
Done, had to relocate 4 out of 89 chunks

$ btrfs filesystem df /path
Data, single: total=68.03GiB, used=64.43GiB
System, RAID1: total=32.00MiB, used=20.00KiB
Metadata, RAID1: total=14.87GiB, used=8.85GiB
GlobalReserve, single: total=512.00MiB, used=0.00B

Just 1 GiB decrease, which possibly means there are block groups with
good utilization. Making the metadata layout more compact would in turn
require updating more metadata structures, i.e. lots of IO. As running
out of metadata space is a more severe problem, it's not necessary to
keep the utilization ratio too high. For the purpose of this example,
let's see the effects of further compaction:

# btrfs balance start -musage=70 /path
Done, had to relocate 13 out of 88 chunks

$ btrfs filesystem df .
Data, single: total=68.03GiB, used=64.43GiB
System, RAID1: total=32.00MiB, used=20.00KiB
Metadata, RAID1: total=11.97GiB, used=8.83GiB
GlobalReserve, single: total=512.00MiB, used=0.00B

GETTING RID OF COMPLETELY UNUSED BLOCK GROUPS
Normally the balance operation needs a work space, to temporarily move
the data before the old block groups gets removed. If there's no work
space, it ends with no space left.

There's a special case when the block groups are completely unused, pos-
sibly left after removing lots of files or deleting snapshots. Removing
empty block groups is automatic since 3.18. The same can be achieved
manually with a notable exception that this operation does not require
the work space. Thus it can be used to reclaim unused block groups to
make it available.

# btrfs balance start -dusage=0 /path

This should lead to decrease in the total numbers in the btrfs filesys-
tem df output.

EXIT STATUS
Unless indicated otherwise below, all btrfs balance subcommands return a
zero exit status if they succeed, and non zero in case of failure.

The pause, cancel, and resume subcommands exit with a status of 2 if
they fail because a balance operation was not running.

The status subcommand exits with a status of 0 if a balance operation is
not running, 1 if the command-line usage is incorrect or a balance oper-
ation is still running, and 2 on other errors.

AVAILABILITY
btrfs is part of btrfs-progs. Please refer to the documentation at ]8;;https://btrfs.readthedocs.io\-
https://btrfs.readthedocs.io]8;;\.