EXT4(5) File Formats Manual EXT4(5)
NAME
ext2 - the second extended file system
ext3 - the third extended file system
ext4 - the fourth extended file system
DESCRIPTION
The second, third, and fourth extended file systems, or ext2, ext3, and
ext4 as they are commonly known, are Linux file systems that have his-
torically been the default file system for many Linux distributions.
They are general purpose file systems that have been designed for exten-
sibility and backwards compatibility. In particular, file systems pre-
viously intended for use with the ext2 and ext3 file systems can be
mounted using the ext4 file system driver, and indeed in many modern
Linux distributions, the ext4 file system driver has been configured to
handle mount requests for ext2 and ext3 file systems.
FILE SYSTEM FEATURES
A file system formatted for ext2, ext3, or ext4 can have some collection
of the following file system feature flags enabled. Some of these fea-
tures are not supported by all implementations of the ext2, ext3, and
ext4 file system drivers, depending on Linux kernel version in use. On
other operating systems, such as the GNU/HURD or FreeBSD, only a very
restrictive set of file system features may be supported in their imple-
mentations of ext2.
64bit
Enables the file system to be larger than 2^32 blocks. This fea-
ture is set automatically, as needed, but it can be useful to
specify this feature explicitly if the file system might need to
be resized larger than 2^32 blocks, even if it was smaller than
that threshold when it was originally created. Note that some
older kernels and older versions of e2fsprogs will not support
file systems with this ext4 feature enabled.
bigalloc
This ext4 feature enables clustered block allocation, so that the
unit of allocation is a power of two number of blocks. That is,
each bit in the what had traditionally been known as the block
allocation bitmap now indicates whether a cluster is in use or
not, where a cluster is by default composed of 16 blocks. This
feature can decrease the time spent on doing block allocation and
brings smaller fragmentation, especially for large files. The
size can be specified using the mke2fs -C option.
Warning: The bigalloc feature is still under development, and may
not be fully supported with your kernel or may have various bugs.
Please see the web page http://ext4.wiki.kernel.org/index.php/Bi-
galloc for details. May clash with delayed allocation (see node-
lalloc mount option).
This feature requires that the extent feature be enabled.
casefold
This ext4 feature provides file system level character encoding
support for directories with the casefold (+F) flag enabled.
This feature is name-preserving on the disk, but it allows appli-
cations to lookup for a file in the file system using an encoding
equivalent version of the file name.
dir_index
Use hashed b-trees to speed up name lookups in large directories.
This feature is supported by ext3 and ext4 file systems, and is
ignored by ext2 file systems.
dir_nlink
Normally, ext4 allows an inode to have no more than 65,000 hard
links. This applies to regular files as well as directories,
which means that there can be no more than 64,998 subdirectories
in a directory (because each of the '.' and '..' entries, as well
as the directory entry for the directory in its parent directory
counts as a hard link). This feature lifts this limit by causing
ext4 to use a link count of 1 to indicate that the number of hard
links to a directory is not known when the link count might ex-
ceed the maximum count limit.
ea_inode
Normally, a file's extended attributes and associated metadata
must fit within the inode or the inode's associated extended at-
tribute block. This feature allows the value of each extended at-
tribute to be placed in the data blocks of a separate inode if
necessary, increasing the limit on the size and number of ex-
tended attributes per file.
encrypt
Enables support for file-system level encryption of data blocks
and file names. The inode metadata (timestamps, file size,
user/group ownership, etc.) is not encrypted.
This feature is most useful on file systems with multiple users,
or where not all files should be encrypted. In many use cases,
especially on single-user systems, encryption at the block device
layer using dm-crypt may provide much better security.
ext_attr
This feature enables the use of extended attributes. This fea-
ture is supported by ext2, ext3, and ext4.
extent
This ext4 feature allows the mapping of logical block numbers for
a particular inode to physical blocks on the storage device to be
stored using an extent tree, which is a more efficient data
structure than the traditional indirect block scheme used by the
ext2 and ext3 file systems. The use of the extent tree decreases
metadata block overhead, improves file system performance, and
decreases the needed to run e2fsck(8) on the file system. (Note:
both extent and extents are accepted as valid names for this fea-
ture for historical/backwards compatibility reasons.)
extra_isize
This ext4 feature reserves a specific amount of space in each in-
ode for extended metadata such as nanosecond timestamps and file
creation time, even if the current kernel does not currently need
to reserve this much space. Without this feature, the kernel
will reserve the amount of space for features it currently needs,
and the rest may be consumed by extended attributes.
For this feature to be useful the inode size must be 256 bytes in
size or larger.
filetype
This feature enables the storage of file type information in di-
rectory entries. This feature is supported by ext2, ext3, and
ext4.
flex_bg
This ext4 feature allows the per-block group metadata (allocation
bitmaps and inode tables) to be placed anywhere on the storage
media. In addition, mke2fs will place the per-block group meta-
data together starting at the first block group of each "flex_bg
group". The size of the flex_bg group can be specified using
the -G option.
has_journal
Create a journal to ensure file system consistency even across
unclean shutdowns. Setting the file system feature is equivalent
to using the -j option with mke2fs or tune2fs. This feature is
supported by ext3 and ext4, and ignored by the ext2 file system
driver.
huge_file
This ext4 feature allows files to be larger than 2 terabytes in
size.
inline_data
Allow data to be stored in the inode and extended attribute area.
journal_dev
This feature is enabled on the superblock found on an external
journal device. The block size for the external journal must be
the same as the file system which uses it.
The external journal device can be used by a file system by spec-
ifying the -J device=<external-device> option to mke2fs(8) or
tune2fs8).
large_dir
This feature increases the limit on the number of files per di-
rectory by raising the maximum size of directories and, for
hashed b-tree directories (see dir_index), the maximum height of
the hashed b-tree used to store the directory entries.
large_file
This feature flag is set automatically by modern kernels when a
file larger than 2 gigabytes is created. Very old kernels could
not handle large files, so this feature flag was used to prohibit
those kernels from mounting file systems that they could not un-
derstand.
metadata_csum
This ext4 feature enables metadata checksumming. This feature
stores checksums for all of the file system metadata (superblock,
group descriptor blocks, inode and block bitmaps, directories,
and extent tree blocks). The checksum algorithm used for the
metadata blocks is different than the one used for group descrip-
tors with the uninit_bg feature. These two features are incom-
patible and metadata_csum will be used preferentially instead of
uninit_bg.
metadata_csum_seed
This feature allows the file system to store the metadata check-
sum seed in the superblock, which allows the administrator to
change the UUID of a file system using the metadata_csum feature
while it is mounted.
meta_bg
This ext4 feature allows file systems to be resized on-line with-
out explicitly needing to reserve space for growth in the size of
the block group descriptors. This scheme is also used to resize
file systems which are larger than 2^32 blocks. It is not recom-
mended that this feature be set when a file system is created,
since this alternate method of storing the block group descrip-
tors will slow down the time needed to mount the file system, and
newer kernels can automatically set this feature as necessary
when doing an online resize and no more reserved space is avail-
able in the resize inode.
mmp
This ext4 feature provides multiple mount protection (MMP). MMP
helps to protect the file system from being multiply mounted and
is useful in shared storage environments.
orphan_file
This ext4 feature fixes a potential scalability bottleneck for
workloads that are doing a large number of truncate or file ex-
tensions in parallel. It is supported by Linux kernels starting
version 5.15, and by e2fsprogs starting with version 1.47.0.
project
This ext4 feature provides project quota support. With this fea-
ture, the project ID of inode will be managed when the file sys-
tem is mounted.
quota
Create quota inodes (inode #3 for userquota and inode #4 for
group quota) and set them in the superblock. With this feature,
the quotas will be enabled automatically when the file system is
mounted.
Causes the quota files (i.e., user.quota and group.quota which
existed in the older quota design) to be hidden inodes.
resize_inode
This file system feature indicates that space has been reserved
so that the block group descriptor table can be extended while
resizing a mounted file system. The online resize operation is
carried out by the kernel, triggered by resize2fs(8). By default
mke2fs will attempt to reserve enough space so that the file sys-
tem may grow to 1024 times its initial size. This can be changed
using the resize extended option.
This feature requires that the sparse_super or sparse_super2 fea-
ture be enabled.
sparse_super
This file system feature is set on all modern ext2, ext3, and
ext4 file systems. It indicates that backup copies of the su-
perblock and block group descriptors are present only in a few
block groups, not all of them.
sparse_super2
This feature indicates that there will only be at most two backup
superblocks and block group descriptors. The block groups used
to store the backup superblock(s) and blockgroup descriptor(s)
are stored in the superblock, but typically, one will be located
at the beginning of block group #1, and one in the last block
group in the file system. This feature is essentially a more ex-
treme version of sparse_super and is designed to allow a much
larger percentage of the disk to have contiguous blocks available
for data files.
stable_inodes
Marks the file system's inode numbers and UUID as stable. re-
size2fs(8) will not allow shrinking a file system with this fea-
ture, nor will tune2fs(8) allow changing its UUID. This feature
allows the use of specialized encryption settings that make use
of the inode numbers and UUID. Note that the encrypt feature
still needs to be enabled separately. stable_inodes is a "com-
pat" feature, so old kernels will allow it.
uninit_bg
This ext4 file system feature indicates that the block group de-
scriptors will be protected using checksums, making it safe for
mke2fs(8) to create a file system without initializing all of the
block groups. The kernel will keep a high watermark of unused
inodes, and initialize inode tables and blocks lazily. This fea-
ture speeds up the time to check the file system using e2fsck(8),
and it also speeds up the time required for mke2fs(8) to create
the file system.
verity
Enables support for verity protected files. Verity files are
readonly, and their data is transparently verified against a
Merkle tree hidden past the end of the file. Using the Merkle
tree's root hash, a verity file can be efficiently authenticated,
independent of the file's size.
This feature is most useful for authenticating important read-
only files on read-write file systems. If the file system itself
is read-only, then using dm-verity to authenticate the entire
block device may provide much better security.
MOUNT OPTIONS
This section describes mount options which are specific to ext2, ext3,
and ext4. Other generic mount options may be used as well; see mount(8)
for details.
Mount options for ext2
The `ext2' file system is the standard Linux file system. Since Linux
2.5.46, for most mount options the default is determined by the file
system superblock. Set them with tune2fs(8).
acl|noacl
Support POSIX Access Control Lists (or not). See the acl(5) man-
ual page.
bsddf|minixdf
Set the behavior for the statfs system call. The minixdf behavior
is to return in the f_blocks field the total number of blocks of
the file system, while the bsddf behavior (which is the default)
is to subtract the overhead blocks used by the ext2 file system
and not available for file storage. Thus
% mount /k -o minixdf; df /k; umount /k
File System 1024-blocks Used Available Capacity Mounted on
/dev/sda6 2630655 86954 2412169 3% /k
% mount /k -o bsddf; df /k; umount /k
File System 1024-blocks Used Available Capacity Mounted on
/dev/sda6 2543714 13 2412169 0% /k
(Note that this example shows that one can add command line op-
tions to the options given in /etc/fstab.)
check=none or nocheck
No checking is done at mount time. This is the default. This is
fast. It is wise to invoke e2fsck(8) every now and then, e.g. at
boot time. The non-default behavior is unsupported (check=normal
and check=strict options have been removed). Note that these
mount options don't have to be supported if ext4 kernel driver is
used for ext2 and ext3 file systems.
debug Print debugging info upon each (re)mount.
errors={continue|remount-ro|panic}
Define the behavior when an error is encountered. (Either ignore
errors and just mark the file system erroneous and continue, or
remount the file system read-only, or panic and halt the system.)
The default is set in the file system superblock, and can be
changed using tune2fs(8).
grpid|bsdgroups and nogrpid|sysvgroups
These options define what group id a newly created file gets.
When grpid is set, it takes the group id of the directory in
which it is created; otherwise (the default) it takes the fsgid
of the current process, unless the directory has the setgid bit
set, in which case it takes the gid from the parent directory,
and also gets the setgid bit set if it is a directory itself.
grpquota|noquota|quota|usrquota
The usrquota (same as quota) mount option enables user quota sup-
port on the file system. grpquota enables group quotas support.
You need the quota utilities to actually enable and manage the
quota system.
nouid32
Disables 32-bit UIDs and GIDs. This is for interoperability with
older kernels which only store and expect 16-bit values.
oldalloc or orlov
Use old allocator or Orlov allocator for new inodes. Orlov is de-
fault.
resgid=n and resuid=n
The ext2 file system reserves a certain percentage of the avail-
able space (by default 5%, see mke2fs(8) and tune2fs(8)). These
options determine who can use the reserved blocks. (Roughly:
whoever has the specified uid, or belongs to the specified
group.)
sb=n Instead of using the normal superblock, use an alternative su-
perblock specified by n. This option is normally used when the
primary superblock has been corrupted. The location of backup
superblocks is dependent on the file system's blocksize, the num-
ber of blocks per group, and features such as sparse_super.
Additional backup superblocks can be determined by using the
mke2fs program using the -n option to print out where the su-
perblocks exist, supposing mke2fs is supplied with arguments that
are consistent with the file system's layout (e.g., blocksize,
blocks per group, sparse_super, etc.).
The block number here uses 1 k units. Thus, if you want to use
logical block 32768 on a file system with 4 k blocks, use
"sb=131072".
user_xattr|nouser_xattr
Support "user." extended attributes (or not).
Mount options for ext3
The ext3 file system is a version of the ext2 file system which has been
enhanced with journaling. It supports the same options as ext2 as well
as the following additions:
journal_dev=devnum/journal_path=path
When the external journal device's major/minor numbers have
changed, these options allow the user to specify the new journal
location. The journal device is identified either through its
new major/minor numbers encoded in devnum, or via a path to the
device.
norecovery/noload
Don't load the journal on mounting. Note that if the file system
was not unmounted cleanly, skipping the journal replay will lead
to the file system containing inconsistencies that can lead to
any number of problems.
data={journal|ordered|writeback}
Specifies the journaling mode for file data. Metadata is always
journaled. To use modes other than ordered on the root file sys-
tem, pass the mode to the kernel as boot parameter, e.g. root-
flags=data=journal.
journal
All data is committed into the journal prior to being
written into the main file system.
ordered
This is the default mode. All data is forced directly out
to the main file system prior to its metadata being com-
mitted to the journal.
writeback
Data ordering is not preserved – data may be written into
the main file system after its metadata has been committed
to the journal. This is rumoured to be the highest-
throughput option. It guarantees internal file system in-
tegrity, however it can allow old data to appear in files
after a crash and journal recovery.
data_err=ignore
Just print an error message if an error occurs in a file data
buffer in ordered mode.
data_err=abort
Abort the journal if an error occurs in a file data buffer in or-
dered mode.
barrier=0 / barrier=1
This disables / enables the use of write barriers in the jbd
code. barrier=0 disables, barrier=1 enables (default). This also
requires an IO stack which can support barriers, and if jbd gets
an error on a barrier write, it will disable barriers again with
a warning. Write barriers enforce proper on-disk ordering of
journal commits, making volatile disk write caches safe to use,
at some performance penalty. If your disks are battery-backed in
one way or another, disabling barriers may safely improve perfor-
mance.
commit=nrsec
Start a journal commit every nrsec seconds. The default value is
5 seconds. Zero means default.
user_xattr
Enable Extended User Attributes. See the attr(5) manual page.
jqfmt={vfsold|vfsv0|vfsv1}
Apart from the old quota system (as in ext2, jqfmt=vfsold aka
version 1 quota) ext3 also supports journaled quotas (version 2
quota). jqfmt=vfsv0 or jqfmt=vfsv1 enables journaled quotas.
Journaled quotas have the advantage that even after a crash no
quota check is required. When the quota file system feature is
enabled, journaled quotas are used automatically, and this mount
option is ignored.
usrjquota=aquota.user|grpjquota=aquota.group
For journaled quotas (jqfmt=vfsv0 or jqfmt=vfsv1), the mount op-
tions usrjquota=aquota.user and grpjquota=aquota.group are re-
quired to tell the quota system which quota database files to
use. When the quota file system feature is enabled, journaled
quotas are used automatically, and this mount option is ignored.
Mount options for ext4
The ext4 file system is an advanced level of the ext3 file system which
incorporates scalability and reliability enhancements for supporting
large file system.
The options journal_dev, journal_path, norecovery, noload, data, commit,
orlov, oldalloc, [no]user_xattr, [no]acl, bsddf, minixdf, debug, errors,
data_err, grpid, bsdgroups, nogrpid, sysvgroups, resgid, resuid, sb,
quota, noquota, nouid32, grpquota, usrquota, usrjquota, grpjquota, and
jqfmt are backwardly compatible with ext3 or ext2.
journal_checksum | nojournal_checksum
The journal_checksum option enables checksumming of the journal
transactions. This will allow the recovery code in e2fsck and
the kernel to detect corruption in the kernel. It is a compatible
change and will be ignored by older kernels.
journal_async_commit
Commit block can be written to disk without waiting for descrip-
tor blocks. If enabled older kernels cannot mount the device.
This will enable 'journal_checksum' internally.
barrier=0 / barrier=1 / barrier / nobarrier
These mount options have the same effect as in ext3. The mount
options "barrier" and "nobarrier" are added for consistency with
other ext4 mount options.
The ext4 file system enables write barriers by default.
inode_readahead_blks=n
This tuning parameter controls the maximum number of inode table
blocks that ext4's inode table readahead algorithm will pre-read
into the buffer cache. The value must be a power of 2. The de-
fault value is 32 blocks.
stripe=n
Number of file system blocks that mballoc will try to use for al-
location size and alignment. For RAID5/6 systems this should be
the number of data disks * RAID chunk size in file system blocks.
delalloc
Deferring block allocation until write-out time.
nodelalloc
Disable delayed allocation. Blocks are allocated when data is
copied from user to page cache.
max_batch_time=usec
Maximum amount of time ext4 should wait for additional file sys-
tem operations to be batch together with a synchronous write op-
eration. Since a synchronous write operation is going to force a
commit and then a wait for the I/O complete, it doesn't cost
much, and can be a huge throughput win, we wait for a small
amount of time to see if any other transactions can piggyback on
the synchronous write. The algorithm used is designed to automat-
ically tune for the speed of the disk, by measuring the amount of
time (on average) that it takes to finish committing a transac-
tion. Call this time the "commit time". If the time that the
transaction has been running is less than the commit time, ext4
will try sleeping for the commit time to see if other operations
will join the transaction. The commit time is capped by the
max_batch_time, which defaults to 15000 µs (15 ms). This opti-
mization can be turned off entirely by setting max_batch_time to
0.
min_batch_time=usec
This parameter sets the commit time (as described above) to be at
least min_batch_time. It defaults to zero microseconds. Increas-
ing this parameter may improve the throughput of multi-threaded,
synchronous workloads on very fast disks, at the cost of increas-
ing latency.
journal_ioprio=prio
The I/O priority (from 0 to 7, where 0 is the highest priority)
which should be used for I/O operations submitted by kjournald2
during a commit operation. This defaults to 3, which is a
slightly higher priority than the default I/O priority.
abort Simulate the effects of calling ext4_abort() for debugging pur-
poses. This is normally used while remounting a file system
which is already mounted.
auto_da_alloc|noauto_da_alloc
Many broken applications don't use fsync() when replacing exist-
ing files via patterns such as
fd = open("foo.new")/write(fd,...)/close(fd)/ rename("foo.new",
"foo")
or worse yet
fd = open("foo", O_TRUNC)/write(fd,...)/close(fd).
If auto_da_alloc is enabled, ext4 will detect the replace-via-re-
name and replace-via-truncate patterns and force that any delayed
allocation blocks are allocated such that at the next journal
commit, in the default data=ordered mode, the data blocks of the
new file are forced to disk before the rename() operation is com-
mitted. This provides roughly the same level of guarantees as
ext3, and avoids the "zero-length" problem that can happen when a
system crashes before the delayed allocation blocks are forced to
disk.
noinit_itable
Do not initialize any uninitialized inode table blocks in the
background. This feature may be used by installation CD's so that
the install process can complete as quickly as possible; the in-
ode table initialization process would then be deferred until the
next time the file system is mounted.
init_itable=n
The lazy itable init code will wait n times the number of mil-
liseconds it took to zero out the previous block group's inode
table. This minimizes the impact on system performance while the
file system's inode table is being initialized.
discard/nodiscard
Controls whether ext4 should issue discard/TRIM commands to the
underlying block device when blocks are freed. This is useful
for SSD devices and sparse/thinly-provisioned LUNs, but it is off
by default until sufficient testing has been done.
block_validity/noblock_validity
This option enables/disables the in-kernel facility for tracking
file system metadata blocks within internal data structures. This
allows multi-block allocator and other routines to quickly locate
extents which might overlap with file system metadata blocks.
This option is intended for debugging purposes and since it nega-
tively affects the performance, it is off by default.
dioread_lock/dioread_nolock
Controls whether or not ext4 should use the DIO read locking. If
the dioread_nolock option is specified ext4 will allocate unini-
tialized extent before buffer write and convert the extent to
initialized after IO completes. This approach allows ext4 code
to avoid using inode mutex, which improves scalability on high
speed storages. However this does not work with data journaling
and dioread_nolock option will be ignored with kernel warning.
Note that dioread_nolock code path is only used for extent-based
files. Because of the restrictions this options comprises it is
off by default (e.g. dioread_lock).
max_dir_size_kb=n
This limits the size of the directories so that any attempt to
expand them beyond the specified limit in kilobytes will cause an
ENOSPC error. This is useful in memory-constrained environments,
where a very large directory can cause severe performance prob-
lems or even provoke the Out Of Memory killer. (For example, if
there is only 512 MiB memory available, a 176 MiB directory may
seriously cramp the system's style.)
i_version
Enable 64-bit inode version support. This option is off by de-
fault.
nombcache
This option disables use of mbcache for extended attribute dedu-
plication. On systems where extended attributes are rarely or
never shared between files, use of mbcache for deduplication adds
unnecessary computational overhead.
prjquota
The prjquota mount option enables project quota support on the
file system. You need the quota utilities to actually enable and
manage the quota system. This mount option requires the project
file system feature.
FILE ATTRIBUTES
The ext2, ext3, and ext4 file systems support setting the following file
attributes on Linux systems using the chattr(1) utility:
a - append only
A - no atime updates
d - no dump
D - synchronous directory updates
i - immutable
S - synchronous updates
u - undeletable
In addition, the ext3 and ext4 file systems support the following flag:
j - data journaling
Finally, the ext4 file system also supports the following flag:
e - extents format
For descriptions of these attribute flags, please refer to the chattr(1)
man page.
KERNEL SUPPORT
This section lists the file system driver (e.g., ext2, ext3, ext4) and
upstream kernel version where a particular file system feature was sup-
ported. Note that in some cases the feature was present in earlier ker-
nel versions, but there were known, serious bugs. In other cases the
feature may still be considered in an experimental state. Finally, note
that some distributions may have backported features into older kernels;
in particular the kernel versions in certain "enterprise distributions"
can be extremely misleading.
filetype ext2, 2.2.0
sparse_super ext2, 2.2.0
large_file ext2, 2.2.0
has_journal ext3, 2.4.15
ext_attr ext2/ext3, 2.6.0
dir_index ext3, 2.6.0
resize_inode ext3, 2.6.10 (online resizing)
64bit ext4, 2.6.28
dir_nlink ext4, 2.6.28
extent ext4, 2.6.28
extra_isize ext4, 2.6.28
flex_bg ext4, 2.6.28
huge_file ext4, 2.6.28
meta_bg ext4, 2.6.28
uninit_bg ext4, 2.6.28
mmp ext4, 3.0
bigalloc ext4, 3.2
quota ext4, 3.6
inline_data ext4, 3.8
sparse_super2 ext4, 3.16
metadata_csum ext4, 3.18
encrypt ext4, 4.1
metadata_csum_seed ext4, 4.4
project ext4, 4.5
ea_inode ext4, 4.13
large_dir ext4, 4.13
casefold ext4, 5.2
verity ext4, 5.4
stable_inodes ext4, 5.5
SEE ALSO
mke2fs(8), mke2fs.conf(5), e2fsck(8), dumpe2fs(8), tune2fs(8), de-
bugfs(8), mount(8), chattr(1)
E2fsprogs version 1.47.2 January 2025 EXT4(5)
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