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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 com‐
monly known, are Linux file systems that have historically been the default file system for
many Linux distributions. They are general purpose file systems that have been designed for
extensibility and backwards compatibility. In particular, file systems previously 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 config‐
ured 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 features are not supported by all implemen‐
tations 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 implementations of ext2.

64bit
Enables the file system to be larger than 2^32 blocks. This feature is set automati‐
cally, 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 sup‐
ported with your kernel or may have various bugs. Please see the web page
http://ext4.wiki.kernel.org/index.php/Bigalloc for details. May clash with delayed
allocation (see nodelalloc mount option).

This feature requires that the extent feature be enabled.

casefold
This ext4 feature provides file system level character encoding support for directo‐
ries with the casefold (+F) flag enabled. This feature is name-preserving on the
disk, but it allows applications to lookup for a file in the file system using an en‐
coding 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 in‐
dicate that the number of hard links to a directory is not known when the link count
might exceed the maximum count limit.

ea_inode
Normally, a file's extended attributes and associated metadata must fit within the in‐
ode or the inode's associated extended attribute block. This feature allows the value
of each extended attribute to be placed in the data blocks of a separate inode if nec‐
essary, increasing the limit on the size and number of extended 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, en‐
cryption at the block device layer using dm-crypt may provide much better security.

ext_attr
This feature enables the use of extended attributes. This feature 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 over‐
head, 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
feature for historical/backwards compatibility reasons.)

extra_isize
This ext4 feature reserves a specific amount of space in each inode for extended meta‐
data 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 directory 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 metadata 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 specifying the -J de‐‐
vice=<external-device> option to mke2fs(8) or tune2fs(8).

large_dir
This feature increases the limit on the number of files per directory 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 gi‐
gabytes 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
understand.

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 descriptors with the
uninit_bg feature. These two features are incompatible and metadata_csum will be used
preferentially instead of uninit_bg.

metadata_csum_seed
This feature allows the file system to store the metadata checksum seed in the su‐
perblock, 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 without 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 rec‐
ommended that this feature be set when a file system is created, since this alternate
method of storing the block group descriptors 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 available in the resize in‐
ode.

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.

project
This ext4 feature provides project quota support. With this feature, the project ID of
inode will be managed when the file system 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 on‐
line resize operation is carried out by the kernel, triggered by resize2fs(8). By de‐
fault mke2fs will attempt to reserve enough space so that the file system 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 feature 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 superblock 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 lo‐
cated at the beginning of block group #1, and one in the last block group in the file
system. This feature is essentially a more extreme version of sparse_super and is de‐
signed to allow a much larger percentage of the disk to have contiguous blocks avail‐
able for data files.

stable_inodes
Marks the file system's inode numbers and UUID as stable. resize2fs(8) will not allow
shrinking a file system with this feature, 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 "compat" feature, so old kernels will allow it.

uninit_bg
This ext4 file system feature indicates that the block group descriptors will be pro‐
tected 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 feature 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. Us‐
ing the Merkle tree's root hash, a verity file can be efficiently authenticated, inde‐
pendent 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 authen‐
ticate 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) manual 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 options 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 support on the file sys‐
tem. 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 default.

resgid=n and resuid=n
The ext2 file system reserves a certain percentage of the available 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 superblock specified by n.
This option is normally used when the primary superblock has been corrupted. The lo‐
cation of backup superblocks is dependent on the file system's blocksize, the number
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 superblocks 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 jour‐
naling. 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 al‐
low 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 de‐
vice.

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 inconsis‐
tencies 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 system, pass the mode to the kernel as boot
parameter, e.g. rootflags=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 committed 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 integrity,
however it can allow old data to appear in files after a crash and journal re‐
covery.

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 ordered mode.

barrier=0 / barrier=1
This disables / enables the use of write barriers in the jbd code. barrier=0 dis‐
ables, 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
performance.

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 options usr‐
jquota=aquota.user and grpjquota=aquota.group are required 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 scala‐
bility 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, gr‐‐
pjquota, 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 ker‐
nel. 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 descriptor blocks. If enabled
older kernels cannot mount the device. This will enable 'journal_checksum' inter‐
nally.

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 default value is 32 blocks.

stripe=n
Number of file system blocks that mballoc will try to use for allocation 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 system operations to be
batch together with a synchronous write operation. 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 de‐
signed to automatically tune for the speed of the disk, by measuring the amount of
time (on average) that it takes to finish committing a transaction. 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 de‐
faults to 15000 µs (15 ms). This optimization 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. Increasing this parameter may im‐
prove the throughput of multi-threaded, synchronous workloads on very fast disks, at
the cost of increasing 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 purposes. 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 existing 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-rename 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 committed. 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 inode 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 milliseconds it took to zero
out the previous block group's inode table. This minimizes the impact on system per‐
formance while the file system's inode table is being initialized.

discard/nodiscard
Controls whether ext4 should issue discard/TRIM commands to the underlying block de‐
vice when blocks are freed. This is useful for SSD devices and sparse/thinly-provi‐
sioned 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 negatively 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 uninitialized 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-con‐
strained 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 MB
memory available, a 176 MB directory may seriously cramp the system's style.)

i_version
Enable 64-bit inode version support. This option is off by default.

nombcache
This option disables use of mbcache for extended attribute deduplication. 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 ver‐
sion where a particular file system feature was supported. Note that in some cases the fea‐
ture was present in earlier kernel 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 ver‐
sions 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), debugfs(8), mount(8),
chattr(1)

E2fsprogs version 1.46.5 December 2021 EXT4(5)