EXT4(5) File Formats Manual EXT4(5)
ext2 - the second extended file system
ext3 - the third extended file system
ext4 - the fourth extended file system
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 ex-
tensibility 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 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 collec-
tion of the following file system feature flags enabled. Some of these
features 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
implementations of ext2.
Enables the file system to be larger than 2^32 blocks. This
feature 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.
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 alloca-
tion 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/in-
dex.php/Bigalloc for details. May clash with delayed allocation
(see nodelalloc mount option).
This feature requires that the extent feature be enabled.
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 ap-
plications to lookup for a file in the file system using an en-
coding equivalent version of the file name.
Use hashed b-trees to speed up name lookups in large directo-
ries. This feature is supported by ext3 and ext4 file systems,
and is ignored by ext2 file systems.
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 di-
rectory 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 exceed the maximum count limit.
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
attribute 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.
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 de-
vice layer using dm-crypt may provide much better security.
This feature enables the use of extended attributes. This fea-
ture is supported by ext2, ext3, and ext4.
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 perfor-
mance, 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
This ext4 feature reserves a specific amount of space in each
inode for extended metadata such as nanosecond timestamps and
file creation time, even if the current kernel does not cur-
rently need to reserve this much space. Without this feature,
the kernel will reserve the amount of space for features it cur-
rently needs, and the rest may be consumed by extended at-
For this feature to be useful the inode size must be 256 bytes
in size or larger.
This feature enables the storage of file type information in di-
rectory entries. This feature is supported by ext2, ext3, and
This ext4 feature allows the per-block group metadata (alloca-
tion 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.
Create a journal to ensure filesystem consistency even across
unclean shutdowns. Setting the filesystem 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
This ext4 feature allows files to be larger than 2 terabytes in
Allow data to be stored in the inode and extended attribute
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 device=<external-device> option to mke2fs(8)
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.
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 pro-
hibit those kernels from mounting file systems that they could
This ext4 feature enables metadata checksumming. This feature
stores checksums for all of the filesystem 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 de-
scriptors with the uninit_bg feature. These two features are
incompatible and metadata_csum will be used preferentially in-
stead of uninit_bg.
This feature allows the filesystem to store the metadata check-
sum seed in the superblock, which allows the administrator to
change the UUID of a filesystem using the metadata_csum feature
while it is mounted.
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 recommended 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 inode.
This ext4 feature provides multiple mount protection (MMP). MMP
helps to protect the filesystem from being multiply mounted and
is useful in shared storage environments.
This ext4 feature provides project quota support. With this fea-
ture, the project ID of inode will be managed when the filesys-
tem is mounted.
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 filesystem is
Causes the quota files (i.e., user.quota and group.quota which
existed in the older quota design) to be hidden inodes.
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 de-
fault mke2fs will attempt to reserve enough space so that the
filesystem 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.
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.
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 de-
scriptor(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 essen-
tially a more extreme version of sparse_super and is designed to
allow a much larger percentage of the disk to have contiguous
blocks available for data files.
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 un-
used 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.
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 authenti-
cated, 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 it-
self is read-only, then using dm-verity to authenticate the en-
tire block device may provide much better security.
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' filesystem is the standard Linux filesystem. Since Linux
2.5.46, for most mount options the default is determined by the
filesystem superblock. Set them with tune2fs(8).
Support POSIX Access Control Lists (or not). See the acl(5)
Set the behavior for the statfs system call. The minixdf behav-
ior is to return in the f_blocks field the total number of
blocks of the filesystem, while the bsddf behavior (which is the
default) is to subtract the overhead blocks used by the ext2
filesystem and not available for file storage. Thus
% mount /k -o minixdf; df /k; umount /k
Filesystem 1024-blocks Used Available Capacity Mounted on
/dev/sda6 2630655 86954 2412169 3% /k
% mount /k -o bsddf; df /k; umount /k
Filesystem 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 ker-
nel driver is used for ext2 and ext3 filesystems.
debug Print debugging info upon each (re)mount.
Define the behavior when an error is encountered. (Either ig-
nore errors and just mark the filesystem erroneous and continue,
or remount the filesystem read-only, or panic and halt the sys-
tem.) The default is set in the filesystem 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.
The usrquota (same as quota) mount option enables user quota
support on the filesystem. grpquota enables group quotas sup-
port. You need the quota utilities to actually enable and manage
the quota system.
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
resgid=n and resuid=n
The ext2 filesystem 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
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 filesystem'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 filesystem's layout (e.g. block-
size, 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 filesystem with 4 k blocks, use
Support "user." extended attributes (or not).
Mount options for ext3
The ext3 filesystem is a version of the ext2 filesystem which has been
enhanced with journaling. It supports the same options as ext2 as well
as the following additions:
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
Don't load the journal on mounting. Note that if the filesystem
was not unmounted cleanly, skipping the journal replay will lead
to the filesystem containing inconsistencies that can lead to
any number of problems.
Specifies the journaling mode for file data. Metadata is always
journaled. To use modes other than ordered on the root filesys-
tem, pass the mode to the kernel as boot parameter, e.g. root-
All data is committed into the journal prior to being
written into the main filesystem.
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.
Data ordering is not preserved - data may be written into
the main filesystem after its metadata has been committed
to the journal. This is rumoured to be the highest-
throughput option. It guarantees internal filesystem in-
tegrity, however it can allow old data to appear in files
after a crash and journal recovery.
Just print an error message if an error occurs in a file data
buffer in ordered mode.
Abort the journal if an error occurs in a file data buffer in
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 im-
Start a journal commit every nrsec seconds. The default value
is 5 seconds. Zero means default.
Enable Extended User Attributes. See the attr(5) manual page.
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 filesystem feature is
enabled, journaled quotas are used automatically, and this mount
option is ignored.
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 filesystem feature is enabled, journaled
quotas are used automatically, and this mount option is ignored.
Mount options for ext4
The ext4 filesystem is an advanced level of the ext3 filesystem which
incorporates scalability and reliability enhancements for supporting
The options journal_dev, journal_path, norecovery, noload, data, com-
mit, orlov, oldalloc, [no]user_xattr, [no]acl, bsddf, minixdf, debug,
errors, data_err, grpid, bsdgroups, nogrpid, sysvgroups, resgid, re-
suid, 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 kernel. It is a compati-
ble change and will be ignored by older kernels.
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 filesystem enables write barriers by default.
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.
Number of filesystem 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 filesystem blocks.
Deferring block allocation until write-out time.
Disable delayed allocation. Blocks are allocated when data is
copied from user to page cache.
Maximum amount of time ext4 should wait for additional filesys-
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 auto-
matically 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 oper-
ations will join the transaction. The commit time is capped by
the max_batch_time, which defaults to 15000 <micro>s (15 ms).
This optimization can be turned off entirely by setting
max_batch_time to 0.
This parameter sets the commit time (as described above) to be
at least min_batch_time. It defaults to zero microseconds. In-
creasing this parameter may improve the throughput of multi-
threaded, synchronous workloads on very fast disks, at the cost
of increasing latency.
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 filesystem
which is already mounted.
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",
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 de-
layed 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.
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 filesystem is mounted.
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
filesystem's inode table is being initialized.
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.
This option enables/disables the in-kernel facility for tracking
filesystem metadata blocks within internal data structures. This
allows multi-block allocator and other routines to quickly lo-
cate extents which might overlap with filesystem metadata
blocks. This option is intended for debugging purposes and since
it negatively affects the performance, it is off by default.
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).
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 environ-
ments, where a very large directory can cause severe performance
problems 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.)
Enable 64-bit inode version support. This option is off by de-
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.
The prjquota mount option enables project quota support on the
filesystem. You need the quota utilities to actually enable and
manage the quota system. This mount option requires the project
The ext2, ext3, and ext4 filesystems 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 filesystems support the following flag:
j - data journaling
Finally, the ext4 filesystem also supports the following flag:
e - extents format
For descriptions of these attribute flags, please refer to the
chattr(1) man page.
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
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 versions in certain "enterprise dis-
tributions" 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
mke2fs(8), mke2fs.conf(5), e2fsck(8), dumpe2fs(8), tune2fs(8), de-
bugfs(8), mount(8), chattr(1)
E2fsprogs version 1.45.6 March 2020 EXT4(5)