The second, third, and fourth extended file systems, or ext2, ext3,
and ext4 as they are commonly 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 configured to handle mount requests for ext2
and ext3 file systems.
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 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 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/Bigalloc for details.
May clash with delayed allocation (see nodelalloc mount
This feature requires that the extent feature be enabled.
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.
This ext4 feature allows more than 65000 subdirectories per
This ext4 feature provides 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
This feature enables the use of extended attributes. This
feature 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 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.)
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
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
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
directory entries. This feature is supported by ext2, ext3,
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.
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 driver.
This ext4 feature allows files to be larger than 2 terabytes
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 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 understand.
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.
Causes the quota files (i.e., user.quota and group.quota which
existed in the older quota design) to be hidden inodes.
This ext4 feature provides project quota support. With this
feature, the project ID of inode will be managed when the
filesystem 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 mounted.
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 filesystem may grow to 1024 times its
initial size. This can be changed using the resize extended
This feature requires that the sparse_super feature be
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.
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 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
descriptors 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 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.
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
behavior 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
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 filesystems.
debug Print debugging info upon each (re)mount.
Define the behavior when an error is encountered. (Either
ignore errors and just mark the filesystem erroneous and
continue, or remount the filesystem read-only, or panic and
halt the system.) 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
The usrquota (same as quota) mount option enables user quota
support on the filesystem. grpquota enables group quotas
support. 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
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
sb=n Instead of block 1, use block n as superblock. This could be
useful when the filesystem has been damaged. (Earlier, copies
of the superblock would be made every 8192 blocks: in block 1,
8193, 16385, ... (and one got thousands of copies on a big
filesystem). Since version 1.08, mke2fs has a -s (sparse
superblock) option to reduce the number of backup superblocks,
and since version 1.15 this is the default. Note that this may
mean that ext2 filesystems created by a recent mke2fs cannot
be mounted r/w under Linux 2.0.*.) 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 "sb=131072".
Support "user." extended attributes (or not).
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 device.
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
filesystem, pass the mode to the kernel as boot parameter,
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 integrity, 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 improve performance.
Sync all data and metadata 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 enables journaled quotas. For journaled
quotas the mount options usrjquota=aquota.user and
grpjquota=aquota.group are required to tell the quota system
which quota database files to use. Journaled quotas have the
advantage that even after a crash no quota check is required.
The ext4 filesystem is an advanced level of the ext3 filesystem which
incorporates scalability and reliability enhancements for supporting
The options journal_dev, 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 andjqfmt are backwardly compatible with ext3 or ext2.
Enable 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.
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' 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 default value is 32 blocks.
Number of filesystem 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 filesystem
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
filesystem 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
designed 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 defaults to 15000 µs (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.
Increasing 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
abort Simulate the effects of calling ext4_abort() for debugging
purposes. This is normally used while remounting a filesystem
which is already mounted.
Many broken applications don't use fsync() when replacing
existing files via patterns such as
fd = open("foo.new")/write(fd,...)/close(fd)/
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.
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
milliseconds 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 options 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 locate 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
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.
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 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
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
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 supported. 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 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
metdata_csum ext4, 3.18
encrypt ext4, 4.1
project ext4, 4.5
This page is part of the e2fsprogs (utilities for ext2/3/4
filesystems) project. Information about the project can be found at
⟨http://e2fsprogs.sourceforge.net/⟩. It is not known how to report
bugs for this man page; if you know, please send a mail to
firstname.lastname@example.org. This page was obtained from the project's
upstream Git repository
⟨git://git.kernel.org/pub/scm/fs/ext2/e2fsprogs.git⟩ on 2016-12-10.
If you discover any rendering problems in this HTML version of the
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E2fsprogs version 1.43.3 September 2016 EXT4(5)