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 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
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
- 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 option).
- This feature requires that the extent feature be
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
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 exceed
the maximum count limit.
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 security.
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
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 extended
For this feature to be useful the inode size must be 256 bytes in size or
This feature enables the storage of file type information in directory
entries. This feature is supported by ext2, ext3, and ext4.
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
This ext4 feature allows files to be larger than 2 terabytes in size.
- 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) or tune2fs(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
- 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
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.
This section describes mount options which are specific to ext2, ext3, and ext4.
Other generic mount options may be used as well; see mount
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
- Support POSIX Access Control Lists (or not). See the
acl(5) manual page.
- 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
% mount /k -o bsddf; df /k; umount /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
- 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
- grpid|bsdgroups and
- 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 support.
You need the quota utilities to actually enable and manage the quota
- 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
- 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
- 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
- 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, e.g.
- 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
- 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 ordered 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 performance.
- Start a journal commit every nrsec seconds. The
default value is 5 seconds. Zero means default.
- Enable Extended User Attributes. See the attr(5)
- 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
The ext4 filesystem is an advanced level of the ext3 filesystem which
incorporates scalability and reliability enhancements for supporting large
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 and jqfmt
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
- 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 /
- 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 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
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
- 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 priority.
- Simulate the effects of calling ext4_abort() for debugging
purposes. This is normally used while remounting a filesystem which is
- 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. 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 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
- 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
- append only
- no atime updates
- no dump
- synchronous directory updates
- synchronous updates
In addition, the ext3 and ext4 filesystems support the following flag:
- data journaling
Finally, the ext4 filesystem also supports the following flag:
- extents format
For descriptions of these attribute flags, please refer to the chattr
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.
- ext2, 2.2.0
- ext2, 2.2.0
- ext2, 2.2.0
- ext3, 2.4.15
- ext2/ext3, 2.6.0
- ext3, 2.6.0
- ext3, 2.6.10 (online resizing)
- ext4, 2.6.28
- ext4, 2.6.28
- ext4, 2.6.28
- ext4, 2.6.28
- ext4, 2.6.28
- ext4, 2.6.28
- ext4, 2.6.28
- ext4, 2.6.28
- ext4, 3.0
- ext4, 3.2
- ext4, 3.6
- ext4, 3.8
- ext4, 3.16
- ext4, 3.18
- ext4, 4.1
- ext4, 4.5