EXT4(5) File Formats Manual EXT4(5)
NAME
ext2 - the second extended file system
ext2 - the third extended file system
ext4 - the fourth extended file system
DESCRIPTION
The second, third, and fourth extended file systems, or ext2, ext3, and ext4 as they are commonly known, are
Linux file systems that have 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 par‐
ticular, 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 handle mount requests for ext2 and ext3 file systems.
FILE SYSTEM FEATURES
A file system formated for ext2, ext3, or ext4 can be have some collection of the follow 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 implementa‐
tions of ext2.
64bit
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.
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 -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.ker‐
nel.org/index.php/Bigalloc for details. May clash with delayed allocation (see nodelalloc‐
mountoption).
This feature requires that the extent features be enabled.
dir_index
Use hashed b-trees to speed up name lookups in large directories. This feature is sup‐
ported by ext3 and ext4 file systems, and is ignored by ext2 file systems.
dir_nlink
This ext4 feature allows more than 65000 subdirectories per directory.
extent
This ext4 feature allows the mapping of logical block numbers for a particular inode to
physical blocks on the storage device to be stored using an extent tree, which is a more
efficient data structure than the traditional indirect block scheme used by the ext2 and
ext3 file systems. The use of the extent tree decreases metadata block overhead, improves
file system performance, and decreases the needed to run e2fsck(8) on the file system.
(Note: both extent and extents are accepted as valid names for this feature for histori‐
This feature enables the use of extended attributes. This feature is supported by ext2,
ext3, and ext4.
filetype
This feature enables the storage 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 bit‐
maps and inode tables) to be placed anywhere on the storage media. In
addition, mke2fs will place the per-block group metadata together start‐
ing 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 filesystem consistency even across unclean
shutdowns. Setting the filesystem feature is equivalent to using the -j
option. 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.
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 device=<external-device> option to mke2fs(8) or tune2fs(8).
large_file
This feature flag is set automatically by modern kernels when a file
larger than 2 gigabytes is created. Very old kernels could not handle
large files, so this feature flag was used to prohibit those kernels
from mounting file systems that they could not understand.
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 recommended that this fea‐
ture be set when a file system is created, since this alternate method
of storing the block group descriptor will slow down the time needed to
mount the file system, and newer kernels can automatically set this fea‐
ture as necessary when doing an online resize and no more reserved space
is available in the resize inode.
mmp
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 file system feature is set on all modern ext2, ext3, and ext4 file
system. It indicates that backup copies of the superblock and block
group descriptors be present only on a few block groups, and not all of
them.
uninit_bg
This ext4 file system feature indicates that the block group descriptors
will be protected using checksums, making it safe for mke2fs(8) to cre‐
ate 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 block 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.
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' 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).
acl|noacl
Support POSIX Access Control Lists (or not).
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 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.
errors={continue|remount-ro|panic}
grpquota|noquota|quota|usrquota
The usrquota (same as quota) mount option enables user quota support on the filesys‐
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 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 specified
group.)
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".
user_xattr|nouser_xattr
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 jour‐
naling. It supports the same options as ext2 as well as the following additions:
journal=update
Update the ext3 filesystem's journal to the current format.
journal=inum
When a journal already exists, this option is ignored. Otherwise, it specifies the
number of the inode which will represent the ext3 filesystem's journal file; ext3
will create a new journal, overwriting the old contents of the file whose inode num‐
ber is inum.
journal_dev=devnum/journal_path=path
When the external journal device's major/minor numbers have changed, these options
allow the user to specify the new journal location. The journal device is identi‐
fied either through its new major/minor numbers encoded in devnum, or via a path to
the device.
norecovery/noload
Don't load the journal on mounting. Note that if the filesystem was not unmounted
cleanly, skipping the journal replay will lead to the filesystem containing incon‐
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 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.
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
Sync all data and metadata every nrsec seconds. The default value is 5 seconds. Zero
means default.
user_xattr
Enable Extended User Attributes. See the attr(5) manual page.
acl Enable POSIX Access Control Lists. See the acl(5) manual page.
usrjquota=aquota.user|grpjquota=aquota.group|jqfmt=vfsv0
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 quo‐
tas. 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.
Mount options for ext4
The ext4 filesystem is an advanced level of the ext3 filesystem which incorporates scala‐
bility and reliability enhancements for supporting large filesystem.
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, grpquota, usrquota usrjquota, grpjquota and jqfmt are
backwardly compatible with ext3 or ext2.
journal_checksum
The ext4 filesystem 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 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.
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 filesystem operations to be
batch together with a synchronous write operation. Since a synchronous write opera‐
tion 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.
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
improve 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 nor‐
mally used while remounting a filesystem which is already mounted.
auto_da_alloc|noauto_da_alloc
Many broken applications don't use fsync() when replacing existing files via pat‐
terns such as
fd = open("foo.new")/write(fd,...)/close(fd)/ rename("foo.new", "foo")
noinit_itable
Do not initialize any uninitialized inode table blocks in the background. This fea‐
ture 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.
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
performance while the filesystem's inode table is being initialized.
discard/nodiscard
Controls whether ext4 should issue discard/TRIM commands to the underlying block
device when blocks are freed. This is useful for SSD devices and sparse/thinly-pro‐
visioned LUNs, but it is off by default until sufficient testing has been done.
nouid32
Disables 32-bit UIDs and GIDs. This is for interoperability with older kernels
which only store and expect 16-bit values.
block_validity/noblock_validity
This options allows to enables/disables the in-kernel facility for tracking filesys‐
tem metadata blocks within internal data structures. This allows multi-block alloca‐
tor and other routines to quickly locate extents which might overlap with filesystem
metadata blocks. This option is intended for debugging purposes and since it nega‐
tively affects the performance, it is off by default.
dioread_lock/dioread_nolock
Controls whether or not ext4 should use the DIO read locking. If the dioread_nolock
option is specified ext4 will allocate 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 mem‐
ory-constrained environments, where a very large directory can cause severe perfor‐
mance 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.)
i_version
Enable 64-bit inode version support. This option is off by default.
FILE ATTRIBUTES
The ext2, ext3, and ext4 filesystems support setting the following file attributes on Linux
systems using the chattr(1) utility:
a - append only
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.
SEE ALSO
mke2fs(8), mke2fs.conf(5), e2fsck(8), dumpe2fs(8), tune2fs(8), debugfs(8), mount(8),
chattr(1)
E2fsprogs version 1.42.9 December 2013 EXT4(5)
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