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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
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 de-
signed for extensibility and backwards compatibility. In particular, file systems previ-
ously 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 collection of the following
file system feature flags enabled. Some of these features are not supported by all imple-
mentations of the ext2, ext3, and ext4 file system drivers, depending on Linux kernel ver-
sion in use. On other operating systems, such as the GNU/HURD or FreeBSD, only a very re-
strictive 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 auto-
matically, 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 alloca-
tion is a power of two number of blocks. That is, each bit in the what had tradi-
tionally 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 fea-
ture can decrease the time spent on doing block allocation and brings smaller frag-
mentation, 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 direc-
tories 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
encoding 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 ap-
plies 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.
ea_inode
Normally, a file's extended attributes and associated metadata must fit within the
inode 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 in-
ode if necessary, increasing the limit on the size and number of extended at-
tributes 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 en-
crypted.
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 secu-
rity.
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 in-
ode 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 meta-
data 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.)
extra_isize
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 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 op-
tion.
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
gigabytes is created. Very old kernels could not handle large files, so this fea-
ture 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 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.
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 envi-
ronments.
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 automati-
cally 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
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 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 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_su-
per and is designed to allow a much larger percentage of the disk to have contigu-
ous blocks available 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
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 water-
mark 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. Using the Merkle tree's root hash, a verity file can be efficiently authen-
ticated, 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 itself is read-only, then using dm-verity
to authenticate 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 de-
fault) it takes the fsgid of the current process, unless the directory has the set-
gid 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
system. grpquota enables group quotas support. You need the quota utilities to ac-
tually 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 de-
fault 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 speci-
fied 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 location 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 sup-
plied with arguments that are consistent with the file system'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 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
journaling. 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
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 file system was not unmounted
cleanly, skipping the journal replay will lead to the file system containing incon-
sistencies 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 sys-
tem after its metadata has been committed to the journal. This is rumoured
to be the highest-throughput option. It guarantees internal file system in-
tegrity, 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 sup-
port barriers, and if jbd gets an error on a barrier write, it will disable barri-
ers again with a warning. Write barriers enforce proper on-disk ordering of jour-
nal 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 barri-
ers 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 en-
ables 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, jour-
naled 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 scal-
ability 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, nogr-
pid, sysvgroups, resgid, resuid, sb, quota, noquota, nouid32, grpquota, usrquota, usr-
jquota, grpjquota, 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 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 en-
abled 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 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 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 <micro>s (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 nor-
mally 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 pat-
terns 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 commit-
ted. 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 al-
location blocks are forced to disk.
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 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 sys-
tem performance while the file system'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-
provisioned 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 meta-
data blocks within internal data structures. This allows multi-block allocator and
other routines to quickly locate extents which might overlap with file system meta-
data 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 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.
nombcache
This option disables use of mbcache for extended attribute deduplication. On sys-
tems where extended attributes are rarely or never shared between files, use of mb-
cache 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
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
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)
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