# ext4

## 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 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.
    


## 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 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.
    
           **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 **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
                  option).
    
                  This feature requires that the **extent** feature be enabled.
    
           **casefold**
                  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 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 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.
    
           **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 inode if necessary, increasing
                  the limit on the size and number of extended attributes 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_ 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.
    
           **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 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.)
    
           **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** option.
    
           **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 device=** <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 feature 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 superblock, 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 environments.
    
           **orphan_file**
                  This ext4 feature fixes a potential scalability bottleneck
                  for workloads that are doing a large number of truncate or
                  file extensions in parallel.  It is supported by Linux
                  kernels starting version 5.15, and by e2fsprogs starting
                  with version 1.47.0.
    
           **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 automatically 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_super and is designed to allow a much larger
                  percentage of the disk to have contiguous 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 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.
    
           **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 authenticated, 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 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.
    
           **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 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 file system 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 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 supplied with
                  arguments that are consistent with the file system's layout
                  (e.g., blocksize, 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 identified 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
                  inconsistencies 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 system after its metadata has
                         been committed to the journal.  This is rumoured to
                         be the highest-throughput option.  It guarantees
                         internal file system 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 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.
    
           **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 enables
                  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, journaled 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 usrjquota=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 scalability 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, nogrpid, sysvgroups,**
           **resgid, resuid, sb, quota, noquota, nouid32, grpquota, usrquota,**
           **usrjquota, 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 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 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 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.
    
           **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 normally 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 patterns 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 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.
    
           **noinit_itable**
                  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 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 system
                  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 metadata blocks within internal data
                  structures. This allows multi-block allocator and other
                  routines to quickly locate extents which might overlap with
                  file system metadata 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 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 MiB memory
                  available, a 176 MiB 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 systems where extended attributes are
                  rarely or never shared between files, use of mbcache 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
    


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