EXT4(5)                                                 File Formats Manual                                                 EXT4(5)

       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 sys‐
       tems that have historically been the default file system for many Linux distributions.  They are general purpose  file  sys‐
       tems 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 sys‐

       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 ver‐
              sions of e2fsprogs will not support file systems with this ext4 feature enabled.

              This ext4 feature enables clustered block allocation, so that the unit of allocation is a  power  of  two  number  of
              blocks.   That  is,  each  bit  in the what had traditionally been known as the block allocation bitmap now indicates
              whether a cluster is in use or not, where a cluster is by default composed of 16 blocks.  This feature  can  decrease
              the  time spent on doing block allocation and brings smaller fragmentation, especially for large files.  The size can
              be specified using the mke2fs -C option.

              Warning: The bigalloc feature is still under development, and may not be fully supported with your kernel or may have
              various  bugs.   Please  see the web page http://ext4.wiki.kernel.org/index.php/Bigalloc for details.  May clash with
              delayed allocation (see nodelalloc mount option).

              This feature requires that the extent feature be enabled.

              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.

              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.

              Normally,  ext4 allows an inode to have no more than 65,000 hard links.  This applies to regular files as well as di‐
              rectories, 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.

              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.

              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.

              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  stor‐
              age  device to be stored using an extent tree, which is a more efficient data structure than the traditional indirect
              block scheme used by the ext2 and ext3 file systems.  The use of the extent tree decreases metadata  block  overhead,
              improves  file  system performance, and decreases the needed to run e2fsck(8) on the file system.  (Note: both extent
              and extents are accepted as valid names for this feature for historical/backwards compatibility reasons.)

              This ext4 feature reserves a specific amount of space in each inode for extended metadata such  as  nanosecond  time‐
              stamps  and file creation time, even if the current kernel does not currently need to reserve this much space.  With‐
              out 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.

              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 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.

              This ext4 feature allows files to be larger than 2 terabytes in size.

              Allow data to be stored in the inode and extended attribute area.

              This feature is enabled on the superblock found on an external journal device.  The block size for the external jour‐
              nal 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 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

              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 sys‐
              tems that they could not understand.

              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  fea‐
              ture.  These two features are incompatible and metadata_csum will be used preferentially instead of uninit_bg.

              This  feature allows the file system to store the metadata checksum seed in the superblock, which allows the adminis‐
              trator to change the UUID of a file system using the metadata_csum feature while it is mounted.

              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 file system from being multiply
              mounted and is useful in shared storage environments.

              This ext4 feature provides project quota support. With this feature, the project ID of inode will be managed when the
              file system 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 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  in‐

              This  file  system feature indicates that space has been reserved so that the block group descriptor table can be ex‐
              tended 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.

              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  typi‐
              cally,  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.

              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 sepa‐
              rately.  stable_inodes is a "compat" feature, so old kernels will allow it.

              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.

              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 effi‐
              ciently 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  se‐

       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  deter‐
       mined by the file system superblock. Set them with tune2fs(8).

              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 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 re‐
              moved). Note that these mount options don't have to be supported if ext4 kernel driver is used for ext2 and ext3 file

       debug  Print debugging info upon each (re)mount.

              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 direc‐
              tory  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 file system. grpquota enables group quo‐
              tas support. You need the quota utilities to actually enable and manage the quota system.

              Disables 32-bit UIDs and GIDs.  This is for interoperability with older kernels which only store  and  expect  16-bit

       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".

              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 op‐
       tions as ext2 as well as the following additions:

              When  the external journal device's major/minor numbers have changed, these options allow the user to specify the new
              journal location.  The journal device is identified either through its new major/minor numbers encoded in devnum,  or
              via a path to the device.

              Don't load the journal on mounting.  Note that if the file system was not unmounted cleanly, skipping the journal re‐
              play will lead to the file system 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 file system, pass the mode to the kernel as boot parameter, e.g. rootflags=data=journal.

                     All data is committed into the journal prior to being written into the main file system.

                     This is the default mode.  All data is forced directly out to the main file system prior to its metadata being
                     committed to the journal.

                     Data ordering is not preserved – data may be written into the main 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.

              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 dis‐
              able 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) manual page.

              Apart from the old quota system (as in ext2, jqfmt=vfsold aka version 1 quota) ext3 also  supports  journaled  quotas
              (version 2 quota). jqfmt=vfsv0 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.

              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  en‐
              abled, 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 enhance‐
       ments 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, gr‐
       pquota, 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 ker‐

              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 con‐
              sistency with other ext4 mount options.

              The ext4 file system 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 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.

              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 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 trans‐
              actions can piggyback on the synchronous write. The algorithm used is designed to automatically tune for the speed of
              the disk, by measuring the amount of time (on average) that it takes to finish committing a  transaction.  Call  this
              time  the  "commit  time".  If the time that the transaction has been running is less than the commit time, ext4 will
              try sleeping for the commit time to see if other operations will join the transaction. The commit time is  capped  by
              the  max_batch_time,  which  defaults  to  15000 µs  (15 ms). This optimization can be turned off entirely by setting
              max_batch_time to 0.

              This parameter sets the commit time (as described above) to be at least min_batch_time. It defaults to zero microsec‐
              onds.  Increasing  this  parameter  may  improve the throughput of multi-threaded, synchronous workloads on very fast
              disks, at the cost of increasing latency.

              The I/O priority (from 0 to 7, where 0 is the highest priority) which should be used for I/O operations submitted  by
              kjournald2  during  a commit operation.  This defaults to 3, which is a slightly higher priority than the default I/O

       abort  Simulate the effects of calling ext4_abort() for debugging purposes.  This is normally used while remounting  a  file
              system which is already mounted.

              Many broken applications don't use fsync() when replacing existing files via patterns such as

              fd = open("foo.new")/write(fd,...)/close(fd)/ 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  be‐
              fore 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 file system 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 file system's inode table  is  being  initial‐

              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  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 per‐
              formance, 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 al‐
              locate  uninitialized  extent before buffer write and convert the extent to initialized after IO completes.  This ap‐
              proach 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 system's style.)

              Enable 64-bit inode version support. This option is off by default.

              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.

              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.

       The ext2, ext3, and ext4 file systems support setting the following file attributes on Linux  systems  using  the  chattr(1)

       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.

       This  section lists the file system driver (e.g., ext2, ext3, ext4) and upstream kernel version where a particular file sys‐
       tem 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 dis‐
       tributions" 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

       mke2fs(8), mke2fs.conf(5), e2fsck(8), dumpe2fs(8), tune2fs(8), debugfs(8), mount(8), chattr(1)

E2fsprogs version 1.47.0                                   February 2023                                                    EXT4(5)