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 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.

       A file system formatted for ext2, ext3, or ext4 can have some
       collection of the following file system feature flags enabled.  Some of
       these features are not supported by all implementations of the ext2,
       ext3, and ext4 file system drivers, depending on Linux kernel version
       in use.  On other operating systems, such as the GNU/HURD or FreeBSD,
       only a very restrictive set of file system features may be supported in
       their implementations of ext2.

              Enables the file system to be larger than 2^32 blocks.  This
              feature is set automatically, as needed, but it can be useful to
              specify this feature explicitly if the file system might need to
              be resized larger than 2^32 blocks, even if it was smaller than
              that threshold when it was originally created.  Note that some
              older kernels and older versions of e2fsprogs will not support
              file systems with this ext4 feature enabled.

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

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

              This feature requires that the extent feature be 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 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.

              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 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.)

              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.

              This feature enables the storage of file type information in
              directory entries.  This feature is supported by ext2, ext3, and

              This ext4 feature allows the per-block group metadata
              (allocation bitmaps and inode tables) to be placed anywhere on
              the storage media.  In addition, mke2fs will place the per-block
              group metadata together starting at the first block group of
              each "flex_bg group".   The size of the flex_bg group can be
              specified using the -G option.

              Create a journal to ensure filesystem consistency even across
              unclean shutdowns.  Setting the filesystem feature is equivalent
              to using the -j option with mke2fs or tune2fs.  This feature is
              supported by ext3 and ext4, and ignored by the ext2 file system

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

              Allow data to be stored in the inode and extended attribute

              This feature is enabled on the superblock found on an external
              journal device.  The block size for the external journal must be
              the same as the file system which uses it.

              The external journal device can be used by a file system by
              specifying the -J device=<external-device> option to mke2fs(8)
              or tune2fs(8).

              This feature 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.

              This feature flag is set automatically by modern kernels when a
              file larger than 2 gigabytes is created.  Very old kernels could
              not handle large files, so this feature flag was used to
              prohibit those kernels from mounting file systems that they
              could not understand.

              This ext4 feature enables metadata checksumming.  This feature
              stores checksums for all of the filesystem 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.

              This feature allows the filesystem to store the metadata
              checksum seed in the superblock, which allows the administrator
              to change the UUID of a filesystem 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 filesystem 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
              filesystem is mounted.

              Create quota inodes (inode #3 for userquota and inode #4 for
              group quota) and set them in the superblock.  With this feature,
              the quotas will be enabled automatically when the filesystem is

              Causes the quota files (i.e., user.quota and group.quota which
              existed in the older quota design) to be hidden inodes.

              This file system feature indicates that space has been reserved
              so that the block group descriptor table can be extended while
              resizing a mounted file system.  The online resize operation is
              carried out by the kernel, triggered by resize2fs(8).  By
              default mke2fs will attempt to reserve enough space so that the
              filesystem may grow to 1024 times its initial size.  This can be
              changed using the resize extended 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 typically, one
              will be located at the beginning of block group #1, and one in
              the last block group in the file system.  This feature is
              essentially a more extreme version of sparse_super and is
              designed to allow a much larger percentage of the disk to have
              contiguous blocks available for data files.

              This ext4 file system feature indicates that the block group
              descriptors will be protected using checksums, making it safe
              for mke2fs(8) to create a file system without initializing all
              of the block groups.  The kernel will keep a high watermark of
              unused inodes, and initialize inode tables and blocks lazily.
              This feature speeds up the time to check the file system using
              e2fsck(8), and it also speeds up the time required for mke2fs(8)
              to create the file system.

              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.

       This section describes mount options which are specific to ext2, ext3,
       and ext4.  Other generic mount options may be used as well; see
       mount(8) for details.

Mount options for ext2
       The `ext2' filesystem is the standard Linux filesystem.  Since Linux
       2.5.46, for most mount options the default is determined by the
       filesystem superblock. Set them with tune2fs(8).

              Support POSIX Access Control Lists (or not).  See the acl(5)
              manual page.

              Set the behavior for the statfs system call. The minixdf
              behavior is to return in the f_blocks field the total number of
              blocks of the filesystem, while the bsddf behavior (which is the
              default) is to subtract the overhead blocks used by the ext2
              filesystem and not available for file storage. Thus

              % mount /k -o minixdf; df /k; umount /k

              Filesystem  1024-blocks   Used  Available  Capacity  Mounted on
              /dev/sda6     2630655    86954   2412169      3%     /k

              % mount /k -o bsddf; df /k; umount /k

              Filesystem  1024-blocks  Used  Available  Capacity  Mounted on
              /dev/sda6     2543714      13   2412169      0%     /k

              (Note that this example shows that one can add command line
              options to the options given in /etc/fstab.)

       check=none or nocheck
              No checking is done at mount time. This is the default. This is
              fast.  It is wise to invoke e2fsck(8) every now and then, e.g.
              at boot time. The non-default behavior is unsupported
              (check=normal and check=strict options have been removed). Note
              that these mount options don't have to be supported if ext4
              kernel driver is used for ext2 and ext3 filesystems.

       debug  Print debugging info upon each (re)mount.

              Define the behavior when an error is encountered.  (Either
              ignore errors and just mark the filesystem erroneous and
              continue, or remount the filesystem read-only, or panic and halt
              the system.)  The default is set in the filesystem superblock,
              and can be changed using 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.

              The usrquota (same as quota) mount option enables user quota
              support on the filesystem. grpquota enables group quotas
              support. You need the quota utilities to actually enable and
              manage the quota system.

              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

       resgid=n and resuid=n
              The ext2 filesystem reserves a certain percentage of the
              available space (by default 5%, see mke2fs(8) and tune2fs(8)).
              These options determine who can use the reserved blocks.
              (Roughly: whoever has the specified uid, or belongs to the
              specified group.)

       sb=n   Instead of 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 filesystem's blocksize,
              the number of blocks per group, and features such as

              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 filesystem'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 filesystem with 4 k blocks, use

              Support "user." extended attributes (or not).

Mount options for ext3
       The ext3 filesystem is a version of the ext2 filesystem which has been
       enhanced with journaling.  It supports the same options 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

              Don't load the journal on mounting.  Note that if the filesystem
              was not unmounted cleanly, skipping the journal replay will lead
              to the filesystem containing inconsistencies that can lead to
              any number of problems.

              Specifies the journaling mode for file data.  Metadata is always
              journaled.  To use modes other than ordered on the root
              filesystem, pass the mode to the kernel as boot parameter, e.g.

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

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

                     Data ordering is not preserved – data may be written into
                     the main filesystem after its metadata has been committed
                     to the journal.  This is rumoured to be the highest-
                     throughput option.  It guarantees internal filesystem
                     integrity, however it can allow old data to appear in
                     files after a crash and journal recovery.

              Just print an error message if an error occurs in a file data
              buffer in ordered mode.

              Abort the journal if an error occurs in a file data buffer in
              ordered mode.

       barrier=0 / barrier=1
              This disables / enables the use of write barriers in the jbd
              code.  barrier=0 disables, barrier=1 enables (default). This
              also requires an IO stack which can support barriers, and if jbd
              gets an error on a barrier write, it will disable barriers again
              with a warning.  Write barriers enforce proper on-disk ordering
              of journal commits, making volatile disk write caches safe to
              use, at some performance penalty.  If your disks are battery-
              backed in one way or another, disabling barriers may safely
              improve performance.

              Start a journal commit every nrsec seconds.  The default value
              is 5 seconds.  Zero means default.

              Enable Extended User Attributes. See the attr(5) 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 filesystem 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 filesystem feature is enabled, journaled
              quotas are used automatically, and this mount option is ignored.

Mount options for ext4
       The ext4 filesystem is an advanced level of the ext3 filesystem which
       incorporates scalability and reliability enhancements for supporting
       large filesystem.

       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.

              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 filesystem enables write barriers by default.

              This tuning parameter controls the maximum number of inode table
              blocks that ext4's inode table readahead algorithm will pre-read
              into the buffer cache.  The value must be a power of 2. The
              default value is 32 blocks.

              Number of filesystem blocks that mballoc will try to use for
              allocation size and alignment. For RAID5/6 systems this should
              be the number of data disks * RAID chunk size in filesystem

              Deferring block allocation until write-out time.

              Disable delayed allocation. Blocks are allocated when data is
              copied from user to page cache.

              Maximum amount of time ext4 should wait for additional
              filesystem operations to be batch together with a synchronous
              write operation. Since a synchronous write operation is going to
              force a commit and then a wait for the I/O complete, it doesn't
              cost much, and can be a huge throughput win, we wait for a small
              amount of time to see if any other transactions can piggyback on
              the synchronous write. The algorithm used is designed to
              automatically tune for the speed of the disk, by measuring the
              amount of time (on average) that it takes to finish committing a
              transaction. Call this time the "commit time".  If the time that
              the transaction has been running is less than the commit time,
              ext4 will try sleeping for the commit time to see if other
              operations will join the transaction. The commit time is capped
              by the max_batch_time, which defaults to 15000 µs (15 ms). This
              optimization can be turned off entirely by setting
              max_batch_time to 0.

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

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

       abort  Simulate the effects of calling ext4_abort() for debugging
              purposes.  This is normally used while remounting a filesystem
              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",

              or worse yet

              fd = open("foo", O_TRUNC)/write(fd,...)/close(fd).

              If auto_da_alloc is enabled, ext4 will detect the replace-via-
              rename and replace-via-truncate patterns and force that any
              delayed allocation blocks are allocated such that at the next
              journal commit, in the default data=ordered mode, the data
              blocks of the new file are forced to disk before the rename()
              operation is committed.  This provides roughly the same level of
              guarantees as ext3, and avoids the "zero-length" problem that
              can happen when a system crashes before the delayed allocation
              blocks are forced to disk.

              Do not initialize any uninitialized inode table blocks in the
              background. This feature may be used by installation CD's so
              that the install process can complete as quickly as possible;
              the inode table initialization process would then be deferred
              until the next time the filesystem is mounted.

              The lazy itable init code will wait n times the number of
              milliseconds it took to zero out the previous block group's
              inode table. This minimizes the impact on system performance
              while the filesystem's inode table is being initialized.

              Controls whether ext4 should issue discard/TRIM commands to the
              underlying block device when blocks are freed.  This is useful
              for SSD devices and sparse/thinly-provisioned LUNs, but it is
              off by default until sufficient testing has been done.

              This option enables/disables the in-kernel facility for tracking
              filesystem metadata blocks within internal data structures. This
              allows multi-block allocator and other routines to quickly
              locate extents which might overlap with filesystem metadata
              blocks. This option is intended for debugging purposes and since
              it negatively affects the performance, it is off by default.

              Controls whether or not ext4 should use the DIO read locking. If
              the dioread_nolock option is specified ext4 will allocate
              uninitialized extent before buffer write and convert the extent
              to initialized after IO completes.  This approach allows ext4
              code to avoid using inode mutex, which improves scalability on
              high speed storages. However this does not work with data
              journaling and dioread_nolock option will be ignored with kernel
              warning.  Note that dioread_nolock code path is only used for
              extent-based files.  Because of the restrictions this options
              comprises it is off by default (e.g. dioread_lock).

              This limits the size of the directories so that any attempt to
              expand them beyond the specified limit in kilobytes will cause
              an ENOSPC error. This is useful in memory-constrained
              environments, where a very large directory can cause severe
              performance problems or even provoke the Out Of Memory killer.
              (For example, if there is only 512 MB memory available, a 176 MB
              directory may seriously cramp the system's style.)

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

              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
              filesystem.  You need the quota utilities to actually enable and
              manage the quota system.  This mount option requires the project
              filesystem feature.

       The ext2, ext3, and ext4 filesystems 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 filesystems support the following flag:

       j - data journaling

       Finally, the ext4 filesystem also supports the following flag:

       e - extents format

       For descriptions of these attribute flags, please refer to the
       chattr(1) man page.

       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

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

E2fsprogs version 1.45.6          March 2020                           EXT4(5)