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          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 ker-
               nels 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 tradi-
               tionally been known as the block allocation bitmap now
               indicates whether a cluster is in use or not, where a
               cluster is by default composed of 16 blocks.  This fea-
               ture can decrease the time spent on doing block alloca-
               tion and brings smaller fragmentation, especially for
               large files.  The size can be specified using the
               mke2fs -C option.

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               Warning: The bigalloc feature is still under develop-
               ment, 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

               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 direc-
               tory 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 asso-
               ciated 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 (times-
               tamps, file size, user/group ownership, etc.) is not

               This feature is most useful on file systems with

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               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 tra-
               ditional 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 informa-
               tion 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 any-
               where 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

               Create a journal to ensure file system consistency even
               across unclean shutdowns.  Setting the file system fea-
               ture is equivalent to using the -j option with mke2fs

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               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 ter-
               abytes 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 exter-
               nal journal must be the same as the file system which
               uses it.

               The external journal device can be used by a file sys-
               tem 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 directo-
               ries 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 ker-
               nels 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 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 fea-
               tures are incompatible and metadata_csum will be used
               preferentially instead of uninit_bg.

               This feature allows the file system to store the meta-
               data 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.

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               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

               This ext4 feature provides multiple mount protection
               (MMP).  MMP helps to protect the file system from being
               multiply mounted and is useful in shared storage envi-

               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 automati-
               cally when the file system is mounted.

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

               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.

               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

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               are present only in a few block groups, not all of

               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.

               Marks the file system's inode numbers and UUID as sta-
               ble.  resize2fs(8) will not allow shrinking a file sys-
               tem with this feature, nor will tune2fs(8) allow chang-
               ing its UUID.  This feature allows the use of special-
               ized 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.

               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 ker-
               nel will keep a high watermark of unused inodes, and
               initialize inode tables and blocks lazily.  This fea-
               ture 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 impor-
               tant 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 pro-
               vide much better security.

          This section describes mount options which are specific to

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          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

               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 tab(#); l2 l2
               r2 l2 l2 l l c r c c l.  File System#1024-
               blocks#Used#Available#Capacity#Mounted on

               % mount /k -o bsddf; df /k; umount /k tab(#); l2 l2 r2
               l2 l2 l l c r c c l.  File System#1024-
               blocks#Used#Available#Capacity#Mounted on

               (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.

               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

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               changed using tune2fs(8).

          grpid|bsdgroups and nogrpid|
               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

               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.

               Disables 32-bit UIDs and GIDs.  This is for interoper-
               ability 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 alterna-
               tive 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".

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               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:

               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 iden-
               tified 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 replay will lead to the file system containing
               inconsistencies that can lead to any number of prob-

               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 en data may be writ-
                    ten 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.

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               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 jour-
               nal commits, making volatile disk write caches safe to
               use, at some performance penalty.  If your disks are
               battery-backed in one way or another, disabling barri-
               ers may safely improve performance.

               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

               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 enabled, journaled quotas
               are used automatically, and this mount option is

     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, oldalloc, [no]user_xattr, [no]acl, bsddf,
          minixdf, debug, data_err, grpid, bsdgroups, nogrpid,

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          sysvgroups, resgid, quota, noquota, nouid32, grpquota,
          usrquota, usrjquota, and jqfmt are backwardly compatible

          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'

          barrier=0 / barrier=1 / barrier /
               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.

               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

               Number of file system blocks that mballoc will try to
               use for allocation size and alignment. For RAID5/6 sys-
               tems 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 syn-
               chronous 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

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               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 opera-
               tions will join the transaction. The commit time is
               capped by the max_batch_time, which defaults to
               15000 [mc]s (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

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

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

               Many broken applications don't use fsync() when replac-
               ing 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 allo-
               cated 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" prob-
               lem that can happen when a system crashes before the

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               delayed allocation blocks are forced to disk.

               Do not initialize any uninitialized inode table blocks
               in the background. This feature may be used by instal-
               lation 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 sys-
               tem performance while the file system's inode table is
               being initialized.

               Controls whether ext4 should issue discard/TRIM com-
               mands 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 nega-
               tively 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 com-
               pletes.  This approach allows ext4 code to avoid using
               inode mutex, which improves scalability on high speed
               storages. However this does not work with data journal-
               ing and dioread_nolock option will be ignored with ker-
               nel 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

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               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 direc-
               tory 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 com-
               putational 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) 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 fol-
          lowing flag:

          j - data journaling

          Finally, the ext4 file system also supports the following

          e - extents format

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          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

     Page 15             E2fsprogs version 1.46.4    (printed 5/26/22)

     EXT4(5)                   (August 2021)                   EXT4(5)

          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)

     Page 16             E2fsprogs version 1.46.4    (printed 5/26/22)