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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 de-
       signed for extensibility and backwards compatibility.  In particular, file systems  previ-
       ously  intended  for use with the ext2 and ext3 file systems can be mounted using the ext4
       file system driver, and indeed in many modern Linux distributions, the  ext4  file  system
       driver has been configured to handle mount requests for ext2 and ext3 file systems.

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

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

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

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

              This feature requires that the extent feature be enabled.

              This  ext4 feature provides file system level character encoding support for direc-
              tories with the casefold (+F) flag enabled.  This feature is name-preserving on the
              disk,  but  it allows applications to lookup for a file in the file system using an
              encoding equivalent version of the file name.

              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 ap-
              plies to regular files as well as directories, which means that  there  can  be  no
              more  than  64,998  subdirectories in a directory (because each of the '.' and '..'
              entries, as well as the directory entry for the directory in its  parent  directory
              counts  as  a  hard  link).  This feature lifts this limit by causing ext4 to use a
              link count of 1 to indicate that the number of hard links to  a  directory  is  not
              known when the link count might exceed the maximum count limit.

              Normally,  a file's extended attributes and associated metadata must fit within the
              inode or the inode's associated extended attribute block. This feature  allows  the
              value  of each extended attribute to be placed in the data blocks of a separate in-
              ode if necessary, increasing the limit on the  size  and  number  of  extended  at-
              tributes per file.

              Enables  support  for  file-system  level encryption of data blocks and file names.
              The inode metadata (timestamps, file size, user/group ownership, etc.) is  not  en-

              This  feature  is most useful on file systems with multiple users, or where not all
              files should be encrypted.  In many use cases, especially on  single-user  systems,
              encryption  at  the block device layer using dm-crypt may provide much better secu-

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

              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 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 op-

              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 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 journal must be the same as the file system which  uses

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

              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 fea-
              ture 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  su-
              perblock,  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 environ-

              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 automati-
              cally when the filesystem 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  filesystem  may
              grow to 1024 times its initial size.  This can be changed using the resize extended

              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_su-
              per  and is designed to allow a much larger percentage of the disk to have contigu-
              ous 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  water-
              mark of unused inodes, and initialize inode tables and blocks lazily.  This feature
              speeds up the time to check the file system using e2fsck(8), and it also speeds  up
              the time required for mke2fs(8) to create the file system.

              Enables  support  for verity protected files.  Verity files are readonly, and their
              data is transparently verified against a Merkle tree hidden past  the  end  of  the
              file.   Using the Merkle tree's root hash, a verity file can be efficiently authen-
              ticated, independent of the file's size.

              This feature is most useful for authenticating important read-only files  on  read-
              write  file  systems.  If the file system itself is read-only, then using dm-verity
              to authenticate the entire block device may provide much better security.

       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

              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 de-
              fault) it takes the fsgid of the current process, unless the directory has the set-
              gid  bit  set,  in  which case it takes the gid from the parent directory, and also
              gets the setgid bit set if it is a directory itself.

              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 default.

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

       sb=n   Instead of using the normal superblock, use an alternative superblock specified  by
              n.   This  option  is normally used when the primary superblock has been corrupted.
              The location of backup superblocks is dependent on the filesystem's blocksize,  the
              number of blocks per group, and features such as sparse_super.

              Additional  backup  superblocks can be determined by using the mke2fs program using
              the -n option to print out where the superblocks exist, supposing  mke2fs  is  sup-
              plied  with arguments that are consistent with the filesystem's layout (e.g. block-
              size, blocks per group, sparse_super, etc.).

              The block number here uses 1 k units. Thus, if you want to use logical block  32768
              on a filesystem with 4 k blocks, use "sb=131072".

              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 jour-
       naling.  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 identi-
              fied 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 filesystem was not unmounted
              cleanly, skipping the journal replay will lead to the filesystem containing  incon-
              sistencies 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. rootflags=data=journal.

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

                     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 filesys-
                     tem after its metadata has been committed to the journal.  This is  rumoured
                     to  be the highest-throughput option.  It guarantees internal filesystem in-
                     tegrity, 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

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

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

              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  en-
              ables 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  usr-
              jquota=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  scala-
       bility 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,  nogr-
       pid,  sysvgroups,  resgid,  resuid,  sb, quota, noquota, nouid32, grpquota, usrquota, usr-
       jquota, grpjquota, and jqfmt are backwardly compatible with ext3 or ext2.

       journal_checksum | nojournal_checksum
              The journal_checksum option enables checksumming of the journal transactions.  This
              will  allow  the recovery code in e2fsck and the kernel to detect corruption in the
              kernel. It is a compatible change and will be ignored by older kernels.

              Commit block can be written to disk without waiting for descriptor blocks.  If  en-
              abled  older  kernels cannot mount the device.  This will enable 'journal_checksum'

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

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

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

       abort  Simulate the effects of calling ext4_abort() for debugging purposes.  This is  nor-
              mally used while remounting a filesystem which is already mounted.

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

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

              or worse yet

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

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

              Do not initialize any uninitialized inode table blocks in the background. This fea-
              ture  may  be used by installation CD's so that the install process can complete as
              quickly as possible; the inode table initialization process would then be  deferred
              until the next time the 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  sys-
              tem 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 meta-
              data blocks within internal data structures. This allows multi-block allocator  and
              other  routines to quickly locate extents which might overlap with filesystem meta-
              data 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 mem-
              ory-constrained environments, where a very large directory can cause severe perfor-
              mance  problems or even provoke the Out Of Memory killer. (For example, if there is
              only 512 MB memory available, a 176 MB directory may seriously cramp  the  system's

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

              This  option  disables use of mbcache for extended attribute deduplication. On sys-
              tems where extended attributes are rarely or never shared between files, use of mb-
              cache for deduplication adds unnecessary computational overhead.

              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),

E2fsprogs version 1.45.5                   January 2020                                   EXT4(5)

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