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EXT4(5)                                File Formats Manual                                EXT4(5)

NAME
       ext2 - the second extended file system
       ext2 - the third extended file system
       ext4 - the fourth extended file system

DESCRIPTION
       The  second,  third, and fourth extended file systems, or ext2, ext3, and ext4 as they are
       commonly known, are Linux file systems that have historically been the default file system
       for  many  Linux  distributions.   They  are  general  purpose file systems that have been
       designed for extensibility and backwards compatibility.  In particular, file systems  pre‐
       viously 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 handle mount requests for ext2 and ext3 file systems.

FILE SYSTEM FEATURES
       A  file  system formated for ext2, ext3, or ext4 can be have some collection of the follow
       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
       restrictive set of file system features may be supported in their implementations of ext2.

                   64bit
                          Enables the file system to be larger than 2^32 blocks.  This feature is
                          set automatically, as needed, but it can be useful to specify this fea‐
                          ture explicitly if the file system might need to be resized larger than
                          2^32 blocks, even if it was smaller than that  threshold  when  it  was
                          originally created.  Note that some older kernels and older versions of
                          e2fsprogs will not support file systems with this ext4 feature enabled.

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

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

                          This feature requires that the extent features be enabled.

                   dir_index
                          Use hashed b-trees to speed up name lookups in large directories.  This
                          feature  is  supported by ext3 and ext4 file systems, and is ignored by
                          ext2 file systems.

                   dir_nlink
                          This ext4 feature allows more than 65000 subdirectories per directory.

                   extent
                          This ext4 feature allows the mapping of logical  block  numbers  for  a
                          particular  inode to physical blocks on the storage device to be stored
                          using an extent tree, which is a more efficient data structure than the
                          traditional  indirect  block scheme used by the ext2 and ext3 file sys‐
                          tems.  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 com‐
                          patibility reasons.)

                   extra_isize
                          This ext4 feature reserves a specific amount of space in each inode for
                          extended metadata such as nanosecond timestamps and file creation time,
                          even if the current kernel does not current need to reserve  this  much
                          space.   Without  this  feature,  the kernel will reserve the amount of
                          space for features currently 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.

                   ext_attr
                          This feature enables the use of extended attributes.  This  feature  is
                          supported by ext2, ext3, and ext4.

                   filetype
                          This  feature  enables  the  storage file type information in directory
                          entries.  This feature is supported by ext2, ext3, and ext4.

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

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

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

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

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

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

                   sparse_super2
                          This  feature  indicates  that  there  will  only  at  most  two backup
                          superblock and block group descriptors.  The block groups used to store
                          the  backup  superblock  and  blockgroup  descriptors 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 is feature is essentially a more extreme version  of  sparse_super
                          and  is  designed  to allow the a much larger percentage of the disk to
                          have contiguous blocks available for data files.

                   meta_bg
                          This ext4 feature allows file systems to  be  resized  on-line  without
                          explicitly needing to reserve space for growth in the size of the block
                          group descriptors.  This scheme is also used  to  resize  file  systems
                          which  are  larger  than  2^32 blocks.  It is not recommended that this
                          feature be set when a file system  is  created,  since  this  alternate
                          method  of  storing  the block group descriptor will slow down the time
                          needed to mount the file system, and newer  kernels  can  automatically
                          set  this  feature as necessary when doing an online resize and no more
                          reserved space is available in the resize inode.

                   mmp
                          This ext4 feature provides multiple mount protection (MMP).  MMP  helps
                          to  protect the filesystem from being multiply mounted and is useful in
                          shared storage environments.

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

                   resize_inode
                          This  file system feature indicates that space has been reserved so the
                          block group descriptor table can be extended  by  the  file  system  is
                          resized  while the file system is mounted.  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 feature be enabled.

                   sparse_super
                          This file system feature is set on all modern ext2, ext3, and ext4 file
                          system.  It indicates that backup copies of the  superblock  and  block
                          group descriptors be present only on a few block groups, and not all of
                          them.

                   uninit_bg
                          This ext4 file system feature indicates that the block  group  descrip‐
                          tors 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 block 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.

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

Mount options for ext2
       The  `ext2'  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).

       acl|noacl
              Support POSIX Access Control Lists (or not).

       bsddf|minixdf
              Set  the  behavior for the statfs system call. The minixdf behavior is to return in
              the f_blocks field the total number of blocks of the 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.

       errors={continue|remount-ro|panic}
              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.

       grpquota|noquota|quota|usrquota
              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.

       nouid32
              Disables 32-bit UIDs and GIDs.  This is for  interoperability  with  older  kernels
              which only store and expect 16-bit values.

       oldalloc or orlov
              Use old allocator or Orlov allocator for new inodes. Orlov is default.

       resgid=n and resuid=n
              The  ext2  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 speci‐
              fied group.)

       sb=n   Instead of block 1, use block n as  superblock.  This  could  be  useful  when  the
              filesystem  has  been  damaged.   (Earlier,  copies of the superblock would be made
              every 8192 blocks: in block 1, 8193, 16385, ... (and one got thousands of copies on
              a  big  filesystem). Since version 1.08, mke2fs has a -s (sparse superblock) option
              to reduce the number of backup superblocks, and since  version  1.15  this  is  the
              default.  Note  that this may mean that ext2 filesystems created by a recent mke2fs
              cannot be mounted r/w under Linux 2.0.*.)  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".

       user_xattr|nouser_xattr
              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:

       journal=update
              Update the ext3 filesystem's journal to the current format.

       journal=inum
              When  a journal already exists, this option is ignored. Otherwise, it specifies the
              number of the inode which will represent the ext3 filesystem's journal  file;  ext3
              will  create  a  new  journal, overwriting the old contents of the file whose inode
              number is inum.

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

       norecovery/noload
              Don't  load the journal on mounting.  Note that if the 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.

       data={journal|ordered|writeback}
              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.

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

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

              writeback
                     Data  ordering is not preserved – data may be written into the main filesys‐
                     tem 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.

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

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

       barrier=0 / barrier=1
              This disables / enables the use of write barriers in the jbd code.  barrier=0  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.

       commit=nrsec
              Sync all data and metadata every nrsec seconds. The default  value  is  5  seconds.
              Zero means default.

       user_xattr
              Enable Extended User Attributes. See the attr(5) manual page.

       acl    Enable POSIX Access Control Lists. See the acl(5) manual page.

       usrjquota=aquota.user|grpjquota=aquota.group|jqfmt=vfsv0
              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  enables  journaled
              quotas.   For   journaled   quotas  the  mount  options  usrjquota=aquota.user  and
              grpjquota=aquota.group are required to tell the quota system which  quota  database
              files  to use. Journaled quotas have the advantage that even after a crash no quota
              check is required.

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, 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, grpquota, usrquota usrjquota, grpjquota and jqfmt are
       backwardly compatible with ext3 or ext2.

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

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

       barrier=0 / barrier=1 / barrier / nobarrier
              These mount options have the same effect as in ext3.  The mount  options  "barrier"
              and "nobarrier" are added for consistency with other ext4 mount options.

              The ext4 filesystem enables write barriers by default.

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

       stripe=n
              Number  of  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.

       delalloc
              Deferring block allocation until write-out time.

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

       max_batch_time=usec
              Maximum amount of time ext4 should wait for additional 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 µs (15 ms). This optimization can be turned
              off entirely by setting max_batch_time to 0.

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

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

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

       auto_da_alloc|noauto_da_alloc
              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
              allocation blocks are forced to disk.

       noinit_itable
              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.

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

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

       nouid32
              Disables  32-bit  UIDs and GIDs.  This is for interoperability  with  older kernels
              which only store and expect 16-bit values.

       block_validity/noblock_validity
              This options  allows  to  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.

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

       max_dir_size_kb=n
              This  limits  the size of the directories so that any attempt to expand them beyond
              the specified limit in kilobytes will cause an ENOSPC error. This is useful in 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
              style.)

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

FILE ATTRIBUTES
       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.

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

E2fsprogs version 1.42.13                    May 2015                                     EXT4(5)

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