Provided by: e2fsprogs_1.45.3-4ubuntu2_amd64 bug


       ext2 - the second extended file system
       ext3 - the third extended file system
       ext4 - the fourth extended file system


       The  second,  third, and fourth extended file systems, or ext2, ext3, and ext4 as they are
       commonly known, are Linux file systems that have historically been the default file system
       for  many  Linux  distributions.   They  are  general  purpose file systems that have been
       designed for extensibility and  backwards  compatibility.   In  particular,  file  systems
       previously  intended  for use with the ext2 and ext3 file systems can be mounted using the
       ext4 file system driver, and indeed in many modern  Linux  distributions,  the  ext4  file
       system driver has been configured to handle mount requests for ext2 and ext3 file systems.


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

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

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

              This feature requires that the extent feature be enabled.

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

              Normally, ext4 allows an inode to have  no  more  than  65,000  hard  links.   This
              applies  to  regular files as well as directories, which means that there can be no
              more than 64,998 subdirectories in a directory (because each of the  '.'  and  '..'
              entries,  as  well as the directory entry for the directory in its parent directory
              counts as a hard link).  This feature lifts this limit by causing  ext4  to  use  a
              link  count  of  1  to indicate that the number of hard links to a directory is not
              known when the link count might exceed the maximum count limit.

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

              This ext4 feature provides file-system level encryption of  data  blocks  and  file
              names.   The  inode metadata (timestamps, file size, user/group ownership, etc.) is
              not encrypted.

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

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

              This ext4 feature allows the mapping of logical  block  numbers  for  a  particular
              inode  to  physical blocks on the storage device to be stored using an extent tree,
              which is a more efficient data structure than the traditional indirect block scheme
              used  by  the  ext2  and  ext3  file systems.  The use of the extent tree decreases
              metadata block overhead, improves file system performance, and decreases the needed
              to  run  e2fsck(8) on the file system.  (Note: both extent and extents are accepted
              as valid names for this feature for historical/backwards compatibility reasons.)

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

              For this feature to be useful the inode size must be 256 bytes in size or larger.

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

              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.

              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
              device=<external-device> option to mke2fs(8) or tune2fs(8).

              This feature increases the limit on the number of files per  directory  by  raising
              the maximum size of directories and, for hashed b-tree directories (see dir_index),
              the maximum height of the hashed b-tree used to store the directory entries.

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

              This ext4 feature enables metadata checksumming.  This feature stores checksums for
              all of the filesystem metadata (superblock,  group  descriptor  blocks,  inode  and
              block  bitmaps,  directories, and extent tree blocks).  The checksum algorithm used
              for the metadata blocks is different than the one used for group  descriptors  with
              the  uninit_bg feature.  These two features are incompatible and metadata_csum will
              be used preferentially instead of uninit_bg.

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

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

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

              This ext4 feature provides project quota support. With this feature, the project ID
              of inode will be managed when the filesystem is mounted.

              Create quota inodes (inode #3 for userquota and inode #4 for group quota)  and  set
              them   in   the  superblock.   With  this  feature,  the  quotas  will  be  enabled
              automatically when the filesystem is 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_super  and is designed to allow a much larger percentage of the disk to have
              contiguous blocks available for data files.

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


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

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

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

       oldalloc or orlov
              Use old allocator or Orlov allocator for new inodes. Orlov is 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
              specified group.)

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

              The  block number here uses 1 k units. Thus, if you want to use logical block 32768
              on a filesystem with 4 k blocks, use "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
       journaling.  It supports the same options as ext2 as well as the following additions:

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

              Specifies the journaling mode for file data.  Metadata is always journaled.  To use
              modes  other  than  ordered  on the root filesystem, pass the mode to the kernel as
              boot parameter, e.g. 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
                     filesystem  after  its  metadata has been committed to the journal.  This is
                     rumoured to  be  the  highest-throughput  option.   It  guarantees  internal
                     filesystem integrity, however it can allow old data to appear in files after
                     a crash and journal recovery.

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

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

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

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

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

              Apart from the old quota system (as in ext2, jqfmt=vfsold aka version 1 quota) ext3
              also  supports  journaled  quotas  (version  2  quota).  jqfmt=vfsv0 or jqfmt=vfsv1
              enables journaled quotas. Journaled quotas have the advantage  that  even  after  a
              crash  no  quota  check  is required. When the quota filesystem feature is enabled,
              journaled quotas are used automatically, and this mount option is ignored.

              For  journaled   quotas   (jqfmt=vfsv0   or   jqfmt=vfsv1),   the   mount   options
              usrjquota=aquota.user  and  are  required to tell the quota
              system which quota database files to use. When  the  quota  filesystem  feature  is
              enabled, journaled quotas are used automatically, and this mount option is ignored.

Mount options for ext4

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

       The options journal_dev, journal_path, norecovery, noload, data, commit, orlov,  oldalloc,
       [no]user_xattr,  [no]acl,  bsddf,  minixdf,  debug,  errors,  data_err,  grpid, bsdgroups,
       nogrpid, sysvgroups, resgid, resuid, sb,  quota,  noquota,  nouid32,  grpquota,  usrquota,
       usrjquota, grpjquota, and jqfmt are backwardly compatible with ext3 or ext2.

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

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

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

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

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

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

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

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

              or worse yet

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

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

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

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

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

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

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

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

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

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

              The prjquota mount option enables project quota support  on  the  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


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