Provided by: e2fsprogs_1.44.1-1_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

              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),