ext4

       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  his-
       torically  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
       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 collec-
       tion of the follow 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.

                   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 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.

                   bigalloc
                          This  ext4  feature  enables clustered block alloca-
                          tion, 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 fragmenta-
                          tion,  especially  for large files.  The size can be
                          specified using the -C option.

                          Warning: The bigalloc feature is still under  devel-
                          opment,  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
                          nodelallocmountoption).

                          This  feature  requires  that the extent features be

                   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 struc-
                          ture 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  compati-
                          bility 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  informa-
                          tion  in  directory  entries.   This feature is sup-
                          ported by ext2, ext3, and ext4.

                   flex_bg
                          This ext4 feature allows the per-block  group  meta-
                          data  (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 option.

                   has_journal
                          Create a journal to  ensure  filesystem  consistency
                          even across unclean shutdowns.  Setting the filesys-
                          tem 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

                   large_file
                          This feature flag is  set  automatically  by  modern
                          kernels  when a file larger than 2 gigabytes is cre-
                          ated.  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 percent-
                          age 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 descrip-
                          tors.  This scheme is also used to resize file  sys-
                          tems  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 neces-
                          sary  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 stor-
                          age 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.

                          Warning:  The  quota feature is still under develop-
                          ment, and may not be fully supported with your  ker-
                          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  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 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.

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



E2fsprogs version 1.42.9         February 2014                         EXT4(5)
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