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EXT4(5)								       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  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
			  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  sys‐
			  tems.

		   dir_nlink
			  This ext4 feature allows more than 65000 subdirecto‐
			  ries 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 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
			  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=<external-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 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‐
			  nel	or   may   have	  various  bugs.   Please  see
			  https://ext4.wiki.kernel.org/index.php/Quota	   for
			  more details.

		   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 oper‐
			  ation	 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	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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