ext2 man page on Archlinux

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EXT4(5)								       EXT4(5)

       ext2 - the second extended file system
       ext2 - 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  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.

       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.

			  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 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

			  This	feature	 requires  that the extent features be

			  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‐

			  This ext4 feature allows more than 65000 subdirecto‐
			  ries per directory.

			  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.)

			  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.

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

			  This feature enables the storage file type  informa‐
			  tion	in  directory  entries.	  This feature is sup‐
			  ported 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 meta‐
			  data	together  starting  at	the
			  first	  block	  group	  of   each
			  "flex_bg group".    The  size	 of
			  the  flex_bg	group can be speci‐
			  fied using the -G option.

			  Create  a   journal	to   ensure
			  filesystem	consistency    even
			  across unclean  shutdowns.   Set‐
			  ting	the  filesystem	 feature is
			  equivalent  to   using   the	 -j
			  option.   This  feature  is  sup‐
			  ported  by  ext3  and	 ext4,	and
			  ignored  by  the ext2 file system

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

			  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 spec‐
			  ifying  the  -J device=<external-
			  device> option  to  mke2fs(8)	 or

			  This feature flag is set automat‐
			  ically 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

			  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 systems which are
			  larger than 2^32 blocks.   It	 is
			  not recommended that this feature
			  be set when a file system is cre‐
			  ated, 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.

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

			  This file  system  feature  indi‐
			  cates	  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, trig‐
			  gered,   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 feature be enabled.

			  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

			  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.

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

E2fsprogs version 1.42.9	 December 2013			       EXT4(5)

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