Provided by: e2fsprogs_1.42.9-3ubuntu1_amd64 bug

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 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 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 collection 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 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 -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  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 systems.

                   dir_nlink
                          This ext4 feature allows more than 65000 subdirectories 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 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.)

                   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 information in directory
                          entries.  This feature is supported by ext2, ext3, and ext4.

                   flex_bg
                          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 option.

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

                   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 percentage 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 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 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.

                   mmp
                          This ext4 feature provides multiple mount protection (MMP).  MMP  helps
                          to  protect the filesystem from being multiply mounted and is useful in
                          shared storage 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 development, and may not be
                          fully supported with your kernel 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 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 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)