Provided by: e2fsprogs_1.42.9-3ubuntu1.3_amd64 
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)