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)