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.
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.
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
for details. May clash with delayed allocation (see
This feature requires that the extent features be
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-
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 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.
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.
This ext4 feature allows files to be larger than 2
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.
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.
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.
This ext4 feature provides multiple mount protection
(MMP). MMP helps to protect the filesystem from
being multiply mounted and is useful in shared stor-
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
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-
to 1024 times its initial size. This can be changed
using the resize extended option.
This feature requires that the sparse_super feature
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.
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
mke2fs(8), mke2fs.conf(5), e2fsck(8), dumpe2fs(8), tune2fs(8),
E2fsprogs version 1.42.9 February 2014 EXT4(5)
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