EXT4(5)EXT4(5)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 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.
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 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
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 sys‐
tems.
dir_nlink
This ext4 feature allows more than 65000 subdirecto‐
ries 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 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.)
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 informa‐
tion in directory entries. This feature is sup‐
ported 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 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.
has_journal
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
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 spec‐
ifying the -J device=<external-
device> option to mke2fs(8) or
tune2fs(8).
large_file
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
understand.
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 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.
mmp
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‐
ronments.
resize_inode
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
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 ALSOmke2fs(8), mke2fs.conf(5), e2fsck(8), dumpe2fs(8),
tune2fs(8), debugfs(8)E2fsprogs version 1.42.9 December 2013 EXT4(5)