EXT4(5) File Formats Manual 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
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 config-
ured 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 use-
ful 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 clus-
ter 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 sys-
tems, 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 fea-
ture, the kernel will reserve the amount of space for features currently it currently needs, and the rest may be con-
sumed 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 ker-
nels 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 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 auto-
matically 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.
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)
E2fsprogs version 1.42.9 December 2013 EXT4(5)