Section: File Formats (5)
Updated: February 2014
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ext2 - the second extended file system
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 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.  


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.

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 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.

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.

This ext4 feature allows more than 65000 subdirectories per directory.

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.)

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 information in directory entries. This feature is supported by ext2, ext3, and ext4.

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.

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.

This ext4 feature allows files to be larger than 2 terabytes in size.

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).

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.

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.

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.

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.

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.

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.

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 file system.


mke2fs(8), mke2fs.conf(5), e2fsck(8), dumpe2fs(8), tune2fs(8), debugfs(8)




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Time: 21:46:17 GMT, January 23, 2018