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   ext3    ( 5 )

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Особенности файловой системы (File system features)

A file system formatted for ext2, ext3, or ext4 can have some collection of the following 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 mke2fs -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 nodelalloc mount option).

This feature requires that the extent feature be enabled.

casefold This ext4 feature provides file system level character encoding support for directories with the casefold (+F) flag enabled. This feature is name-preserving on the disk, but it allows applications to lookup for a file in the file system using an encoding equivalent version of the file name.

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 Normally, ext4 allows an inode to have no more than 65,000 hard links. This applies to regular files as well as directories, which means that there can be no more than 64,998 subdirectories in a directory (because each of the '.' and '..' entries, as well as the directory entry for the directory in its parent directory counts as a hard link). This feature lifts this limit by causing ext4 to use a link count of 1 to indicate that the number of hard links to a directory is not known when the link count might exceed the maximum count limit.

ea_inode Normally, a file's extended attributes and associated metadata must fit within the inode or the inode's associated extended attribute block. This feature allows the value of each extended attribute to be placed in the data blocks of a separate inode if necessary, increasing the limit on the size and number of extended attributes per file.

encrypt Enables support for file-system level encryption of data blocks and file names. The inode metadata (timestamps, file size, user/group ownership, etc.) is not encrypted.

This feature is most useful on file systems with multiple users, or where not all files should be encrypted. In many use cases, especially on single-user systems, encryption at the block device layer using dm-crypt may provide much better security.

ext_attr This feature enables the use of extended attributes. This feature is supported by ext2, ext3, and ext4.

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 currently need to reserve this much space. Without this feature, the kernel will reserve the amount of space for features 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.

filetype This feature enables the storage of 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 file system consistency even across unclean shutdowns. Setting the file system feature is equivalent to using the -j option with mke2fs or tune2fs. 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.

inline_data Allow data to be stored in the inode and extended attribute area.

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_dir This feature increases the limit on the number of files per directory by raising the maximum size of directories and, for hashed b-tree directories (see dir_index), the maximum height of the hashed b-tree used to store the directory entries.

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.

metadata_csum This ext4 feature enables metadata checksumming. This feature stores checksums for all of the file system metadata (superblock, group descriptor blocks, inode and block bitmaps, directories, and extent tree blocks). The checksum algorithm used for the metadata blocks is different than the one used for group descriptors with the uninit_bg feature. These two features are incompatible and metadata_csum will be used preferentially instead of uninit_bg.

metadata_csum_seed This feature allows the file system to store the metadata checksum seed in the superblock, which allows the administrator to change the UUID of a file system using the metadata_csum feature while it is mounted.

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 descriptors 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 file system from being multiply mounted and is useful in shared storage environments.

project This ext4 feature provides project quota support. With this feature, the project ID of inode will be managed when the file system is mounted.

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 file system is mounted.

Causes the quota files (i.e., user.quota and group.quota which existed in the older quota design) to be hidden inodes.

resize_inode This file system feature indicates that space has been reserved so that the block group descriptor table can be extended while resizing a mounted file system. 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 file system may grow to 1024 times its initial size. This can be changed using the resize extended option.

This feature requires that the sparse_super or sparse_super2 feature be enabled.

sparse_super This file system feature is set on all modern ext2, ext3, and ext4 file systems. It indicates that backup copies of the superblock and block group descriptors are present only in a few block groups, not all of them.

sparse_super2 This feature indicates that there will only be at most two backup superblocks and block group descriptors. The block groups used to store the backup superblock(s) and blockgroup descriptor(s) 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 feature is essentially a more extreme version of sparse_super and is designed to allow a much larger percentage of the disk to have contiguous blocks available for data files.

stable_inodes Marks the file system's inode numbers and UUID as stable. resize2fs(8) will not allow shrinking a file system with this feature, nor will tune2fs(8) allow changing its UUID. This feature allows the use of specialized encryption settings that make use of the inode numbers and UUID. Note that the encrypt feature still needs to be enabled separately. stable_inodes is a "compat" feature, so old kernels will allow it.

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 blocks 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.

verity Enables support for verity protected files. Verity files are readonly, and their data is transparently verified against a Merkle tree hidden past the end of the file. Using the Merkle tree's root hash, a verity file can be efficiently authenticated, independent of the file's size.

This feature is most useful for authenticating important read-only files on read-write file systems. If the file system itself is read-only, then using dm-verity to authenticate the entire block device may provide much better security.