Copyright 1993, 1994, 1995 by Theodore Ts'o. All Rights Reserved.
This file may be copied under the terms of the GNU Public License.
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 -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 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 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 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 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 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.
.TP
.B metadata_csum
.br
This ext4 feature enables metadata checksumming. This feature stores
checksums for all of the filesystem 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
.B uninit_bg
feature, these two features are incompatible and
.B metadata_csum
will be used preferentially instead of
.BR uninit_bg .
.br
.B Future feature, available in e2fsprogs 1.43-WIP
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 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 filesystem from being multiply mounted and is useful in shared storage environments. @QUOTA_MAN_COMMENT@.TP @QUOTA_MAN_COMMENT@.B quota @QUOTA_MAN_COMMENT@.br @QUOTA_MAN_COMMENT@Create quota inodes (inode #3 for userquota and inode @QUOTA_MAN_COMMENT@#4 for group quota) and set them in the superblock. @QUOTA_MAN_COMMENT@With this feature, the quotas will be enabled @QUOTA_MAN_COMMENT@automatically when the filesystem is mounted. @QUOTA_MAN_COMMENT@.IP @QUOTA_MAN_COMMENT@Causes the quota files (i.e., user.quota and @QUOTA_MAN_COMMENT (at] group.quota which existed @QUOTA_MAN_COMMENT@in the older quota design) to be hidden inodes. @QUOTA_MAN_COMMENT@.IP @QUOTA_MAN_COMMENT@.B Warning: @QUOTA_MAN_COMMENT@The quota feature is still under development, @QUOTA_MAN_COMMENT@and may not be fully supported with your kernel @QUOTA_MAN_COMMENT@or may have various bugs. Please @QUOTA_MAN_COMMENT@see https://ext4.wiki.kernel.org/index.php/Quota @QUOTA_MAN_COMMENT@for more details.
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.
