mirror of https://github.com/vitalif/e2fsprogs
264 lines
9.6 KiB
Groff
264 lines
9.6 KiB
Groff
.\" -*- nroff -*-
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.\" Copyright 1993, 1994, 1995 by Theodore Ts'o. All Rights Reserved.
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.\" This file may be copied under the terms of the GNU Public License.
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.\"
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.TH EXT4 5 "@E2FSPROGS_MONTH@ @E2FSPROGS_YEAR@" "E2fsprogs version @E2FSPROGS_VERSION@"
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.SH NAME
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ext2 \- the second extended file system
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.br
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ext2 \- the third extended file system
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.br
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ext4 \- the fourth extended file system
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.SH DESCRIPTION
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The second, third, and fourth extended file systems, or ext2, ext3, and
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ext4 as they are commonly known, are Linux file systems that have
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historically been the default file system for many Linux distributions.
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They are general purpose file systems that have been designed for
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extensibility and backwards compatibility. In particular, file systems
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previously intended for use with the ext2 and ext3 file systems can be
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mounted using the ext4 file system driver, and indeed in many modern
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Linux distributions, the ext4 file system driver has been configured
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handle mount requests for ext2 and ext3 file systems.
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.SH FILE SYSTEM FEATURES
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A file system formated for ext2, ext3, or ext4 can be have some
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collection of the follow file system feature flags enabled. Some of
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these features are not supported by all implementations of the ext2,
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ext3, and ext4 file system drivers, depending on Linux kernel version in
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use. On other operating systems, such as the GNU/HURD or FreeBSD, only
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a very restrictive set of file system features may be supported in their
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implementations of ext2.
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.RS 1.2i
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.TP
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.B 64bit
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.br
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Enables the file system to be larger than 2^32 blocks. This feature is set
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automatically, as needed, but it can be useful to specify this feature
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explicitly if the file system might need to be resized larger than 2^32
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blocks, even if it was smaller than that threshold when it was
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originally created. Note that some older kernels and older versions
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of e2fsprogs will not support file systems with this ext4 feature enabled.
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.TP
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.B bigalloc
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.br
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This ext4 feature enables clustered block allocation, so that the unit of
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allocation is a power of two number of blocks. That is, each bit in the
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what had traditionally been known as the block allocation bitmap now
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indicates whether a cluster is in use or not, where a cluster is by
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default composed of 16 blocks. This feature can decrease the time
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spent on doing block allocation and brings smaller fragmentation, especially
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for large files. The size can be specified using the
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.B \-C option.
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.IP
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.B Warning:
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The bigalloc feature is still under development, and may not be fully
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supported with your kernel or may have various bugs. Please see the web
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page http://ext4.wiki.kernel.org/index.php/Bigalloc for details.
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May clash with delayed allocation (see
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.BR nodelalloc mount option).
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.IP
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This feature requires that the
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.B extent
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features be enabled.
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.TP
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.B dir_index
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.br
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Use hashed b-trees to speed up name lookups in large directories. This
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feature is supported by ext3 and ext4 file systems, and is ignored by
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ext2 file systems.
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.TP
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.B dir_nlink
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.br
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This ext4 feature allows more than 65000 subdirectories per directory.
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.TP
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.B extent
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.br
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This ext4 feature allows the mapping of logical block numbers for a
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particular inode to physical blocks on the storage device to be stored
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using an extent tree, which is a more efficient data structure than the
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traditional indirect block scheme used by the ext2 and ext3 file
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systems. The use of the extent tree decreases metadata block overhead,
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improves file system performance, and decreases the needed to run
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.BR e2fsck (8)
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on the file system.
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(Note: both
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.B extent
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and
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.B extents
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are accepted as valid names for this feature for
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historical/backwards compatibility reasons.)
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.TP
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.B extra_isize
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.br
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This ext4 feature reserves a specific amount of space in each inode for
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extended metadata such as nanosecond timestamps and file creation time,
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even if the current kernel does not current need to reserve this much
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space. Without this feature, the kernel will reserve the amount of
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space for features currently it currently needs, and the rest may be
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consumed by extended attributes.
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For this feature to be useful the inode size must be 256 bytes in size
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or larger.
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.TP
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.B ext_attr
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.br
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This feature enables the use of extended attributes. This feature is
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supported by ext2, ext3, and ext4.
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.TP
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.B filetype
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.br
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This feature enables the storage file type information in directory
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entries. This feature is supported by ext2, ext3, and ext4.
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.TP
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.B flex_bg
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.br
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This ext4 feature allows the per-block group metadata (allocation
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bitmaps
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and inode tables)
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to be placed anywhere on the storage media. In addition,
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.B mke2fs
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will place the per-block group metadata together starting at the first
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block group of each "flex_bg group". The size of the flex_bg group
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can be specified using the
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.B \-G
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option.
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.TP
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.B has_journal
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.br
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Create a journal to ensure filesystem consistency even across unclean
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shutdowns. Setting the filesystem feature is equivalent to using the
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.B \-j
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option. This feature is supported by ext3 and ext4, and ignored by the
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ext2 file system driver.
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.TP
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.B huge_file
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.br
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This ext4 feature allows files to be larger than 2 terabytes in size.
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.TP
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.B journal_dev
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.br
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This feature is enabled on the superblock found on an external journal
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device. The block size for the external journal must be the same as the
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file system which uses it.
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.IP
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The external journal device can be used by a file system by specifying
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the
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.B \-J
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.BR device= <external-device>
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option to
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.BR mke2fs (8)
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or
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.BR tune2fs(8).
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.TP
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.B inline_data
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Allow data to be stored in the inode and extended attribute area
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.TP
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.B large_file
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.br
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This feature flag is set automatically by modern kernels when a file
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larger than 2 gigabytes is created. Very old kernels could not
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handle large files, so this feature flag was used to prohibit those
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kernels from mounting file systems that they could not understand.
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.\" .TP
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.\" .B metadata_csum
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.\" .br
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.\" This ext4 feature enables metadata checksumming. This feature stores
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.\" checksums for all of the filesystem metadata (superblock, group
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.\" descriptor blocks, inode and block bitmaps, directories, and
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.\" extent tree blocks). The checksum algorithm used for the metadata
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.\" blocks is different than the one used for group descriptors with the
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.\" .B uninit_bg
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.\" feature, these two features are incompatible and
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.\" .B metadata_csum
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.\" will be used preferentially instead of
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.\" .BR uninit_bg .
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.\" .br
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.\" .B Future feature, available in e2fsprogs 1.43-WIP
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.TP
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.B sparse_super2
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.br
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This feature indicates that there will only at most two backup
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superblock and block group descriptors. The block groups used to store
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the backup superblock and blockgroup descriptors are stored in the
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superblock, but typically, one will be located at the beginning of block
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group #1, and one in the last block group in the file system. This is
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feature is essentially a more extreme version of sparse_super and is
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designed to allow the a much larger percentage of the disk to have
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contiguous blocks available for data files.
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.TP
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.B meta_bg
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.br
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This ext4 feature allows file systems to be resized on-line without explicitly
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needing to reserve space for growth in the size of the block group
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descriptors. This scheme is also used to resize file systems which are
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larger than 2^32 blocks. It is not recommended that this feature be set
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when a file system is created, since this alternate method of storing
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the block group descriptor will slow down the time needed to mount the
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file system, and newer kernels can automatically set this feature as
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necessary when doing an online resize and no more reserved space is
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available in the resize inode.
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.TP
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.B mmp
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.br
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This ext4 feature provides multiple mount protection (MMP). MMP helps to
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protect the filesystem from being multiply mounted and is useful in
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shared storage environments.
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@QUOTA_MAN_COMMENT@.TP
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@QUOTA_MAN_COMMENT@.B quota
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@QUOTA_MAN_COMMENT@.br
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@QUOTA_MAN_COMMENT@Create quota inodes (inode #3 for userquota and inode
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@QUOTA_MAN_COMMENT@#4 for group quota) and set them in the superblock.
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@QUOTA_MAN_COMMENT@With this feature, the quotas will be enabled
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@QUOTA_MAN_COMMENT@automatically when the filesystem is mounted.
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@QUOTA_MAN_COMMENT@.IP
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@QUOTA_MAN_COMMENT@Causes the quota files (i.e., user.quota and
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@QUOTA_MAN_COMMENT@group.quota which existed
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@QUOTA_MAN_COMMENT@in the older quota design) to be hidden inodes.
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.TP
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.B resize_inode
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.br
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This file system feature indicates that space has been reserved so
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the block group descriptor table can be extended by the file system is
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resized while the file system is mounted. The online resize operation
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is carried out by the kernel, triggered, by
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.BR resize2fs (8).
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By default
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.B mke2fs
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will attempt to reserve enough space so that the
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filesystem may grow to 1024 times its initial size. This can be changed
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using the
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.B resize
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extended option.
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.IP
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This feature requires that the
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.B sparse_super
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feature be enabled.
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.TP
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.B sparse_super
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.br
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This file system feature is set on all modern ext2, ext3, and ext4 file
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system. It indicates that backup copies of the superblock and block
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group descriptors be present only on a few block groups, and not all of
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them.
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.TP
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.B uninit_bg
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.br
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This ext4 file system feature indicates that the block group descriptors
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will be protected using checksums, making it safe for
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.BR mke2fs (8)
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to create a file system without initializing all of the block groups.
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The kernel will keep a high watermark of unused inodes, and initialize
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inode tables and block lazily. This feature speeds up the time to check
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the file system using
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.BR e2fsck (8),
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and it also speeds up the time required for
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.BR mke2fs (8)
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to create the file system.
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.RE
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.SH SEE ALSO
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.BR mke2fs (8),
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.BR mke2fs.conf (5),
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.BR e2fsck (8),
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.BR dumpe2fs (8),
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.BR tune2fs (8),
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.BR debugfs (8)
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