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diff --git a/misc/ext4.5.in b/misc/ext4.5.in new file mode 100644 index 00000000..134c19f9 --- /dev/null +++ b/misc/ext4.5.in @@ -0,0 +1,260 @@ +.\" -*- nroff -*- +.\" Copyright 1993, 1994, 1995 by Theodore Ts'o. All Rights Reserved. +.\" This file may be copied under the terms of the GNU Public License. +.\" +.TH EXT4 5 "@E2FSPROGS_MONTH@ @E2FSPROGS_YEAR@" "E2fsprogs version @E2FSPROGS_VERSION@" +.SH NAME +ext2 \- the second extended file system +.br +ext2 \- the third extended file system +.br +ext4 \- the fourth extended file system +.SH 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 configured +handle mount requests for ext2 and ext3 file systems. +.SH 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. +.RS 1.2i +.TP +.B 64bit +.br +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. +.TP +.B bigalloc +.br +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 +.B \-C option. +.IP +.B 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 +.BR nodelalloc mount option). +.IP +This feature requires that the +.B extent +features be enabled. +.TP +.B dir_index +.br +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. +.TP +.B dir_nlink +.br +This ext4 feature allows more than 65000 subdirectories per directory. +.TP +.B extent +.br +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 +.BR e2fsck (8) +on the file system. +(Note: both +.B extent +and +.B extents +are accepted as valid names for this feature for +historical/backwards compatibility reasons.) +.TP +.B extra_isize +.br +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. +.TP +.B ext_attr +.br +This feature enables the use of extended attributes. This feature is +supported by ext2, ext3, and ext4. +.TP +.B filetype +.br +This feature enables the storage file type information in directory +entries. This feature is supported by ext2, ext3, and ext4. +.TP +.B flex_bg +.br +This ext4 feature allows the per-block group metadata (allocation +bitmaps +and inode tables) +to be placed anywhere on the storage media. In addition, +.B 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 +.B \-G +option. +.TP +.B has_journal +.br +Create a journal to ensure filesystem consistency even across unclean +shutdowns. Setting the filesystem feature is equivalent to using the +.B \-j +option. This feature is supported by ext3 and ext4, and ignored by the +ext2 file system driver. +.TP +.B huge_file +.br +This ext4 feature allows files to be larger than 2 terabytes in size. +.TP +.B journal_dev +.br +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. +.IP +The external journal device can be used by a file system by specifying +the +.B \-J +.BR device= <external-device> +option to +.BR mke2fs (8) +or +.BR tune2fs(8). +.TP +.B large_file +.br +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 +.TP +.B sparse_super2 +.br +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. +.TP +.B meta_bg +.br +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. +.TP +.B mmp +.br +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@group.quota which existed +@QUOTA_MAN_COMMENT@in the older quota design) to be hidden inodes. +.TP +.B resize_inode +.br +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 +.BR resize2fs (8). +By default +.B 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 +.B resize +extended option. +.IP +This feature requires that the +.B sparse_super +feature be enabled. +.TP +.B sparse_super +.br +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. +.TP +.B uninit_bg +.br +This ext4 file system feature indicates that the block group descriptors +will be protected using checksums, making it safe for +.BR 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 +.BR e2fsck (8), +and it also speeds up the time required for +.BR mke2fs (8) +to create the file system. +.RE +.SH SEE ALSO +.BR mke2fs (8), +.BR mke2fs.conf (5), +.BR e2fsck (8), +.BR dumpe2fs (8), +.BR tune2fs (8), +.BR debugfs (8) |