Adding upstream version 1.47.0.upstream/1.47.0upstream

Signed-off-by: Daniel Baumann <daniel.baumann@progress-linux.org>

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+.\" -*- 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
+ext3 \- 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
+to handle mount requests for ext2 and ext3 file systems.
+.SH 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.
+.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 mke2fs \-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
+.B nodelalloc
+mount option).
+.IP
+This feature requires that the
+.B extent
+feature be enabled.
+.TP
+.B casefold
+.br
+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.
+.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
+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.
+.TP
+.B ea_inode
+.br
+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.
+.TP
+.B encrypt
+.br
+Enables support for file-system level encryption of data blocks and file
+names.  The inode metadata (timestamps, file size, user/group ownership,
+etc.) is
+.I not
+encrypted.
+.IP
+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.
+.TP
+.B ext_attr
+.br
+This feature enables the use of extended attributes.  This feature is
+supported by ext2, ext3, and ext4.
+.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 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.
+.TP
+.B filetype
+.br
+This feature enables the storage of 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 file system consistency even across unclean
+shutdowns.  Setting the file system feature is equivalent to using the
+.B \-j
+option with
+.BR mke2fs " or " tune2fs.
+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 inline_data
+Allow data to be stored in the inode and extended attribute area.
+.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_dir
+.br
+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
+.BR dir_index ),
+the maximum height of the hashed b-tree used to store the directory entries.
+.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 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
+.B uninit_bg
+feature.  These two features are incompatible and
+.B metadata_csum
+will be used preferentially instead of
+.BR uninit_bg .
+.TP
+.B metadata_csum_seed
+.br
+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
+.B metadata_csum
+feature while it is mounted.
+.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 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.
+.TP
+.B mmp
+.br
+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.
+.TP
+.B project
+.br
+This ext4 feature provides project quota support. With this feature,
+the project ID of inode will be managed when the file system is mounted.
+.TP
+.B quota
+.br
+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.
+.IP
+Causes the quota files (i.e., user.quota and
+group.quota which existed
+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 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
+.BR resize2fs (8).
+By default
+.B 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
+.B resize
+extended option.
+.IP
+This feature requires that the
+.B sparse_super
+or
+.B sparse_super2
+feature be enabled.
+.TP
+.B sparse_super
+.br
+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.
+.TP
+.B sparse_super2
+.br
+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.
+.TP
+.B stable_inodes
+.br
+Marks the file system's inode numbers and UUID as stable.
+.BR resize2fs (8)
+will not allow shrinking a file system with this feature, nor
+will
+.BR 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
+.B encrypt
+feature still needs to be enabled separately.
+.B stable_inodes
+is a "compat" feature, so old kernels will allow it.
+.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 blocks 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.
+.TP
+.B verity
+.br
+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.
+.IP
+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.
+.SH MOUNT OPTIONS
+This section describes mount options which are specific to ext2, ext3,
+and ext4.  Other generic mount options may be used as well; see
+.BR mount (8)
+for details.
+.SH "Mount options for ext2"
+The `ext2' file system is the standard Linux file system.
+Since Linux 2.5.46, for most mount options the default
+is determined by the file system superblock. Set them with
+.BR tune2fs (8).
+.TP
+.BR acl | noacl
+Support POSIX Access Control Lists (or not).  See the
+.BR acl (5)
+manual page.
+.TP
+.BR bsddf | minixdf
+Set the behavior for the
+.I statfs
+system call. The
+.B minixdf
+behavior is to return in the
+.I f_blocks
+field the total number of blocks of the file system, while the
+.B bsddf
+behavior (which is the default) is to subtract the overhead blocks
+used by the ext2 file system and not available for file storage. Thus
+.sp 1
+% mount /k \-o minixdf; df /k; umount /k
+.TS
+tab(#);
+l2 l2 r2 l2 l2 l
+l c r c c l.
+File System#1024-blocks#Used#Available#Capacity#Mounted on
+/dev/sda6#2630655#86954#2412169#3%#/k
+.TE
+.sp 1
+% mount /k \-o bsddf; df /k; umount /k
+.TS
+tab(#);
+l2 l2 r2 l2 l2 l
+l c r c c l.
+File System#1024-blocks#Used#Available#Capacity#Mounted on
+/dev/sda6#2543714#13#2412169#0%#/k
+.TE
+.sp 1
+(Note that this example shows that one can add command line options
+to the options given in
+.IR /etc/fstab .)
+.TP
+.BR check=none " or " nocheck
+No checking is done at mount time. This is the default. This is fast.
+It is wise to invoke
+.BR e2fsck (8)
+every now and then, e.g.\& at boot time. The non-default behavior is unsupported
+(check=normal and check=strict options have been removed). Note that these mount options
+don't have to be supported if ext4 kernel driver is used for ext2 and ext3 file systems.
+.TP
+.B debug
+Print debugging info upon each (re)mount.
+.TP
+.BR errors= { continue | remount-ro | panic }
+Define the behavior when an error is encountered.
+(Either ignore errors and just mark the file system erroneous and continue,
+or remount the file system read-only, or panic and halt the system.)
+The default is set in the file system superblock, and can be
+changed using
+.BR tune2fs (8).
+.TP
+.BR grpid | bsdgroups " and " nogrpid | sysvgroups
+These options define what group id a newly created file gets.
+When
+.B grpid
+is set, it takes the group id of the directory in which it is created;
+otherwise (the default) it takes the fsgid of the current process, unless
+the directory has the setgid bit set, in which case it takes the gid
+from the parent directory, and also gets the setgid bit set
+if it is a directory itself.
+.TP
+.BR grpquota | noquota | quota | usrquota
+The usrquota (same as quota) mount option enables user quota support on the
+file system. grpquota enables group quotas support. You need the quota utilities
+to actually enable and manage the quota system.
+.TP
+.B nouid32
+Disables 32-bit UIDs and GIDs.  This is for interoperability with older
+kernels which only store and expect 16-bit values.
+.TP
+.BR oldalloc " or " orlov
+Use old allocator or Orlov allocator for new inodes. Orlov is default.
+.TP
+\fBresgid=\fP\,\fIn\fP and \fBresuid=\fP\,\fIn\fP
+The ext2 file system reserves a certain percentage of the available
+space (by default 5%, see
+.BR mke2fs (8)
+and
+.BR tune2fs (8)).
+These options determine who can use the reserved blocks.
+(Roughly: whoever has the specified uid, or belongs to the specified group.)
+.TP
+.BI sb= n
+Instead of using the normal superblock, use an alternative superblock
+specified by
+.IR n .
+This option is normally used when the primary superblock has been
+corrupted.  The location of backup superblocks is dependent on the
+file system's blocksize, the number of blocks per group, and features
+such as
+.BR sparse_super .
+.IP
+Additional backup superblocks can be determined by using the
+.B mke2fs
+program using the
+.B \-n
+option to print out where the superblocks exist, supposing
+.B mke2fs
+is supplied with arguments that are consistent with the file system's layout
+(e.g. blocksize, blocks per group,
+.BR sparse_super ,
+etc.).
+.IP
+The block number here uses 1\ k units. Thus, if you want to use logical
+block 32768 on a file system with 4\ k blocks, use "sb=131072".
+.TP
+.BR user_xattr | nouser_xattr
+Support "user." extended attributes (or not).
+
+
+.SH "Mount options for ext3"
+The ext3 file system is a version of the ext2 file system which has been
+enhanced with journaling.  It supports the same options as ext2 as
+well as the following additions:
+.TP
+.BR journal_dev=devnum / journal_path=path
+When the external journal device's major/minor numbers
+have changed, these options allow the user to specify
+the new journal location.  The journal device is
+identified either through its new major/minor numbers encoded
+in devnum, or via a path to the device.
+.TP
+.BR norecovery / noload
+Don't load the journal on mounting.  Note that
+if the file system was not unmounted cleanly,
+skipping the journal replay will lead to the
+file system containing inconsistencies that can
+lead to any number of problems.
+.TP
+.BR data= { journal | ordered | writeback }
+Specifies the journaling mode for file data.  Metadata is always journaled.
+To use modes other than
+.B ordered
+on the root file system, pass the mode to the kernel as boot parameter, e.g.\&
+.IR rootflags=data=journal .
+.RS
+.TP
+.B journal
+All data is committed into the journal prior to being written into the
+main file system.
+.TP
+.B ordered
+This is the default mode.  All data is forced directly out to the main file
+system prior to its metadata being committed to the journal.
+.TP
+.B writeback
+Data ordering is not preserved \(en data may be written into the main
+file system after its metadata has been committed to the journal.
+This is rumoured to be the highest-throughput option.  It guarantees
+internal file system integrity, however it can allow old data to appear
+in files after a crash and journal recovery.
+.RE
+.TP
+.B data_err=ignore
+Just print an error message if an error occurs in a file data buffer in
+ordered mode.
+.TP
+.B data_err=abort
+Abort the journal if an error occurs in a file data buffer in ordered mode.
+.TP
+.BR barrier=0 " / " barrier=1 "
+This disables / enables the use of write barriers in the jbd code.  barrier=0
+disables, barrier=1 enables (default). This also requires an IO stack which can
+support barriers, and if jbd gets an error on a barrier write, it will disable
+barriers again with a warning.  Write barriers enforce proper on-disk ordering
+of journal commits, making volatile disk write caches safe to use, at some
+performance penalty.  If your disks are battery-backed in one way or another,
+disabling barriers may safely improve performance.
+.TP
+.BI commit= nrsec
+Start a journal commit every
+.I nrsec
+seconds.  The default value is 5 seconds.  Zero means default.
+.TP
+.B user_xattr
+Enable Extended User Attributes. See the
+.BR attr (5)
+manual page.
+.TP
+.BR jqfmt= { vfsold | vfsv0 | vfsv1 }
+Apart from the old quota system (as in ext2, jqfmt=vfsold aka version 1 quota)
+ext3 also supports journaled quotas (version 2 quota). jqfmt=vfsv0 or
+jqfmt=vfsv1 enables journaled quotas. Journaled quotas have the advantage that
+even after a crash no quota check is required. When the
+.B quota
+file system feature is enabled, journaled quotas are used automatically, and
+this mount option is ignored.
+.TP
+.BR usrjquota=aquota.user | grpjquota=aquota.group
+For journaled quotas (jqfmt=vfsv0 or jqfmt=vfsv1), the mount options
+usrjquota=aquota.user and grpjquota=aquota.group are required to tell the
+quota system which quota database files to use. When the
+.B quota
+file system feature is enabled, journaled quotas are used automatically, and
+this mount option is ignored.
+
+.SH "Mount options for ext4"
+The ext4 file system is an advanced level of the ext3 file system which
+incorporates scalability and reliability enhancements for supporting large
+file system.
+
+The options
+.B journal_dev, journal_path, norecovery, noload, data, commit, orlov,
+.B oldalloc, [no]user_xattr, [no]acl, bsddf, minixdf, debug, errors,
+.B data_err, grpid, bsdgroups, nogrpid, sysvgroups, resgid, resuid, sb,
+.B quota, noquota, nouid32, grpquota, usrquota, usrjquota, grpjquota,
+.B and jqfmt are backwardly compatible with ext3 or ext2.
+.TP
+.B journal_checksum | nojournal_checksum
+The journal_checksum option enables checksumming of the journal transactions.
+This will allow the recovery code in e2fsck and the kernel to detect corruption
+in the kernel. It is a compatible change and will be ignored by older kernels.
+.TP
+.B journal_async_commit
+Commit block can be written to disk without waiting for descriptor blocks. If
+enabled older kernels cannot mount the device.
+This will enable 'journal_checksum' internally.
+.TP
+.BR barrier=0 " / " barrier=1 " / " barrier " / " nobarrier
+These mount options have the same effect as in ext3.  The mount options
+"barrier" and "nobarrier" are added for consistency with other ext4 mount
+options.
+
+The ext4 file system enables write barriers by default.
+.TP
+.BI inode_readahead_blks= n
+This tuning parameter controls the maximum number of inode table blocks that
+ext4's inode table readahead algorithm will pre-read into the buffer cache.
+The value must be a power of 2. The default value is 32 blocks.
+.TP
+.BI stripe= n
+Number of file system blocks that mballoc will try to use for allocation size
+and alignment. For RAID5/6 systems this should be the number of data disks *
+RAID chunk size in file system blocks.
+.TP
+.B delalloc
+Deferring block allocation until write-out time.
+.TP
+.B nodelalloc
+Disable delayed allocation. Blocks are allocated when data is copied from user
+to page cache.
+.TP
+.BI max_batch_time= usec
+Maximum amount of time ext4 should wait for additional file system operations to
+be batch together with a synchronous write operation. Since a synchronous
+write operation is going to force a commit and then a wait for the I/O
+complete, it doesn't cost much, and can be a huge throughput win, we wait for a
+small amount of time to see if any other transactions can piggyback on the
+synchronous write. The algorithm used is designed to automatically tune for
+the speed of the disk, by measuring the amount of time (on average) that it
+takes to finish committing a transaction. Call this time the "commit time".
+If the time that the transaction has been running is less than the commit time,
+ext4 will try sleeping for the commit time to see if other operations will join
+the transaction. The commit time is capped by the max_batch_time, which
+defaults to 15000\ \[mc]s (15\ ms). This optimization can be turned off entirely by
+setting max_batch_time to 0.
+.TP
+.BI min_batch_time= usec
+This parameter sets the commit time (as described above) to be at least
+min_batch_time. It defaults to zero microseconds. Increasing this parameter
+may improve the throughput of multi-threaded, synchronous workloads on very
+fast disks, at the cost of increasing latency.
+.TP
+.BI journal_ioprio= prio
+The I/O priority (from 0 to 7, where 0 is the highest priority) which should be
+used for I/O operations submitted by kjournald2 during a commit operation.
+This defaults to 3, which is a slightly higher priority than the default I/O
+priority.
+.TP
+.B abort
+Simulate the effects of calling ext4_abort() for
+debugging purposes.  This is normally used while
+remounting a file system which is already mounted.
+.TP
+.BR auto_da_alloc | noauto_da_alloc
+Many broken applications don't use fsync() when
+replacing existing files via patterns such as
+
+fd = open("foo.new")/write(fd,...)/close(fd)/ rename("foo.new", "foo")
+
+or worse yet
+
+fd = open("foo", O_TRUNC)/write(fd,...)/close(fd).
+
+If auto_da_alloc is enabled, ext4 will detect the replace-via-rename and
+replace-via-truncate patterns and force that any delayed allocation blocks are
+allocated such that at the next journal commit, in the default data=ordered
+mode, the data blocks of the new file are forced to disk before the rename()
+operation is committed.  This provides roughly the same level of guarantees as
+ext3, and avoids the "zero-length" problem that can happen when a system
+crashes before the delayed allocation blocks are forced to disk.
+.TP
+.B noinit_itable
+Do not initialize any uninitialized inode table blocks in the background. This
+feature may be used by installation CD's so that the install process can
+complete as quickly as possible; the inode table initialization process would
+then be deferred until the next time the file system is mounted.
+.TP
+.B init_itable=n
+The lazy itable init code will wait n times the number of milliseconds it took
+to zero out the previous block group's inode table. This minimizes the impact on
+system performance while the file system's inode table is being initialized.
+.TP
+.BR discard / nodiscard
+Controls whether ext4 should issue discard/TRIM commands to the underlying
+block device when blocks are freed.  This is useful for SSD devices and
+sparse/thinly-provisioned LUNs, but it is off by default until sufficient
+testing has been done.
+.TP
+.BR block_validity / noblock_validity
+This option enables/disables the in-kernel facility for tracking
+file system metadata blocks within internal data structures. This allows multi-\c
+block allocator and other routines to quickly locate extents which might
+overlap with file system metadata blocks. This option is intended for debugging
+purposes and since it negatively affects the performance, it is off by default.
+.TP
+.BR dioread_lock / dioread_nolock
+Controls whether or not ext4 should use the DIO read locking. If the
+dioread_nolock option is specified ext4 will allocate uninitialized extent
+before buffer write and convert the extent to initialized after IO completes.
+This approach allows ext4 code to avoid using inode mutex, which improves
+scalability on high speed storages. However this does not work with data
+journaling and dioread_nolock option will be ignored with kernel warning.
+Note that dioread_nolock code path is only used for extent-based files.
+Because of the restrictions this options comprises it is off by default
+(e.g.\& dioread_lock).
+.TP
+.B max_dir_size_kb=n
+This limits the size of the directories so that any attempt to expand them
+beyond the specified limit in kilobytes will cause an ENOSPC error. This is
+useful in memory-constrained environments, where a very large directory can
+cause severe performance problems or even provoke the Out Of Memory killer. (For
+example, if there is only 512\ MB memory available, a 176\ MB directory may
+seriously cramp the system's style.)
+.TP
+.B i_version
+Enable 64-bit inode version support. This option is off by default.
+.TP
+.B nombcache
+This option disables use of mbcache for extended attribute deduplication. On
+systems where extended attributes are rarely or never shared between files,
+use of mbcache for deduplication adds unnecessary computational overhead.
+.TP
+.B prjquota
+The prjquota mount option enables project quota support on the file system.
+You need the quota utilities to actually enable and manage the quota system.
+This mount option requires the
+.B project
+file system feature.
+
+.SH FILE ATTRIBUTES
+The ext2, ext3, and ext4 file systems support setting the following file
+attributes on Linux systems using the
+.BR chattr (1)
+utility:
+.sp
+.BR a " - append only"
+.sp
+.BR A " - no atime updates"
+.sp
+.BR d " - no dump"
+.sp
+.BR D " - synchronous directory updates"
+.sp
+.BR i " - immutable"
+.sp
+.BR S " - synchronous updates"
+.sp
+.BR u " - undeletable"
+.sp
+In addition, the ext3 and ext4 file systems support the following flag:
+.sp
+.BR j " - data journaling"
+.sp
+Finally, the ext4 file system also supports the following flag:
+.sp
+.BR e " - extents format"
+.sp
+For descriptions of these attribute flags, please refer to the
+.BR chattr (1)
+man page.
+.SH KERNEL SUPPORT
+This section lists the file system driver (e.g., ext2, ext3, ext4) and
+upstream kernel version where a particular file system feature was
+supported.  Note that in some cases the feature was present in earlier
+kernel versions, but there were known, serious bugs.  In other cases the
+feature may still be considered in an experimental state.  Finally, note
+that some distributions may have backported features into older kernels;
+in particular the kernel versions in certain "enterprise distributions"
+can be extremely misleading.
+.IP "\fBfiletype\fR" 2in
+ext2, 2.2.0
+.IP "\fBsparse_super\fR" 2in
+ext2, 2.2.0
+.IP "\fBlarge_file\fR" 2in
+ext2, 2.2.0
+.IP "\fBhas_journal\fR" 2in
+ext3, 2.4.15
+.IP "\fBext_attr\fR" 2in
+ext2/ext3, 2.6.0
+.IP "\fBdir_index\fR" 2in
+ext3, 2.6.0
+.IP "\fBresize_inode\fR" 2in
+ext3, 2.6.10 (online resizing)
+.IP "\fB64bit\fR" 2in
+ext4, 2.6.28
+.IP "\fBdir_nlink\fR" 2in
+ext4, 2.6.28
+.IP "\fBextent\fR" 2in
+ext4, 2.6.28
+.IP "\fBextra_isize\fR" 2in
+ext4, 2.6.28
+.IP "\fBflex_bg\fR" 2in
+ext4, 2.6.28
+.IP "\fBhuge_file\fR" 2in
+ext4, 2.6.28
+.IP "\fBmeta_bg\fR" 2in
+ext4, 2.6.28
+.IP "\fBuninit_bg\fR" 2in
+ext4, 2.6.28
+.IP "\fBmmp\fR" 2in
+ext4, 3.0
+.IP "\fBbigalloc\fR" 2in
+ext4, 3.2
+.IP "\fBquota\fR" 2in
+ext4, 3.6
+.IP "\fBinline_data\fR" 2in
+ext4, 3.8
+.IP "\fBsparse_super2\fR" 2in
+ext4, 3.16
+.IP "\fBmetadata_csum\fR" 2in
+ext4, 3.18
+.IP "\fBencrypt\fR" 2in
+ext4, 4.1
+.IP "\fBmetadata_csum_seed\fR" 2i
+ext4, 4.4
+.IP "\fBproject\fR" 2i
+ext4, 4.5
+.IP "\fBea_inode\fR" 2i
+ext4, 4.13
+.IP "\fBlarge_dir\fR" 2i
+ext4, 4.13
+.IP "\fBcasefold\fR" 2i
+ext4, 5.2
+.IP "\fBverity\fR" 2i
+ext4, 5.4
+.IP "\fBstable_inodes\fR" 2i
+ext4, 5.5
+.SH SEE ALSO
+.BR mke2fs (8),
+.BR mke2fs.conf (5),
+.BR e2fsck (8),
+.BR dumpe2fs (8),
+.BR tune2fs (8),
+.BR debugfs (8),
+.BR mount (8),
+.BR chattr (1)