2 .\" Copyright 1993, 1994, 1995 by Theodore Ts'o. All Rights Reserved.
3 .\" This file may be copied under the terms of the GNU Public License.
5 .TH EXT4 5 "@E2FSPROGS_MONTH@ @E2FSPROGS_YEAR@" "E2fsprogs version @E2FSPROGS_VERSION@"
7 ext2 \- the second extended file system
9 ext3 \- the third extended file system
11 ext4 \- the fourth extended file system
13 The second, third, and fourth extended file systems, or ext2, ext3, and
14 ext4 as they are commonly known, are Linux file systems that have
15 historically been the default file system for many Linux distributions.
16 They are general purpose file systems that have been designed for
17 extensibility and backwards compatibility. In particular, file systems
18 previously intended for use with the ext2 and ext3 file systems can be
19 mounted using the ext4 file system driver, and indeed in many modern
20 Linux distributions, the ext4 file system driver has been configured
21 to handle mount requests for ext2 and ext3 file systems.
22 .SH FILE SYSTEM FEATURES
23 A file system formatted for ext2, ext3, or ext4 can have some
24 collection of the following file system feature flags enabled. Some of
25 these features are not supported by all implementations of the ext2,
26 ext3, and ext4 file system drivers, depending on Linux kernel version in
27 use. On other operating systems, such as the GNU/HURD or FreeBSD, only
28 a very restrictive set of file system features may be supported in their
29 implementations of ext2.
33 Enables the file system to be larger than 2^32 blocks. This feature is set
34 automatically, as needed, but it can be useful to specify this feature
35 explicitly if the file system might need to be resized larger than 2^32
36 blocks, even if it was smaller than that threshold when it was
37 originally created. Note that some older kernels and older versions
38 of e2fsprogs will not support file systems with this ext4 feature enabled.
42 This ext4 feature enables clustered block allocation, so that the unit of
43 allocation is a power of two number of blocks. That is, each bit in the
44 what had traditionally been known as the block allocation bitmap now
45 indicates whether a cluster is in use or not, where a cluster is by
46 default composed of 16 blocks. This feature can decrease the time
47 spent on doing block allocation and brings smaller fragmentation, especially
48 for large files. The size can be specified using the
53 The bigalloc feature is still under development, and may not be fully
54 supported with your kernel or may have various bugs. Please see the web
55 page http://ext4.wiki.kernel.org/index.php/Bigalloc for details.
56 May clash with delayed allocation (see
60 This feature requires that the
66 Use hashed b-trees to speed up name lookups in large directories. This
67 feature is supported by ext3 and ext4 file systems, and is ignored by
72 Normally, ext4 allows an inode to have no more than 65,000 hard links.
73 This applies to regular files as well as directories, which means that
74 there can be no more than 64,998 subdirectories in a directory (because
75 each of the '.' and '..' entries, as well as the directory entry for the
76 directory in its parent directory counts as a hard link). This feature
77 lifts this limit by causing ext4 to use a link count of 1 to indicate
78 that the number of hard links to a directory is not known when the link
79 count might exceed the maximum count limit.
83 Normally, a file's extended attributes and associated metadata must fit within
84 the inode or the inode's associated extended attribute block. This feature
85 allows the value of each extended attribute to be placed in the data blocks of a
86 separate inode if necessary, increasing the limit on the size and number of
87 extended attributes per file.
91 This ext4 feature provides file-system level encryption of data blocks
92 and file names. The inode metadata (timestamps, file size, user/group
97 This feature is most useful on file systems with multiple users, or
98 where not all files should be encrypted. In many use cases, especially
99 on single-user systems, encryption at the block device layer using
100 dm-crypt may provide much better security.
104 This feature enables the use of extended attributes. This feature is
105 supported by ext2, ext3, and ext4.
109 This ext4 feature allows the mapping of logical block numbers for a
110 particular inode to physical blocks on the storage device to be stored
111 using an extent tree, which is a more efficient data structure than the
112 traditional indirect block scheme used by the ext2 and ext3 file
113 systems. The use of the extent tree decreases metadata block overhead,
114 improves file system performance, and decreases the needed to run
121 are accepted as valid names for this feature for
122 historical/backwards compatibility reasons.)
126 This ext4 feature reserves a specific amount of space in each inode for
127 extended metadata such as nanosecond timestamps and file creation time,
128 even if the current kernel does not currently need to reserve this much
129 space. Without this feature, the kernel will reserve the amount of
130 space for features it currently needs, and the rest may be
131 consumed by extended attributes.
133 For this feature to be useful the inode size must be 256 bytes in size
138 This feature enables the storage of file type information in directory
139 entries. This feature is supported by ext2, ext3, and ext4.
143 This ext4 feature allows the per-block group metadata (allocation
146 to be placed anywhere on the storage media. In addition,
148 will place the per-block group metadata together starting at the first
149 block group of each "flex_bg group". The size of the flex_bg group
150 can be specified using the
156 Create a journal to ensure filesystem consistency even across unclean
157 shutdowns. Setting the filesystem feature is equivalent to using the
160 .BR mke2fs " or " tune2fs.
161 This feature is supported by ext3 and ext4, and ignored by the
162 ext2 file system driver.
166 This ext4 feature allows files to be larger than 2 terabytes in size.
169 Allow data to be stored in the inode and extended attribute area.
173 This feature is enabled on the superblock found on an external journal
174 device. The block size for the external journal must be the same as the
175 file system which uses it.
177 The external journal device can be used by a file system by specifying
180 .BR device= <external-device>
188 This feature increases the limit on the number of files per directory by
189 raising the maximum size of directories and, for hashed b-tree directories (see
191 the maximum height of the hashed b-tree used to store the directory entries.
195 This feature flag is set automatically by modern kernels when a file
196 larger than 2 gigabytes is created. Very old kernels could not
197 handle large files, so this feature flag was used to prohibit those
198 kernels from mounting file systems that they could not understand.
202 This ext4 feature enables metadata checksumming. This feature stores
203 checksums for all of the filesystem metadata (superblock, group
204 descriptor blocks, inode and block bitmaps, directories, and
205 extent tree blocks). The checksum algorithm used for the metadata
206 blocks is different than the one used for group descriptors with the
208 feature. These two features are incompatible and
210 will be used preferentially instead of
213 .B metadata_csum_seed
215 This feature allows the filesystem to store the metadata checksum seed in the
216 superblock, which allows the administrator to change the UUID of a filesystem
219 feature while it is mounted.
223 This ext4 feature allows file systems to be resized on-line without explicitly
224 needing to reserve space for growth in the size of the block group
225 descriptors. This scheme is also used to resize file systems which are
226 larger than 2^32 blocks. It is not recommended that this feature be set
227 when a file system is created, since this alternate method of storing
228 the block group descriptors will slow down the time needed to mount the
229 file system, and newer kernels can automatically set this feature as
230 necessary when doing an online resize and no more reserved space is
231 available in the resize inode.
235 This ext4 feature provides multiple mount protection (MMP). MMP helps to
236 protect the filesystem from being multiply mounted and is useful in
237 shared storage environments.
241 This ext4 feature provides project quota support. With this feature,
242 the project ID of inode will be managed when the filesystem is mounted.
246 Create quota inodes (inode #3 for userquota and inode
247 #4 for group quota) and set them in the superblock.
248 With this feature, the quotas will be enabled
249 automatically when the filesystem is mounted.
251 Causes the quota files (i.e., user.quota and
252 group.quota which existed
253 in the older quota design) to be hidden inodes.
257 This file system feature indicates that space has been reserved so that
258 the block group descriptor table can be extended while resizing a mounted
259 file system. The online resize operation
260 is carried out by the kernel, triggered by
264 will attempt to reserve enough space so that the
265 filesystem may grow to 1024 times its initial size. This can be changed
270 This feature requires that the
278 This file system feature is set on all modern ext2, ext3, and ext4 file
279 systems. It indicates that backup copies of the superblock and block
280 group descriptors are present only in a few block groups, not all of
285 This feature indicates that there will only be at most two backup
286 superblocks and block group descriptors. The block groups used to store
287 the backup superblock(s) and blockgroup descriptor(s) are stored in the
288 superblock, but typically, one will be located at the beginning of block
289 group #1, and one in the last block group in the file system. This
290 feature is essentially a more extreme version of sparse_super and is
291 designed to allow a much larger percentage of the disk to have
292 contiguous blocks available for data files.
296 This ext4 file system feature indicates that the block group descriptors
297 will be protected using checksums, making it safe for
299 to create a file system without initializing all of the block groups.
300 The kernel will keep a high watermark of unused inodes, and initialize
301 inode tables and blocks lazily. This feature speeds up the time to check
302 the file system using
304 and it also speeds up the time required for
306 to create the file system.
308 This section describes mount options which are specific to ext2, ext3,
309 and ext4. Other generic mount options may be used as well; see
312 .SH "Mount options for ext2"
313 The `ext2' filesystem is the standard Linux filesystem.
314 Since Linux 2.5.46, for most mount options the default
315 is determined by the filesystem superblock. Set them with
319 Support POSIX Access Control Lists (or not). See the
324 Set the behavior for the
328 behavior is to return in the
330 field the total number of blocks of the filesystem, while the
332 behavior (which is the default) is to subtract the overhead blocks
333 used by the ext2 filesystem and not available for file storage. Thus
335 % mount /k \-o minixdf; df /k; umount /k
340 Filesystem#1024-blocks#Used#Available#Capacity#Mounted on
341 /dev/sda6#2630655#86954#2412169#3%#/k
344 % mount /k \-o bsddf; df /k; umount /k
349 Filesystem#1024-blocks#Used#Available#Capacity#Mounted on
350 /dev/sda6#2543714#13#2412169#0%#/k
353 (Note that this example shows that one can add command line options
354 to the options given in
357 .BR check=none " or " nocheck
358 No checking is done at mount time. This is the default. This is fast.
361 every now and then, e.g.\& at boot time. The non-default behavior is unsupported
362 (check=normal and check=strict options have been removed). Note that these mount options
363 don't have to be supported if ext4 kernel driver is used for ext2 and ext3 filesystems.
366 Print debugging info upon each (re)mount.
368 .BR errors= { continue | remount-ro | panic }
369 Define the behavior when an error is encountered.
370 (Either ignore errors and just mark the filesystem erroneous and continue,
371 or remount the filesystem read-only, or panic and halt the system.)
372 The default is set in the filesystem superblock, and can be
376 .BR grpid | bsdgroups " and " nogrpid | sysvgroups
377 These options define what group id a newly created file gets.
380 is set, it takes the group id of the directory in which it is created;
381 otherwise (the default) it takes the fsgid of the current process, unless
382 the directory has the setgid bit set, in which case it takes the gid
383 from the parent directory, and also gets the setgid bit set
384 if it is a directory itself.
386 .BR grpquota | noquota | quota | usrquota
387 The usrquota (same as quota) mount option enables user quota support on the
388 filesystem. grpquota enables group quotas support. You need the quota utilities
389 to actually enable and manage the quota system.
392 Disables 32-bit UIDs and GIDs. This is for interoperability with older
393 kernels which only store and expect 16-bit values.
395 .BR oldalloc " or " orlov
396 Use old allocator or Orlov allocator for new inodes. Orlov is default.
398 \fBresgid=\fP\,\fIn\fP and \fBresuid=\fP\,\fIn\fP
399 The ext2 filesystem reserves a certain percentage of the available
400 space (by default 5%, see
404 These options determine who can use the reserved blocks.
405 (Roughly: whoever has the specified uid, or belongs to the specified group.)
408 Instead of using the normal superblock, use an alternative superblock
411 This option is normally used when the primary superblock has been
412 corrupted. The location of backup superblocks is dependent on the
413 filesystem's blocksize, the number of blocks per group, and features
417 Additional backup superblocks can be determined by using the
421 option to print out where the superblocks exist, supposing
423 is supplied with arguments that are consistent with the filesystem's layout
424 (e.g. blocksize, blocks per group,
428 The block number here uses 1\ k units. Thus, if you want to use logical
429 block 32768 on a filesystem with 4\ k blocks, use "sb=131072".
431 .BR user_xattr | nouser_xattr
432 Support "user." extended attributes (or not).
435 .SH "Mount options for ext3"
436 The ext3 filesystem is a version of the ext2 filesystem which has been
437 enhanced with journaling. It supports the same options as ext2 as
438 well as the following additions:
440 .BR journal_dev=devnum / journal_path=path
441 When the external journal device's major/minor numbers
442 have changed, these options allow the user to specify
443 the new journal location. The journal device is
444 identified either through its new major/minor numbers encoded
445 in devnum, or via a path to the device.
447 .BR norecovery / noload
448 Don't load the journal on mounting. Note that
449 if the filesystem was not unmounted cleanly,
450 skipping the journal replay will lead to the
451 filesystem containing inconsistencies that can
452 lead to any number of problems.
454 .BR data= { journal | ordered | writeback }
455 Specifies the journaling mode for file data. Metadata is always journaled.
456 To use modes other than
458 on the root filesystem, pass the mode to the kernel as boot parameter, e.g.\&
459 .IR rootflags=data=journal .
463 All data is committed into the journal prior to being written into the
467 This is the default mode. All data is forced directly out to the main file
468 system prior to its metadata being committed to the journal.
471 Data ordering is not preserved \(en data may be written into the main
472 filesystem after its metadata has been committed to the journal.
473 This is rumoured to be the highest-throughput option. It guarantees
474 internal filesystem integrity, however it can allow old data to appear
475 in files after a crash and journal recovery.
479 Just print an error message if an error occurs in a file data buffer in
483 Abort the journal if an error occurs in a file data buffer in ordered mode.
485 .BR barrier=0 " / " barrier=1 "
486 This disables / enables the use of write barriers in the jbd code. barrier=0
487 disables, barrier=1 enables (default). This also requires an IO stack which can
488 support barriers, and if jbd gets an error on a barrier write, it will disable
489 barriers again with a warning. Write barriers enforce proper on-disk ordering
490 of journal commits, making volatile disk write caches safe to use, at some
491 performance penalty. If your disks are battery-backed in one way or another,
492 disabling barriers may safely improve performance.
495 Start a journal commit every
497 seconds. The default value is 5 seconds. Zero means default.
500 Enable Extended User Attributes. See the
504 .BR jqfmt= { vfsold | vfsv0 | vfsv1 }
505 Apart from the old quota system (as in ext2, jqfmt=vfsold aka version 1 quota)
506 ext3 also supports journaled quotas (version 2 quota). jqfmt=vfsv0 or
507 jqfmt=vfsv1 enables journaled quotas. Journaled quotas have the advantage that
508 even after a crash no quota check is required. When the
510 filesystem feature is enabled, journaled quotas are used automatically, and
511 this mount option is ignored.
513 .BR usrjquota=aquota.user | grpjquota=aquota.group
514 For journaled quotas (jqfmt=vfsv0 or jqfmt=vfsv1), the mount options
515 usrjquota=aquota.user and grpjquota=aquota.group are required to tell the
516 quota system which quota database files to use. When the
518 filesystem feature is enabled, journaled quotas are used automatically, and
519 this mount option is ignored.
521 .SH "Mount options for ext4"
522 The ext4 filesystem is an advanced level of the ext3 filesystem which
523 incorporates scalability and reliability enhancements for supporting large
527 .B journal_dev, journal_path, norecovery, noload, data, commit, orlov,
528 .B oldalloc, [no]user_xattr, [no]acl, bsddf, minixdf, debug, errors,
529 .B data_err, grpid, bsdgroups, nogrpid, sysvgroups, resgid, resuid, sb,
530 .B quota, noquota, nouid32, grpquota, usrquota, usrjquota, grpjquota,
531 .B and jqfmt are backwardly compatible with ext3 or ext2.
533 .B journal_checksum | nojournal_checksum
534 The journal_checksum option enables checksumming of the journal transactions.
535 This will allow the recovery code in e2fsck and the kernel to detect corruption
536 in the kernel. It is a compatible change and will be ignored by older kernels.
538 .B journal_async_commit
539 Commit block can be written to disk without waiting for descriptor blocks. If
540 enabled older kernels cannot mount the device.
541 This will enable 'journal_checksum' internally.
543 .BR barrier=0 " / " barrier=1 " / " barrier " / " nobarrier
544 These mount options have the same effect as in ext3. The mount options
545 "barrier" and "nobarrier" are added for consistency with other ext4 mount
548 The ext4 filesystem enables write barriers by default.
550 .BI inode_readahead_blks= n
551 This tuning parameter controls the maximum number of inode table blocks that
552 ext4's inode table readahead algorithm will pre-read into the buffer cache.
553 The value must be a power of 2. The default value is 32 blocks.
556 Number of filesystem blocks that mballoc will try to use for allocation size
557 and alignment. For RAID5/6 systems this should be the number of data disks *
558 RAID chunk size in filesystem blocks.
561 Deferring block allocation until write-out time.
564 Disable delayed allocation. Blocks are allocated when data is copied from user
567 .BI max_batch_time= usec
568 Maximum amount of time ext4 should wait for additional filesystem operations to
569 be batch together with a synchronous write operation. Since a synchronous
570 write operation is going to force a commit and then a wait for the I/O
571 complete, it doesn't cost much, and can be a huge throughput win, we wait for a
572 small amount of time to see if any other transactions can piggyback on the
573 synchronous write. The algorithm used is designed to automatically tune for
574 the speed of the disk, by measuring the amount of time (on average) that it
575 takes to finish committing a transaction. Call this time the "commit time".
576 If the time that the transaction has been running is less than the commit time,
577 ext4 will try sleeping for the commit time to see if other operations will join
578 the transaction. The commit time is capped by the max_batch_time, which
579 defaults to 15000\ \[mc]s (15\ ms). This optimization can be turned off entirely by
580 setting max_batch_time to 0.
582 .BI min_batch_time= usec
583 This parameter sets the commit time (as described above) to be at least
584 min_batch_time. It defaults to zero microseconds. Increasing this parameter
585 may improve the throughput of multi-threaded, synchronous workloads on very
586 fast disks, at the cost of increasing latency.
588 .BI journal_ioprio= prio
589 The I/O priority (from 0 to 7, where 0 is the highest priority) which should be
590 used for I/O operations submitted by kjournald2 during a commit operation.
591 This defaults to 3, which is a slightly higher priority than the default I/O
595 Simulate the effects of calling ext4_abort() for
596 debugging purposes. This is normally used while
597 remounting a filesystem which is already mounted.
599 .BR auto_da_alloc | noauto_da_alloc
600 Many broken applications don't use fsync() when
601 replacing existing files via patterns such as
603 fd = open("foo.new")/write(fd,...)/close(fd)/ rename("foo.new", "foo")
607 fd = open("foo", O_TRUNC)/write(fd,...)/close(fd).
609 If auto_da_alloc is enabled, ext4 will detect the replace-via-rename and
610 replace-via-truncate patterns and force that any delayed allocation blocks are
611 allocated such that at the next journal commit, in the default data=ordered
612 mode, the data blocks of the new file are forced to disk before the rename()
613 operation is committed. This provides roughly the same level of guarantees as
614 ext3, and avoids the "zero-length" problem that can happen when a system
615 crashes before the delayed allocation blocks are forced to disk.
618 Do not initialize any uninitialized inode table blocks in the background. This
619 feature may be used by installation CD's so that the install process can
620 complete as quickly as possible; the inode table initialization process would
621 then be deferred until the next time the filesystem is mounted.
624 The lazy itable init code will wait n times the number of milliseconds it took
625 to zero out the previous block group's inode table. This minimizes the impact on
626 system performance while the filesystem's inode table is being initialized.
628 .BR discard / nodiscard
629 Controls whether ext4 should issue discard/TRIM commands to the underlying
630 block device when blocks are freed. This is useful for SSD devices and
631 sparse/thinly-provisioned LUNs, but it is off by default until sufficient
632 testing has been done.
634 .BR block_validity / noblock_validity
635 This option enables/disables the in-kernel facility for tracking
636 filesystem metadata blocks within internal data structures. This allows multi-\c
637 block allocator and other routines to quickly locate extents which might
638 overlap with filesystem metadata blocks. This option is intended for debugging
639 purposes and since it negatively affects the performance, it is off by default.
641 .BR dioread_lock / dioread_nolock
642 Controls whether or not ext4 should use the DIO read locking. If the
643 dioread_nolock option is specified ext4 will allocate uninitialized extent
644 before buffer write and convert the extent to initialized after IO completes.
645 This approach allows ext4 code to avoid using inode mutex, which improves
646 scalability on high speed storages. However this does not work with data
647 journaling and dioread_nolock option will be ignored with kernel warning.
648 Note that dioread_nolock code path is only used for extent-based files.
649 Because of the restrictions this options comprises it is off by default
650 (e.g.\& dioread_lock).
653 This limits the size of the directories so that any attempt to expand them
654 beyond the specified limit in kilobytes will cause an ENOSPC error. This is
655 useful in memory-constrained environments, where a very large directory can
656 cause severe performance problems or even provoke the Out Of Memory killer. (For
657 example, if there is only 512\ MB memory available, a 176\ MB directory may
658 seriously cramp the system's style.)
661 Enable 64-bit inode version support. This option is off by default.
664 This option disables use of mbcache for extended attribute deduplication. On
665 systems where extended attributes are rarely or never shared between files,
666 use of mbcache for deduplication adds unnecessary computational overhead.
669 The prjquota mount option enables project quota support on the filesystem.
670 You need the quota utilities to actually enable and manage the quota system.
671 This mount option requires the
676 The ext2, ext3, and ext4 filesystems support setting the following file
677 attributes on Linux systems using the
681 .BR a " - append only"
683 .BR A " - no atime updates"
687 .BR D " - synchronous directory updates"
691 .BR S " - synchronous updates"
693 .BR u " - undeletable"
695 In addition, the ext3 and ext4 filesystems support the following flag:
697 .BR j " - data journaling"
699 Finally, the ext4 filesystem also supports the following flag:
701 .BR e " - extents format"
703 For descriptions of these attribute flags, please refer to the
707 This section lists the file system driver (e.g., ext2, ext3, ext4) and
708 upstream kernel version where a particular file system feature was
709 supported. Note that in some cases the feature was present in earlier
710 kernel versions, but there were known, serious bugs. In other cases the
711 feature may still be considered in an experimental state. Finally, note
712 that some distributions may have backported features into older kernels;
713 in particular the kernel versions in certain "enterprise distributions"
714 can be extremely misleading.
715 .IP "\fBfiletype\fR" 2in
717 .IP "\fBsparse_super\fR" 2in
719 .IP "\fBlarge_file\fR" 2in
721 .IP "\fBhas_journal\fR" 2in
723 .IP "\fBext_attr\fR" 2in
725 .IP "\fBdir_index\fR" 2in
727 .IP "\fBresize_inode\fR" 2in
728 ext3, 2.6.10 (online resizing)
729 .IP "\fB64bit\fR" 2in
731 .IP "\fBdir_nlink\fR" 2in
733 .IP "\fBextent\fR" 2in
735 .IP "\fBextra_isize\fR" 2in
737 .IP "\fBflex_bg\fR" 2in
739 .IP "\fBhuge_file\fR" 2in
741 .IP "\fBmeta_bg\fR" 2in
743 .IP "\fBuninit_bg\fR" 2in
747 .IP "\fBbigalloc\fR" 2in
749 .IP "\fBquota\fR" 2in
751 .IP "\fBinline_data\fR" 2in
753 .IP "\fBsparse_super2\fR" 2in
755 .IP "\fBmetadata_csum\fR" 2in
757 .IP "\fBencrypt\fR" 2in
759 .IP "\fBmetadata_csum_seed\fR" 2in
761 .IP "\fBproject\fR" 2in
763 .IP "\fBea_inode\fR" 2in
765 .IP "\fBlarge_dir\fR" 2in
projects / thirdparty / e2fsprogs.git / blob