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1.. SPDX-License-Identifier: GPL-2.0
2
3========================
4ext4 General Information
5========================
6
7Ext4 is an advanced level of the ext3 filesystem which incorporates
8scalability and reliability enhancements for supporting large filesystems
9(64 bit) in keeping with increasing disk capacities and state-of-the-art
10feature requirements.
11
12Mailing list: linux-ext4@vger.kernel.org
13Web site: http://ext4.wiki.kernel.org
14
15
16Quick usage instructions
17========================
18
19Note: More extensive information for getting started with ext4 can be
20found at the ext4 wiki site at the URL:
21http://ext4.wiki.kernel.org/index.php/Ext4_Howto
22
23 - The latest version of e2fsprogs can be found at:
24
25 https://www.kernel.org/pub/linux/kernel/people/tytso/e2fsprogs/
26
27 or
28
29 http://sourceforge.net/project/showfiles.php?group_id=2406
30
31 or grab the latest git repository from:
32
33 https://git.kernel.org/pub/scm/fs/ext2/e2fsprogs.git
34
35 - Create a new filesystem using the ext4 filesystem type:
36
37 # mke2fs -t ext4 /dev/hda1
38
39 Or to configure an existing ext3 filesystem to support extents:
40
41 # tune2fs -O extents /dev/hda1
42
43 If the filesystem was created with 128 byte inodes, it can be
44 converted to use 256 byte for greater efficiency via:
45
46 # tune2fs -I 256 /dev/hda1
47
48 - Mounting:
49
50 # mount -t ext4 /dev/hda1 /wherever
51
52 - When comparing performance with other filesystems, it's always
53 important to try multiple workloads; very often a subtle change in a
54 workload parameter can completely change the ranking of which
55 filesystems do well compared to others. When comparing versus ext3,
56 note that ext4 enables write barriers by default, while ext3 does
57 not enable write barriers by default. So it is useful to use
58 explicitly specify whether barriers are enabled or not when via the
59 '-o barriers=[0|1]' mount option for both ext3 and ext4 filesystems
60 for a fair comparison. When tuning ext3 for best benchmark numbers,
61 it is often worthwhile to try changing the data journaling mode; '-o
62 data=writeback' can be faster for some workloads. (Note however that
63 running mounted with data=writeback can potentially leave stale data
64 exposed in recently written files in case of an unclean shutdown,
65 which could be a security exposure in some situations.) Configuring
66 the filesystem with a large journal can also be helpful for
67 metadata-intensive workloads.
68
69Features
70========
71
72Currently Available
73-------------------
74
75* ability to use filesystems > 16TB (e2fsprogs support not available yet)
76* extent format reduces metadata overhead (RAM, IO for access, transactions)
77* extent format more robust in face of on-disk corruption due to magics,
78* internal redundancy in tree
79* improved file allocation (multi-block alloc)
80* lift 32000 subdirectory limit imposed by i_links_count[1]
81* nsec timestamps for mtime, atime, ctime, create time
82* inode version field on disk (NFSv4, Lustre)
83* reduced e2fsck time via uninit_bg feature
84* journal checksumming for robustness, performance
85* persistent file preallocation (e.g for streaming media, databases)
86* ability to pack bitmaps and inode tables into larger virtual groups via the
87 flex_bg feature
88* large file support
89* inode allocation using large virtual block groups via flex_bg
90* delayed allocation
91* large block (up to pagesize) support
92* efficient new ordered mode in JBD2 and ext4 (avoid using buffer head to force
93 the ordering)
94* Case-insensitive file name lookups
95
96[1] Filesystems with a block size of 1k may see a limit imposed by the
97directory hash tree having a maximum depth of two.
98
99case-insensitive file name lookups
100======================================================
101
102The case-insensitive file name lookup feature is supported on a
103per-directory basis, allowing the user to mix case-insensitive and
104case-sensitive directories in the same filesystem. It is enabled by
105flipping the +F inode attribute of an empty directory. The
106case-insensitive string match operation is only defined when we know how
107text in encoded in a byte sequence. For that reason, in order to enable
108case-insensitive directories, the filesystem must have the
109casefold feature, which stores the filesystem-wide encoding
110model used. By default, the charset adopted is the latest version of
111Unicode (12.1.0, by the time of this writing), encoded in the UTF-8
112form. The comparison algorithm is implemented by normalizing the
113strings to the Canonical decomposition form, as defined by Unicode,
114followed by a byte per byte comparison.
115
116The case-awareness is name-preserving on the disk, meaning that the file
117name provided by userspace is a byte-per-byte match to what is actually
118written in the disk. The Unicode normalization format used by the
119kernel is thus an internal representation, and not exposed to the
120userspace nor to the disk, with the important exception of disk hashes,
121used on large case-insensitive directories with DX feature. On DX
122directories, the hash must be calculated using the casefolded version of
123the filename, meaning that the normalization format used actually has an
124impact on where the directory entry is stored.
125
126When we change from viewing filenames as opaque byte sequences to seeing
127them as encoded strings we need to address what happens when a program
128tries to create a file with an invalid name. The Unicode subsystem
129within the kernel leaves the decision of what to do in this case to the
130filesystem, which select its preferred behavior by enabling/disabling
131the strict mode. When Ext4 encounters one of those strings and the
132filesystem did not require strict mode, it falls back to considering the
133entire string as an opaque byte sequence, which still allows the user to
134operate on that file, but the case-insensitive lookups won't work.
135
136Options
137=======
138
139When mounting an ext4 filesystem, the following option are accepted:
140(*) == default
141
142 ro
143 Mount filesystem read only. Note that ext4 will replay the journal (and
144 thus write to the partition) even when mounted "read only". The mount
145 options "ro,noload" can be used to prevent writes to the filesystem.
146
147 journal_checksum
148 Enable checksumming of the journal transactions. This will allow the
149 recovery code in e2fsck and the kernel to detect corruption in the
150 kernel. It is a compatible change and will be ignored by older
151 kernels.
152
153 journal_async_commit
154 Commit block can be written to disk without waiting for descriptor
155 blocks. If enabled older kernels cannot mount the device. This will
156 enable 'journal_checksum' internally.
157
158 journal_path=path, journal_dev=devnum
159 When the external journal device's major/minor numbers have changed,
160 these options allow the user to specify the new journal location. The
161 journal device is identified through either its new major/minor numbers
162 encoded in devnum, or via a path to the device.
163
164 norecovery, noload
165 Don't load the journal on mounting. Note that if the filesystem was
166 not unmounted cleanly, skipping the journal replay will lead to the
167 filesystem containing inconsistencies that can lead to any number of
168 problems.
169
170 data=journal
171 All data are committed into the journal prior to being written into the
172 main file system. Enabling this mode will disable delayed allocation
173 and O_DIRECT support.
174
175 data=ordered (*)
176 All data are forced directly out to the main file system prior to its
177 metadata being committed to the journal.
178
179 data=writeback
180 Data ordering is not preserved, data may be written into the main file
181 system after its metadata has been committed to the journal.
182
183 commit=nrsec (*)
184 Ext4 can be told to sync all its data and metadata every 'nrsec'
185 seconds. The default value is 5 seconds. This means that if you lose
186 your power, you will lose as much as the latest 5 seconds of work (your
187 filesystem will not be damaged though, thanks to the journaling). This
188 default value (or any low value) will hurt performance, but it's good
189 for data-safety. Setting it to 0 will have the same effect as leaving
190 it at the default (5 seconds). Setting it to very large values will
191 improve performance.
192
193 barrier=<0|1(*)>, barrier(*), nobarrier
194 This enables/disables the use of write barriers in the jbd code.
195 barrier=0 disables, barrier=1 enables. This also requires an IO stack
196 which can support barriers, and if jbd gets an error on a barrier
197 write, it will disable again with a warning. Write barriers enforce
198 proper on-disk ordering of journal commits, making volatile disk write
199 caches safe to use, at some performance penalty. If your disks are
200 battery-backed in one way or another, disabling barriers may safely
201 improve performance. The mount options "barrier" and "nobarrier" can
202 also be used to enable or disable barriers, for consistency with other
203 ext4 mount options.
204
205 inode_readahead_blks=n
206 This tuning parameter controls the maximum number of inode table blocks
207 that ext4's inode table readahead algorithm will pre-read into the
208 buffer cache. The default value is 32 blocks.
209
210 nouser_xattr
211 Disables Extended User Attributes. See the attr(5) manual page for
212 more information about extended attributes.
213
214 noacl
215 This option disables POSIX Access Control List support. If ACL support
216 is enabled in the kernel configuration (CONFIG_EXT4_FS_POSIX_ACL), ACL
217 is enabled by default on mount. See the acl(5) manual page for more
218 information about acl.
219
220 bsddf (*)
221 Make 'df' act like BSD.
222
223 minixdf
224 Make 'df' act like Minix.
225
226 debug
227 Extra debugging information is sent to syslog.
228
229 abort
230 Simulate the effects of calling ext4_abort() for debugging purposes.
231 This is normally used while remounting a filesystem which is already
232 mounted.
233
234 errors=remount-ro
235 Remount the filesystem read-only on an error.
236
237 errors=continue
238 Keep going on a filesystem error.
239
240 errors=panic
241 Panic and halt the machine if an error occurs. (These mount options
242 override the errors behavior specified in the superblock, which can be
243 configured using tune2fs)
244
245 data_err=ignore(*)
246 Just print an error message if an error occurs in a file data buffer in
247 ordered mode.
248 data_err=abort
249 Abort the journal if an error occurs in a file data buffer in ordered
250 mode.
251
252 grpid | bsdgroups
253 New objects have the group ID of their parent.
254
255 nogrpid (*) | sysvgroups
256 New objects have the group ID of their creator.
257
258 resgid=n
259 The group ID which may use the reserved blocks.
260
261 resuid=n
262 The user ID which may use the reserved blocks.
263
264 sb=
265 Use alternate superblock at this location.
266
267 quota, noquota, grpquota, usrquota
268 These options are ignored by the filesystem. They are used only by
269 quota tools to recognize volumes where quota should be turned on. See
270 documentation in the quota-tools package for more details
271 (http://sourceforge.net/projects/linuxquota).
272
273 jqfmt=<quota type>, usrjquota=<file>, grpjquota=<file>
274 These options tell filesystem details about quota so that quota
275 information can be properly updated during journal replay. They replace
276 the above quota options. See documentation in the quota-tools package
277 for more details (http://sourceforge.net/projects/linuxquota).
278
279 stripe=n
280 Number of filesystem blocks that mballoc will try to use for allocation
281 size and alignment. For RAID5/6 systems this should be the number of
282 data disks * RAID chunk size in file system blocks.
283
284 delalloc (*)
285 Defer block allocation until just before ext4 writes out the block(s)
286 in question. This allows ext4 to better allocation decisions more
287 efficiently.
288
289 nodelalloc
290 Disable delayed allocation. Blocks are allocated when the data is
291 copied from userspace to the page cache, either via the write(2) system
292 call or when an mmap'ed page which was previously unallocated is
293 written for the first time.
294
295 max_batch_time=usec
296 Maximum amount of time ext4 should wait for additional filesystem
297 operations to be batch together with a synchronous write operation.
298 Since a synchronous write operation is going to force a commit and then
299 a wait for the I/O complete, it doesn't cost much, and can be a huge
300 throughput win, we wait for a small amount of time to see if any other
301 transactions can piggyback on the synchronous write. The algorithm
302 used is designed to automatically tune for the speed of the disk, by
303 measuring the amount of time (on average) that it takes to finish
304 committing a transaction. Call this time the "commit time". If the
305 time that the transaction has been running is less than the commit
306 time, ext4 will try sleeping for the commit time to see if other
307 operations will join the transaction. The commit time is capped by
308 the max_batch_time, which defaults to 15000us (15ms). This
309 optimization can be turned off entirely by setting max_batch_time to 0.
310
311 min_batch_time=usec
312 This parameter sets the commit time (as described above) to be at least
313 min_batch_time. It defaults to zero microseconds. Increasing this
314 parameter may improve the throughput of multi-threaded, synchronous
315 workloads on very fast disks, at the cost of increasing latency.
316
317 journal_ioprio=prio
318 The I/O priority (from 0 to 7, where 0 is the highest priority) which
319 should be used for I/O operations submitted by kjournald2 during a
320 commit operation. This defaults to 3, which is a slightly higher
321 priority than the default I/O priority.
322
323 auto_da_alloc(*), noauto_da_alloc
324 Many broken applications don't use fsync() when replacing existing
325 files via patterns such as fd = open("foo.new")/write(fd,..)/close(fd)/
326 rename("foo.new", "foo"), or worse yet, fd = open("foo",
327 O_TRUNC)/write(fd,..)/close(fd). If auto_da_alloc is enabled, ext4
328 will detect the replace-via-rename and replace-via-truncate patterns
329 and force that any delayed allocation blocks are allocated such that at
330 the next journal commit, in the default data=ordered mode, the data
331 blocks of the new file are forced to disk before the rename() operation
332 is committed. This provides roughly the same level of guarantees as
333 ext3, and avoids the "zero-length" problem that can happen when a
334 system crashes before the delayed allocation blocks are forced to disk.
335
336 noinit_itable
337 Do not initialize any uninitialized inode table blocks in the
338 background. This feature may be used by installation CD's so that the
339 install process can complete as quickly as possible; the inode table
340 initialization process would then be deferred until the next time the
341 file system is unmounted.
342
343 init_itable=n
344 The lazy itable init code will wait n times the number of milliseconds
345 it took to zero out the previous block group's inode table. This
346 minimizes the impact on the system performance while file system's
347 inode table is being initialized.
348
349 discard, nodiscard(*)
350 Controls whether ext4 should issue discard/TRIM commands to the
351 underlying block device when blocks are freed. This is useful for SSD
352 devices and sparse/thinly-provisioned LUNs, but it is off by default
353 until sufficient testing has been done.
354
355 nouid32
356 Disables 32-bit UIDs and GIDs. This is for interoperability with
357 older kernels which only store and expect 16-bit values.
358
359 block_validity(*), noblock_validity
360 These options enable or disable the in-kernel facility for tracking
361 filesystem metadata blocks within internal data structures. This
362 allows multi- block allocator and other routines to notice bugs or
363 corrupted allocation bitmaps which cause blocks to be allocated which
364 overlap with filesystem metadata blocks.
365
366 dioread_lock, dioread_nolock
367 Controls whether or not ext4 should use the DIO read locking. If the
368 dioread_nolock option is specified ext4 will allocate uninitialized
369 extent before buffer write and convert the extent to initialized after
370 IO completes. This approach allows ext4 code to avoid using inode
371 mutex, which improves scalability on high speed storages. However this
372 does not work with data journaling and dioread_nolock option will be
373 ignored with kernel warning. Note that dioread_nolock code path is only
374 used for extent-based files. Because of the restrictions this options
375 comprises it is off by default (e.g. dioread_lock).
376
377 max_dir_size_kb=n
378 This limits the size of directories so that any attempt to expand them
379 beyond the specified limit in kilobytes will cause an ENOSPC error.
380 This is useful in memory constrained environments, where a very large
381 directory can cause severe performance problems or even provoke the Out
382 Of Memory killer. (For example, if there is only 512mb memory
383 available, a 176mb directory may seriously cramp the system's style.)
384
385 i_version
386 Enable 64-bit inode version support. This option is off by default.
387
388 dax
389 Use direct access (no page cache). See
390 Documentation/filesystems/dax.txt. Note that this option is
391 incompatible with data=journal.
392
393Data Mode
394=========
395There are 3 different data modes:
396
397* writeback mode
398
399 In data=writeback mode, ext4 does not journal data at all. This mode provides
400 a similar level of journaling as that of XFS, JFS, and ReiserFS in its default
401 mode - metadata journaling. A crash+recovery can cause incorrect data to
402 appear in files which were written shortly before the crash. This mode will
403 typically provide the best ext4 performance.
404
405* ordered mode
406
407 In data=ordered mode, ext4 only officially journals metadata, but it logically
408 groups metadata information related to data changes with the data blocks into
409 a single unit called a transaction. When it's time to write the new metadata
410 out to disk, the associated data blocks are written first. In general, this
411 mode performs slightly slower than writeback but significantly faster than
412 journal mode.
413
414* journal mode
415
416 data=journal mode provides full data and metadata journaling. All new data is
417 written to the journal first, and then to its final location. In the event of
418 a crash, the journal can be replayed, bringing both data and metadata into a
419 consistent state. This mode is the slowest except when data needs to be read
420 from and written to disk at the same time where it outperforms all others
421 modes. Enabling this mode will disable delayed allocation and O_DIRECT
422 support.
423
424/proc entries
425=============
426
427Information about mounted ext4 file systems can be found in
428/proc/fs/ext4. Each mounted filesystem will have a directory in
429/proc/fs/ext4 based on its device name (i.e., /proc/fs/ext4/hdc or
430/proc/fs/ext4/dm-0). The files in each per-device directory are shown
431in table below.
432
433Files in /proc/fs/ext4/<devname>
434
435 mb_groups
436 details of multiblock allocator buddy cache of free blocks
437
438/sys entries
439============
440
441Information about mounted ext4 file systems can be found in
442/sys/fs/ext4. Each mounted filesystem will have a directory in
443/sys/fs/ext4 based on its device name (i.e., /sys/fs/ext4/hdc or
444/sys/fs/ext4/dm-0). The files in each per-device directory are shown
445in table below.
446
447Files in /sys/fs/ext4/<devname>:
448
449(see also Documentation/ABI/testing/sysfs-fs-ext4)
450
451 delayed_allocation_blocks
452 This file is read-only and shows the number of blocks that are dirty in
453 the page cache, but which do not have their location in the filesystem
454 allocated yet.
455
456 inode_goal
457 Tuning parameter which (if non-zero) controls the goal inode used by
458 the inode allocator in preference to all other allocation heuristics.
459 This is intended for debugging use only, and should be 0 on production
460 systems.
461
462 inode_readahead_blks
463 Tuning parameter which controls the maximum number of inode table
464 blocks that ext4's inode table readahead algorithm will pre-read into
465 the buffer cache.
466
467 lifetime_write_kbytes
468 This file is read-only and shows the number of kilobytes of data that
469 have been written to this filesystem since it was created.
470
471 max_writeback_mb_bump
472 The maximum number of megabytes the writeback code will try to write
473 out before move on to another inode.
474
475 mb_group_prealloc
476 The multiblock allocator will round up allocation requests to a
477 multiple of this tuning parameter if the stripe size is not set in the
478 ext4 superblock
479
480 mb_max_to_scan
481 The maximum number of extents the multiblock allocator will search to
482 find the best extent.
483
484 mb_min_to_scan
485 The minimum number of extents the multiblock allocator will search to
486 find the best extent.
487
488 mb_order2_req
489 Tuning parameter which controls the minimum size for requests (as a
490 power of 2) where the buddy cache is used.
491
492 mb_stats
493 Controls whether the multiblock allocator should collect statistics,
494 which are shown during the unmount. 1 means to collect statistics, 0
495 means not to collect statistics.
496
497 mb_stream_req
498 Files which have fewer blocks than this tunable parameter will have
499 their blocks allocated out of a block group specific preallocation
500 pool, so that small files are packed closely together. Each large file
501 will have its blocks allocated out of its own unique preallocation
502 pool.
503
504 session_write_kbytes
505 This file is read-only and shows the number of kilobytes of data that
506 have been written to this filesystem since it was mounted.
507
508 reserved_clusters
509 This is RW file and contains number of reserved clusters in the file
510 system which will be used in the specific situations to avoid costly
511 zeroout, unexpected ENOSPC, or possible data loss. The default is 2% or
512 4096 clusters, whichever is smaller and this can be changed however it
513 can never exceed number of clusters in the file system. If there is not
514 enough space for the reserved space when mounting the file mount will
515 _not_ fail.
516
517Ioctls
518======
519
520There is some Ext4 specific functionality which can be accessed by applications
521through the system call interfaces. The list of all Ext4 specific ioctls are
522shown in the table below.
523
524Table of Ext4 specific ioctls
525
526 EXT4_IOC_GETFLAGS
527 Get additional attributes associated with inode. The ioctl argument is
528 an integer bitfield, with bit values described in ext4.h. This ioctl is
529 an alias for FS_IOC_GETFLAGS.
530
531 EXT4_IOC_SETFLAGS
532 Set additional attributes associated with inode. The ioctl argument is
533 an integer bitfield, with bit values described in ext4.h. This ioctl is
534 an alias for FS_IOC_SETFLAGS.
535
536 EXT4_IOC_GETVERSION, EXT4_IOC_GETVERSION_OLD
537 Get the inode i_generation number stored for each inode. The
538 i_generation number is normally changed only when new inode is created
539 and it is particularly useful for network filesystems. The '_OLD'
540 version of this ioctl is an alias for FS_IOC_GETVERSION.
541
542 EXT4_IOC_SETVERSION, EXT4_IOC_SETVERSION_OLD
543 Set the inode i_generation number stored for each inode. The '_OLD'
544 version of this ioctl is an alias for FS_IOC_SETVERSION.
545
546 EXT4_IOC_GROUP_EXTEND
547 This ioctl has the same purpose as the resize mount option. It allows
548 to resize filesystem to the end of the last existing block group,
549 further resize has to be done with resize2fs, either online, or
550 offline. The argument points to the unsigned logn number representing
551 the filesystem new block count.
552
553 EXT4_IOC_MOVE_EXT
554 Move the block extents from orig_fd (the one this ioctl is pointing to)
555 to the donor_fd (the one specified in move_extent structure passed as
556 an argument to this ioctl). Then, exchange inode metadata between
557 orig_fd and donor_fd. This is especially useful for online
558 defragmentation, because the allocator has the opportunity to allocate
559 moved blocks better, ideally into one contiguous extent.
560
561 EXT4_IOC_GROUP_ADD
562 Add a new group descriptor to an existing or new group descriptor
563 block. The new group descriptor is described by ext4_new_group_input
564 structure, which is passed as an argument to this ioctl. This is
565 especially useful in conjunction with EXT4_IOC_GROUP_EXTEND, which
566 allows online resize of the filesystem to the end of the last existing
567 block group. Those two ioctls combined is used in userspace online
568 resize tool (e.g. resize2fs).
569
570 EXT4_IOC_MIGRATE
571 This ioctl operates on the filesystem itself. It converts (migrates)
572 ext3 indirect block mapped inode to ext4 extent mapped inode by walking
573 through indirect block mapping of the original inode and converting
574 contiguous block ranges into ext4 extents of the temporary inode. Then,
575 inodes are swapped. This ioctl might help, when migrating from ext3 to
576 ext4 filesystem, however suggestion is to create fresh ext4 filesystem
577 and copy data from the backup. Note, that filesystem has to support
578 extents for this ioctl to work.
579
580 EXT4_IOC_ALLOC_DA_BLKS
581 Force all of the delay allocated blocks to be allocated to preserve
582 application-expected ext3 behaviour. Note that this will also start
583 triggering a write of the data blocks, but this behaviour may change in
584 the future as it is not necessary and has been done this way only for
585 sake of simplicity.
586
587 EXT4_IOC_RESIZE_FS
588 Resize the filesystem to a new size. The number of blocks of resized
589 filesystem is passed in via 64 bit integer argument. The kernel
590 allocates bitmaps and inode table, the userspace tool thus just passes
591 the new number of blocks.
592
593 EXT4_IOC_SWAP_BOOT
594 Swap i_blocks and associated attributes (like i_blocks, i_size,
595 i_flags, ...) from the specified inode with inode EXT4_BOOT_LOADER_INO
596 (#5). This is typically used to store a boot loader in a secure part of
597 the filesystem, where it can't be changed by a normal user by accident.
598 The data blocks of the previous boot loader will be associated with the
599 given inode.
600
601References
602==========
603
604kernel source: <file:fs/ext4/>
605 <file:fs/jbd2/>
606
607programs: http://e2fsprogs.sourceforge.net/
608
609useful links: http://fedoraproject.org/wiki/ext3-devel
610 http://www.bullopensource.org/ext4/
611 http://ext4.wiki.kernel.org/index.php/Main_Page
612 http://fedoraproject.org/wiki/Features/Ext4
1.. SPDX-License-Identifier: GPL-2.0
2
3========================
4ext4 General Information
5========================
6
7Ext4 is an advanced level of the ext3 filesystem which incorporates
8scalability and reliability enhancements for supporting large filesystems
9(64 bit) in keeping with increasing disk capacities and state-of-the-art
10feature requirements.
11
12Mailing list: linux-ext4@vger.kernel.org
13Web site: http://ext4.wiki.kernel.org
14
15
16Quick usage instructions
17========================
18
19Note: More extensive information for getting started with ext4 can be
20found at the ext4 wiki site at the URL:
21http://ext4.wiki.kernel.org/index.php/Ext4_Howto
22
23 - The latest version of e2fsprogs can be found at:
24
25 https://www.kernel.org/pub/linux/kernel/people/tytso/e2fsprogs/
26
27 or
28
29 http://sourceforge.net/project/showfiles.php?group_id=2406
30
31 or grab the latest git repository from:
32
33 https://git.kernel.org/pub/scm/fs/ext2/e2fsprogs.git
34
35 - Create a new filesystem using the ext4 filesystem type:
36
37 # mke2fs -t ext4 /dev/hda1
38
39 Or to configure an existing ext3 filesystem to support extents:
40
41 # tune2fs -O extents /dev/hda1
42
43 If the filesystem was created with 128 byte inodes, it can be
44 converted to use 256 byte for greater efficiency via:
45
46 # tune2fs -I 256 /dev/hda1
47
48 - Mounting:
49
50 # mount -t ext4 /dev/hda1 /wherever
51
52 - When comparing performance with other filesystems, it's always
53 important to try multiple workloads; very often a subtle change in a
54 workload parameter can completely change the ranking of which
55 filesystems do well compared to others. When comparing versus ext3,
56 note that ext4 enables write barriers by default, while ext3 does
57 not enable write barriers by default. So it is useful to use
58 explicitly specify whether barriers are enabled or not when via the
59 '-o barriers=[0|1]' mount option for both ext3 and ext4 filesystems
60 for a fair comparison. When tuning ext3 for best benchmark numbers,
61 it is often worthwhile to try changing the data journaling mode; '-o
62 data=writeback' can be faster for some workloads. (Note however that
63 running mounted with data=writeback can potentially leave stale data
64 exposed in recently written files in case of an unclean shutdown,
65 which could be a security exposure in some situations.) Configuring
66 the filesystem with a large journal can also be helpful for
67 metadata-intensive workloads.
68
69Features
70========
71
72Currently Available
73-------------------
74
75* ability to use filesystems > 16TB (e2fsprogs support not available yet)
76* extent format reduces metadata overhead (RAM, IO for access, transactions)
77* extent format more robust in face of on-disk corruption due to magics,
78* internal redundancy in tree
79* improved file allocation (multi-block alloc)
80* lift 32000 subdirectory limit imposed by i_links_count[1]
81* nsec timestamps for mtime, atime, ctime, create time
82* inode version field on disk (NFSv4, Lustre)
83* reduced e2fsck time via uninit_bg feature
84* journal checksumming for robustness, performance
85* persistent file preallocation (e.g for streaming media, databases)
86* ability to pack bitmaps and inode tables into larger virtual groups via the
87 flex_bg feature
88* large file support
89* inode allocation using large virtual block groups via flex_bg
90* delayed allocation
91* large block (up to pagesize) support
92* efficient new ordered mode in JBD2 and ext4 (avoid using buffer head to force
93 the ordering)
94* Case-insensitive file name lookups
95* file-based encryption support (fscrypt)
96* file-based verity support (fsverity)
97
98[1] Filesystems with a block size of 1k may see a limit imposed by the
99directory hash tree having a maximum depth of two.
100
101case-insensitive file name lookups
102======================================================
103
104The case-insensitive file name lookup feature is supported on a
105per-directory basis, allowing the user to mix case-insensitive and
106case-sensitive directories in the same filesystem. It is enabled by
107flipping the +F inode attribute of an empty directory. The
108case-insensitive string match operation is only defined when we know how
109text in encoded in a byte sequence. For that reason, in order to enable
110case-insensitive directories, the filesystem must have the
111casefold feature, which stores the filesystem-wide encoding
112model used. By default, the charset adopted is the latest version of
113Unicode (12.1.0, by the time of this writing), encoded in the UTF-8
114form. The comparison algorithm is implemented by normalizing the
115strings to the Canonical decomposition form, as defined by Unicode,
116followed by a byte per byte comparison.
117
118The case-awareness is name-preserving on the disk, meaning that the file
119name provided by userspace is a byte-per-byte match to what is actually
120written in the disk. The Unicode normalization format used by the
121kernel is thus an internal representation, and not exposed to the
122userspace nor to the disk, with the important exception of disk hashes,
123used on large case-insensitive directories with DX feature. On DX
124directories, the hash must be calculated using the casefolded version of
125the filename, meaning that the normalization format used actually has an
126impact on where the directory entry is stored.
127
128When we change from viewing filenames as opaque byte sequences to seeing
129them as encoded strings we need to address what happens when a program
130tries to create a file with an invalid name. The Unicode subsystem
131within the kernel leaves the decision of what to do in this case to the
132filesystem, which select its preferred behavior by enabling/disabling
133the strict mode. When Ext4 encounters one of those strings and the
134filesystem did not require strict mode, it falls back to considering the
135entire string as an opaque byte sequence, which still allows the user to
136operate on that file, but the case-insensitive lookups won't work.
137
138Options
139=======
140
141When mounting an ext4 filesystem, the following option are accepted:
142(*) == default
143
144 ro
145 Mount filesystem read only. Note that ext4 will replay the journal (and
146 thus write to the partition) even when mounted "read only". The mount
147 options "ro,noload" can be used to prevent writes to the filesystem.
148
149 journal_checksum
150 Enable checksumming of the journal transactions. This will allow the
151 recovery code in e2fsck and the kernel to detect corruption in the
152 kernel. It is a compatible change and will be ignored by older
153 kernels.
154
155 journal_async_commit
156 Commit block can be written to disk without waiting for descriptor
157 blocks. If enabled older kernels cannot mount the device. This will
158 enable 'journal_checksum' internally.
159
160 journal_path=path, journal_dev=devnum
161 When the external journal device's major/minor numbers have changed,
162 these options allow the user to specify the new journal location. The
163 journal device is identified through either its new major/minor numbers
164 encoded in devnum, or via a path to the device.
165
166 norecovery, noload
167 Don't load the journal on mounting. Note that if the filesystem was
168 not unmounted cleanly, skipping the journal replay will lead to the
169 filesystem containing inconsistencies that can lead to any number of
170 problems.
171
172 data=journal
173 All data are committed into the journal prior to being written into the
174 main file system. Enabling this mode will disable delayed allocation
175 and O_DIRECT support.
176
177 data=ordered (*)
178 All data are forced directly out to the main file system prior to its
179 metadata being committed to the journal.
180
181 data=writeback
182 Data ordering is not preserved, data may be written into the main file
183 system after its metadata has been committed to the journal.
184
185 commit=nrsec (*)
186 This setting limits the maximum age of the running transaction to
187 'nrsec' seconds. The default value is 5 seconds. This means that if
188 you lose your power, you will lose as much as the latest 5 seconds of
189 metadata changes (your filesystem will not be damaged though, thanks
190 to the journaling). This default value (or any low value) will hurt
191 performance, but it's good for data-safety. Setting it to 0 will have
192 the same effect as leaving it at the default (5 seconds). Setting it
193 to very large values will improve performance. Note that due to
194 delayed allocation even older data can be lost on power failure since
195 writeback of those data begins only after time set in
196 /proc/sys/vm/dirty_expire_centisecs.
197
198 barrier=<0|1(*)>, barrier(*), nobarrier
199 This enables/disables the use of write barriers in the jbd code.
200 barrier=0 disables, barrier=1 enables. This also requires an IO stack
201 which can support barriers, and if jbd gets an error on a barrier
202 write, it will disable again with a warning. Write barriers enforce
203 proper on-disk ordering of journal commits, making volatile disk write
204 caches safe to use, at some performance penalty. If your disks are
205 battery-backed in one way or another, disabling barriers may safely
206 improve performance. The mount options "barrier" and "nobarrier" can
207 also be used to enable or disable barriers, for consistency with other
208 ext4 mount options.
209
210 inode_readahead_blks=n
211 This tuning parameter controls the maximum number of inode table blocks
212 that ext4's inode table readahead algorithm will pre-read into the
213 buffer cache. The default value is 32 blocks.
214
215 nouser_xattr
216 Disables Extended User Attributes. See the attr(5) manual page for
217 more information about extended attributes.
218
219 noacl
220 This option disables POSIX Access Control List support. If ACL support
221 is enabled in the kernel configuration (CONFIG_EXT4_FS_POSIX_ACL), ACL
222 is enabled by default on mount. See the acl(5) manual page for more
223 information about acl.
224
225 bsddf (*)
226 Make 'df' act like BSD.
227
228 minixdf
229 Make 'df' act like Minix.
230
231 debug
232 Extra debugging information is sent to syslog.
233
234 abort
235 Simulate the effects of calling ext4_abort() for debugging purposes.
236 This is normally used while remounting a filesystem which is already
237 mounted.
238
239 errors=remount-ro
240 Remount the filesystem read-only on an error.
241
242 errors=continue
243 Keep going on a filesystem error.
244
245 errors=panic
246 Panic and halt the machine if an error occurs. (These mount options
247 override the errors behavior specified in the superblock, which can be
248 configured using tune2fs)
249
250 data_err=ignore(*)
251 Just print an error message if an error occurs in a file data buffer in
252 ordered mode.
253 data_err=abort
254 Abort the journal if an error occurs in a file data buffer in ordered
255 mode.
256
257 grpid | bsdgroups
258 New objects have the group ID of their parent.
259
260 nogrpid (*) | sysvgroups
261 New objects have the group ID of their creator.
262
263 resgid=n
264 The group ID which may use the reserved blocks.
265
266 resuid=n
267 The user ID which may use the reserved blocks.
268
269 sb=
270 Use alternate superblock at this location.
271
272 quota, noquota, grpquota, usrquota
273 These options are ignored by the filesystem. They are used only by
274 quota tools to recognize volumes where quota should be turned on. See
275 documentation in the quota-tools package for more details
276 (http://sourceforge.net/projects/linuxquota).
277
278 jqfmt=<quota type>, usrjquota=<file>, grpjquota=<file>
279 These options tell filesystem details about quota so that quota
280 information can be properly updated during journal replay. They replace
281 the above quota options. See documentation in the quota-tools package
282 for more details (http://sourceforge.net/projects/linuxquota).
283
284 stripe=n
285 Number of filesystem blocks that mballoc will try to use for allocation
286 size and alignment. For RAID5/6 systems this should be the number of
287 data disks * RAID chunk size in file system blocks.
288
289 delalloc (*)
290 Defer block allocation until just before ext4 writes out the block(s)
291 in question. This allows ext4 to better allocation decisions more
292 efficiently.
293
294 nodelalloc
295 Disable delayed allocation. Blocks are allocated when the data is
296 copied from userspace to the page cache, either via the write(2) system
297 call or when an mmap'ed page which was previously unallocated is
298 written for the first time.
299
300 max_batch_time=usec
301 Maximum amount of time ext4 should wait for additional filesystem
302 operations to be batch together with a synchronous write operation.
303 Since a synchronous write operation is going to force a commit and then
304 a wait for the I/O complete, it doesn't cost much, and can be a huge
305 throughput win, we wait for a small amount of time to see if any other
306 transactions can piggyback on the synchronous write. The algorithm
307 used is designed to automatically tune for the speed of the disk, by
308 measuring the amount of time (on average) that it takes to finish
309 committing a transaction. Call this time the "commit time". If the
310 time that the transaction has been running is less than the commit
311 time, ext4 will try sleeping for the commit time to see if other
312 operations will join the transaction. The commit time is capped by
313 the max_batch_time, which defaults to 15000us (15ms). This
314 optimization can be turned off entirely by setting max_batch_time to 0.
315
316 min_batch_time=usec
317 This parameter sets the commit time (as described above) to be at least
318 min_batch_time. It defaults to zero microseconds. Increasing this
319 parameter may improve the throughput of multi-threaded, synchronous
320 workloads on very fast disks, at the cost of increasing latency.
321
322 journal_ioprio=prio
323 The I/O priority (from 0 to 7, where 0 is the highest priority) which
324 should be used for I/O operations submitted by kjournald2 during a
325 commit operation. This defaults to 3, which is a slightly higher
326 priority than the default I/O priority.
327
328 auto_da_alloc(*), noauto_da_alloc
329 Many broken applications don't use fsync() when replacing existing
330 files via patterns such as fd = open("foo.new")/write(fd,..)/close(fd)/
331 rename("foo.new", "foo"), or worse yet, fd = open("foo",
332 O_TRUNC)/write(fd,..)/close(fd). If auto_da_alloc is enabled, ext4
333 will detect the replace-via-rename and replace-via-truncate patterns
334 and force that any delayed allocation blocks are allocated such that at
335 the next journal commit, in the default data=ordered mode, the data
336 blocks of the new file are forced to disk before the rename() operation
337 is committed. This provides roughly the same level of guarantees as
338 ext3, and avoids the "zero-length" problem that can happen when a
339 system crashes before the delayed allocation blocks are forced to disk.
340
341 noinit_itable
342 Do not initialize any uninitialized inode table blocks in the
343 background. This feature may be used by installation CD's so that the
344 install process can complete as quickly as possible; the inode table
345 initialization process would then be deferred until the next time the
346 file system is unmounted.
347
348 init_itable=n
349 The lazy itable init code will wait n times the number of milliseconds
350 it took to zero out the previous block group's inode table. This
351 minimizes the impact on the system performance while file system's
352 inode table is being initialized.
353
354 discard, nodiscard(*)
355 Controls whether ext4 should issue discard/TRIM commands to the
356 underlying block device when blocks are freed. This is useful for SSD
357 devices and sparse/thinly-provisioned LUNs, but it is off by default
358 until sufficient testing has been done.
359
360 nouid32
361 Disables 32-bit UIDs and GIDs. This is for interoperability with
362 older kernels which only store and expect 16-bit values.
363
364 block_validity(*), noblock_validity
365 These options enable or disable the in-kernel facility for tracking
366 filesystem metadata blocks within internal data structures. This
367 allows multi- block allocator and other routines to notice bugs or
368 corrupted allocation bitmaps which cause blocks to be allocated which
369 overlap with filesystem metadata blocks.
370
371 dioread_lock, dioread_nolock
372 Controls whether or not ext4 should use the DIO read locking. If the
373 dioread_nolock option is specified ext4 will allocate uninitialized
374 extent before buffer write and convert the extent to initialized after
375 IO completes. This approach allows ext4 code to avoid using inode
376 mutex, which improves scalability on high speed storages. However this
377 does not work with data journaling and dioread_nolock option will be
378 ignored with kernel warning. Note that dioread_nolock code path is only
379 used for extent-based files. Because of the restrictions this options
380 comprises it is off by default (e.g. dioread_lock).
381
382 max_dir_size_kb=n
383 This limits the size of directories so that any attempt to expand them
384 beyond the specified limit in kilobytes will cause an ENOSPC error.
385 This is useful in memory constrained environments, where a very large
386 directory can cause severe performance problems or even provoke the Out
387 Of Memory killer. (For example, if there is only 512mb memory
388 available, a 176mb directory may seriously cramp the system's style.)
389
390 i_version
391 Enable 64-bit inode version support. This option is off by default.
392
393 dax
394 Use direct access (no page cache). See
395 Documentation/filesystems/dax.rst. Note that this option is
396 incompatible with data=journal.
397
398 inlinecrypt
399 When possible, encrypt/decrypt the contents of encrypted files using the
400 blk-crypto framework rather than filesystem-layer encryption. This
401 allows the use of inline encryption hardware. The on-disk format is
402 unaffected. For more details, see
403 Documentation/block/inline-encryption.rst.
404
405Data Mode
406=========
407There are 3 different data modes:
408
409* writeback mode
410
411 In data=writeback mode, ext4 does not journal data at all. This mode provides
412 a similar level of journaling as that of XFS, JFS, and ReiserFS in its default
413 mode - metadata journaling. A crash+recovery can cause incorrect data to
414 appear in files which were written shortly before the crash. This mode will
415 typically provide the best ext4 performance.
416
417* ordered mode
418
419 In data=ordered mode, ext4 only officially journals metadata, but it logically
420 groups metadata information related to data changes with the data blocks into
421 a single unit called a transaction. When it's time to write the new metadata
422 out to disk, the associated data blocks are written first. In general, this
423 mode performs slightly slower than writeback but significantly faster than
424 journal mode.
425
426* journal mode
427
428 data=journal mode provides full data and metadata journaling. All new data is
429 written to the journal first, and then to its final location. In the event of
430 a crash, the journal can be replayed, bringing both data and metadata into a
431 consistent state. This mode is the slowest except when data needs to be read
432 from and written to disk at the same time where it outperforms all others
433 modes. Enabling this mode will disable delayed allocation and O_DIRECT
434 support.
435
436/proc entries
437=============
438
439Information about mounted ext4 file systems can be found in
440/proc/fs/ext4. Each mounted filesystem will have a directory in
441/proc/fs/ext4 based on its device name (i.e., /proc/fs/ext4/hdc or
442/proc/fs/ext4/dm-0). The files in each per-device directory are shown
443in table below.
444
445Files in /proc/fs/ext4/<devname>
446
447 mb_groups
448 details of multiblock allocator buddy cache of free blocks
449
450/sys entries
451============
452
453Information about mounted ext4 file systems can be found in
454/sys/fs/ext4. Each mounted filesystem will have a directory in
455/sys/fs/ext4 based on its device name (i.e., /sys/fs/ext4/hdc or
456/sys/fs/ext4/dm-0). The files in each per-device directory are shown
457in table below.
458
459Files in /sys/fs/ext4/<devname>:
460
461(see also Documentation/ABI/testing/sysfs-fs-ext4)
462
463 delayed_allocation_blocks
464 This file is read-only and shows the number of blocks that are dirty in
465 the page cache, but which do not have their location in the filesystem
466 allocated yet.
467
468 inode_goal
469 Tuning parameter which (if non-zero) controls the goal inode used by
470 the inode allocator in preference to all other allocation heuristics.
471 This is intended for debugging use only, and should be 0 on production
472 systems.
473
474 inode_readahead_blks
475 Tuning parameter which controls the maximum number of inode table
476 blocks that ext4's inode table readahead algorithm will pre-read into
477 the buffer cache.
478
479 lifetime_write_kbytes
480 This file is read-only and shows the number of kilobytes of data that
481 have been written to this filesystem since it was created.
482
483 max_writeback_mb_bump
484 The maximum number of megabytes the writeback code will try to write
485 out before move on to another inode.
486
487 mb_group_prealloc
488 The multiblock allocator will round up allocation requests to a
489 multiple of this tuning parameter if the stripe size is not set in the
490 ext4 superblock
491
492 mb_max_to_scan
493 The maximum number of extents the multiblock allocator will search to
494 find the best extent.
495
496 mb_min_to_scan
497 The minimum number of extents the multiblock allocator will search to
498 find the best extent.
499
500 mb_order2_req
501 Tuning parameter which controls the minimum size for requests (as a
502 power of 2) where the buddy cache is used.
503
504 mb_stats
505 Controls whether the multiblock allocator should collect statistics,
506 which are shown during the unmount. 1 means to collect statistics, 0
507 means not to collect statistics.
508
509 mb_stream_req
510 Files which have fewer blocks than this tunable parameter will have
511 their blocks allocated out of a block group specific preallocation
512 pool, so that small files are packed closely together. Each large file
513 will have its blocks allocated out of its own unique preallocation
514 pool.
515
516 session_write_kbytes
517 This file is read-only and shows the number of kilobytes of data that
518 have been written to this filesystem since it was mounted.
519
520 reserved_clusters
521 This is RW file and contains number of reserved clusters in the file
522 system which will be used in the specific situations to avoid costly
523 zeroout, unexpected ENOSPC, or possible data loss. The default is 2% or
524 4096 clusters, whichever is smaller and this can be changed however it
525 can never exceed number of clusters in the file system. If there is not
526 enough space for the reserved space when mounting the file mount will
527 _not_ fail.
528
529Ioctls
530======
531
532Ext4 implements various ioctls which can be used by applications to access
533ext4-specific functionality. An incomplete list of these ioctls is shown in the
534table below. This list includes truly ext4-specific ioctls (``EXT4_IOC_*``) as
535well as ioctls that may have been ext4-specific originally but are now supported
536by some other filesystem(s) too (``FS_IOC_*``).
537
538Table of Ext4 ioctls
539
540 FS_IOC_GETFLAGS
541 Get additional attributes associated with inode. The ioctl argument is
542 an integer bitfield, with bit values described in ext4.h.
543
544 FS_IOC_SETFLAGS
545 Set additional attributes associated with inode. The ioctl argument is
546 an integer bitfield, with bit values described in ext4.h.
547
548 EXT4_IOC_GETVERSION, EXT4_IOC_GETVERSION_OLD
549 Get the inode i_generation number stored for each inode. The
550 i_generation number is normally changed only when new inode is created
551 and it is particularly useful for network filesystems. The '_OLD'
552 version of this ioctl is an alias for FS_IOC_GETVERSION.
553
554 EXT4_IOC_SETVERSION, EXT4_IOC_SETVERSION_OLD
555 Set the inode i_generation number stored for each inode. The '_OLD'
556 version of this ioctl is an alias for FS_IOC_SETVERSION.
557
558 EXT4_IOC_GROUP_EXTEND
559 This ioctl has the same purpose as the resize mount option. It allows
560 to resize filesystem to the end of the last existing block group,
561 further resize has to be done with resize2fs, either online, or
562 offline. The argument points to the unsigned logn number representing
563 the filesystem new block count.
564
565 EXT4_IOC_MOVE_EXT
566 Move the block extents from orig_fd (the one this ioctl is pointing to)
567 to the donor_fd (the one specified in move_extent structure passed as
568 an argument to this ioctl). Then, exchange inode metadata between
569 orig_fd and donor_fd. This is especially useful for online
570 defragmentation, because the allocator has the opportunity to allocate
571 moved blocks better, ideally into one contiguous extent.
572
573 EXT4_IOC_GROUP_ADD
574 Add a new group descriptor to an existing or new group descriptor
575 block. The new group descriptor is described by ext4_new_group_input
576 structure, which is passed as an argument to this ioctl. This is
577 especially useful in conjunction with EXT4_IOC_GROUP_EXTEND, which
578 allows online resize of the filesystem to the end of the last existing
579 block group. Those two ioctls combined is used in userspace online
580 resize tool (e.g. resize2fs).
581
582 EXT4_IOC_MIGRATE
583 This ioctl operates on the filesystem itself. It converts (migrates)
584 ext3 indirect block mapped inode to ext4 extent mapped inode by walking
585 through indirect block mapping of the original inode and converting
586 contiguous block ranges into ext4 extents of the temporary inode. Then,
587 inodes are swapped. This ioctl might help, when migrating from ext3 to
588 ext4 filesystem, however suggestion is to create fresh ext4 filesystem
589 and copy data from the backup. Note, that filesystem has to support
590 extents for this ioctl to work.
591
592 EXT4_IOC_ALLOC_DA_BLKS
593 Force all of the delay allocated blocks to be allocated to preserve
594 application-expected ext3 behaviour. Note that this will also start
595 triggering a write of the data blocks, but this behaviour may change in
596 the future as it is not necessary and has been done this way only for
597 sake of simplicity.
598
599 EXT4_IOC_RESIZE_FS
600 Resize the filesystem to a new size. The number of blocks of resized
601 filesystem is passed in via 64 bit integer argument. The kernel
602 allocates bitmaps and inode table, the userspace tool thus just passes
603 the new number of blocks.
604
605 EXT4_IOC_SWAP_BOOT
606 Swap i_blocks and associated attributes (like i_blocks, i_size,
607 i_flags, ...) from the specified inode with inode EXT4_BOOT_LOADER_INO
608 (#5). This is typically used to store a boot loader in a secure part of
609 the filesystem, where it can't be changed by a normal user by accident.
610 The data blocks of the previous boot loader will be associated with the
611 given inode.
612
613References
614==========
615
616kernel source: <file:fs/ext4/>
617 <file:fs/jbd2/>
618
619programs: http://e2fsprogs.sourceforge.net/
620
621useful links: https://fedoraproject.org/wiki/ext3-devel
622 http://www.bullopensource.org/ext4/
623 http://ext4.wiki.kernel.org/index.php/Main_Page
624 https://fedoraproject.org/wiki/Features/Ext4