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1
2Ext4 Filesystem
3===============
4
5Ext4 is an advanced level of the ext3 filesystem which incorporates
6scalability and reliability enhancements for supporting large filesystems
7(64 bit) in keeping with increasing disk capacities and state-of-the-art
8feature requirements.
9
10Mailing list: linux-ext4@vger.kernel.org
11Web site: http://ext4.wiki.kernel.org
12
13
141. Quick usage instructions:
15===========================
16
17Note: More extensive information for getting started with ext4 can be
18 found at the ext4 wiki site at the URL:
19 http://ext4.wiki.kernel.org/index.php/Ext4_Howto
20
21 - Compile and install the latest version of e2fsprogs (as of this
22 writing version 1.41.3) from:
23
24 http://sourceforge.net/project/showfiles.php?group_id=2406
25
26 or
27
28 https://www.kernel.org/pub/linux/kernel/people/tytso/e2fsprogs/
29
30 or grab the latest git repository from:
31
32 git://git.kernel.org/pub/scm/fs/ext2/e2fsprogs.git
33
34 - Note that it is highly important to install the mke2fs.conf file
35 that comes with the e2fsprogs 1.41.x sources in /etc/mke2fs.conf. If
36 you have edited the /etc/mke2fs.conf file installed on your system,
37 you will need to merge your changes with the version from e2fsprogs
38 1.41.x.
39
40 - Create a new filesystem using the ext4 filesystem type:
41
42 # mke2fs -t ext4 /dev/hda1
43
44 Or to configure an existing ext3 filesystem to support extents:
45
46 # tune2fs -O extents /dev/hda1
47
48 If the filesystem was created with 128 byte inodes, it can be
49 converted to use 256 byte for greater efficiency via:
50
51 # tune2fs -I 256 /dev/hda1
52
53 (Note: we currently do not have tools to convert an ext4
54 filesystem back to ext3; so please do not do try this on production
55 filesystems.)
56
57 - Mounting:
58
59 # mount -t ext4 /dev/hda1 /wherever
60
61 - When comparing performance with other filesystems, it's always
62 important to try multiple workloads; very often a subtle change in a
63 workload parameter can completely change the ranking of which
64 filesystems do well compared to others. When comparing versus ext3,
65 note that ext4 enables write barriers by default, while ext3 does
66 not enable write barriers by default. So it is useful to use
67 explicitly specify whether barriers are enabled or not when via the
68 '-o barriers=[0|1]' mount option for both ext3 and ext4 filesystems
69 for a fair comparison. When tuning ext3 for best benchmark numbers,
70 it is often worthwhile to try changing the data journaling mode; '-o
71 data=writeback' can be faster for some workloads. (Note however that
72 running mounted with data=writeback can potentially leave stale data
73 exposed in recently written files in case of an unclean shutdown,
74 which could be a security exposure in some situations.) Configuring
75 the filesystem with a large journal can also be helpful for
76 metadata-intensive workloads.
77
782. Features
79===========
80
812.1 Currently available
82
83* ability to use filesystems > 16TB (e2fsprogs support not available yet)
84* extent format reduces metadata overhead (RAM, IO for access, transactions)
85* extent format more robust in face of on-disk corruption due to magics,
86* internal redundancy in tree
87* improved file allocation (multi-block alloc)
88* lift 32000 subdirectory limit imposed by i_links_count[1]
89* nsec timestamps for mtime, atime, ctime, create time
90* inode version field on disk (NFSv4, Lustre)
91* reduced e2fsck time via uninit_bg feature
92* journal checksumming for robustness, performance
93* persistent file preallocation (e.g for streaming media, databases)
94* ability to pack bitmaps and inode tables into larger virtual groups via the
95 flex_bg feature
96* large file support
97* inode allocation using large virtual block groups via flex_bg
98* delayed allocation
99* large block (up to pagesize) support
100* efficient new ordered mode in JBD2 and ext4 (avoid using buffer head to force
101 the ordering)
102
103[1] Filesystems with a block size of 1k may see a limit imposed by the
104directory hash tree having a maximum depth of two.
105
1062.2 Candidate features for future inclusion
107
108* online defrag (patches available but not well tested)
109* reduced mke2fs time via lazy itable initialization in conjunction with
110 the uninit_bg feature (capability to do this is available in e2fsprogs
111 but a kernel thread to do lazy zeroing of unused inode table blocks
112 after filesystem is first mounted is required for safety)
113
114There are several others under discussion, whether they all make it in is
115partly a function of how much time everyone has to work on them. Features like
116metadata checksumming have been discussed and planned for a bit but no patches
117exist yet so I'm not sure they're in the near-term roadmap.
118
119The big performance win will come with mballoc, delalloc and flex_bg
120grouping of bitmaps and inode tables. Some test results available here:
121
122 - http://www.bullopensource.org/ext4/20080818-ffsb/ffsb-write-2.6.27-rc1.html
123 - http://www.bullopensource.org/ext4/20080818-ffsb/ffsb-readwrite-2.6.27-rc1.html
124
1253. Options
126==========
127
128When mounting an ext4 filesystem, the following option are accepted:
129(*) == default
130
131ro Mount filesystem read only. Note that ext4 will
132 replay the journal (and thus write to the
133 partition) even when mounted "read only". The
134 mount options "ro,noload" can be used to prevent
135 writes to the filesystem.
136
137journal_checksum Enable checksumming of the journal transactions.
138 This will allow the recovery code in e2fsck and the
139 kernel to detect corruption in the kernel. It is a
140 compatible change and will be ignored by older kernels.
141
142journal_async_commit Commit block can be written to disk without waiting
143 for descriptor blocks. If enabled older kernels cannot
144 mount the device. This will enable 'journal_checksum'
145 internally.
146
147journal_path=path
148journal_dev=devnum When the external journal device's major/minor numbers
149 have changed, these options allow the user to specify
150 the new journal location. The journal device is
151 identified through either its new major/minor numbers
152 encoded in devnum, or via a path to the device.
153
154norecovery Don't load the journal on mounting. Note that
155noload if the filesystem was not unmounted cleanly,
156 skipping the journal replay will lead to the
157 filesystem containing inconsistencies that can
158 lead to any number of problems.
159
160data=journal All data are committed into the journal prior to being
161 written into the main file system. Enabling
162 this mode will disable delayed allocation and
163 O_DIRECT support.
164
165data=ordered (*) All data are forced directly out to the main file
166 system prior to its metadata being committed to the
167 journal.
168
169data=writeback Data ordering is not preserved, data may be written
170 into the main file system after its metadata has been
171 committed to the journal.
172
173commit=nrsec (*) Ext4 can be told to sync all its data and metadata
174 every 'nrsec' seconds. The default value is 5 seconds.
175 This means that if you lose your power, you will lose
176 as much as the latest 5 seconds of work (your
177 filesystem will not be damaged though, thanks to the
178 journaling). This default value (or any low value)
179 will hurt performance, but it's good for data-safety.
180 Setting it to 0 will have the same effect as leaving
181 it at the default (5 seconds).
182 Setting it to very large values will improve
183 performance.
184
185barrier=<0|1(*)> This enables/disables the use of write barriers in
186barrier(*) the jbd code. barrier=0 disables, barrier=1 enables.
187nobarrier This also requires an IO stack which can support
188 barriers, and if jbd gets an error on a barrier
189 write, it will disable again with a warning.
190 Write barriers enforce proper on-disk ordering
191 of journal commits, making volatile disk write caches
192 safe to use, at some performance penalty. If
193 your disks are battery-backed in one way or another,
194 disabling barriers may safely improve performance.
195 The mount options "barrier" and "nobarrier" can
196 also be used to enable or disable barriers, for
197 consistency with other ext4 mount options.
198
199inode_readahead_blks=n This tuning parameter controls the maximum
200 number of inode table blocks that ext4's inode
201 table readahead algorithm will pre-read into
202 the buffer cache. The default value is 32 blocks.
203
204nouser_xattr Disables Extended User Attributes. See the
205 attr(5) manual page for more information about
206 extended attributes.
207
208noacl This option disables POSIX Access Control List
209 support. If ACL support is enabled in the kernel
210 configuration (CONFIG_EXT4_FS_POSIX_ACL), ACL is
211 enabled by default on mount. See the acl(5) manual
212 page for more information about acl.
213
214bsddf (*) Make 'df' act like BSD.
215minixdf Make 'df' act like Minix.
216
217debug Extra debugging information is sent to syslog.
218
219abort Simulate the effects of calling ext4_abort() for
220 debugging purposes. This is normally used while
221 remounting a filesystem which is already mounted.
222
223errors=remount-ro Remount the filesystem read-only on an error.
224errors=continue Keep going on a filesystem error.
225errors=panic Panic and halt the machine if an error occurs.
226 (These mount options override the errors behavior
227 specified in the superblock, which can be configured
228 using tune2fs)
229
230data_err=ignore(*) Just print an error message if an error occurs
231 in a file data buffer in ordered mode.
232data_err=abort Abort the journal if an error occurs in a file
233 data buffer in ordered mode.
234
235grpid New objects have the group ID of their parent.
236bsdgroups
237
238nogrpid (*) New objects have the group ID of their creator.
239sysvgroups
240
241resgid=n The group ID which may use the reserved blocks.
242
243resuid=n The user ID which may use the reserved blocks.
244
245sb=n Use alternate superblock at this location.
246
247quota These options are ignored by the filesystem. They
248noquota are used only by quota tools to recognize volumes
249grpquota where quota should be turned on. See documentation
250usrquota in the quota-tools package for more details
251 (http://sourceforge.net/projects/linuxquota).
252
253jqfmt=<quota type> These options tell filesystem details about quota
254usrjquota=<file> so that quota information can be properly updated
255grpjquota=<file> during journal replay. They replace the above
256 quota options. See documentation in the quota-tools
257 package for more details
258 (http://sourceforge.net/projects/linuxquota).
259
260stripe=n Number of filesystem blocks that mballoc will try
261 to use for allocation size and alignment. For RAID5/6
262 systems this should be the number of data
263 disks * RAID chunk size in file system blocks.
264
265delalloc (*) Defer block allocation until just before ext4
266 writes out the block(s) in question. This
267 allows ext4 to better allocation decisions
268 more efficiently.
269nodelalloc Disable delayed allocation. Blocks are allocated
270 when the data is copied from userspace to the
271 page cache, either via the write(2) system call
272 or when an mmap'ed page which was previously
273 unallocated is written for the first time.
274
275max_batch_time=usec Maximum amount of time ext4 should wait for
276 additional filesystem operations to be batch
277 together with a synchronous write operation.
278 Since a synchronous write operation is going to
279 force a commit and then a wait for the I/O
280 complete, it doesn't cost much, and can be a
281 huge throughput win, we wait for a small amount
282 of time to see if any other transactions can
283 piggyback on the synchronous write. The
284 algorithm used is designed to automatically tune
285 for the speed of the disk, by measuring the
286 amount of time (on average) that it takes to
287 finish committing a transaction. Call this time
288 the "commit time". If the time that the
289 transaction has been running is less than the
290 commit time, ext4 will try sleeping for the
291 commit time to see if other operations will join
292 the transaction. The commit time is capped by
293 the max_batch_time, which defaults to 15000us
294 (15ms). This optimization can be turned off
295 entirely by setting max_batch_time to 0.
296
297min_batch_time=usec This parameter sets the commit time (as
298 described above) to be at least min_batch_time.
299 It defaults to zero microseconds. Increasing
300 this parameter may improve the throughput of
301 multi-threaded, synchronous workloads on very
302 fast disks, at the cost of increasing latency.
303
304journal_ioprio=prio The I/O priority (from 0 to 7, where 0 is the
305 highest priority) which should be used for I/O
306 operations submitted by kjournald2 during a
307 commit operation. This defaults to 3, which is
308 a slightly higher priority than the default I/O
309 priority.
310
311auto_da_alloc(*) Many broken applications don't use fsync() when
312noauto_da_alloc replacing existing files via patterns such as
313 fd = open("foo.new")/write(fd,..)/close(fd)/
314 rename("foo.new", "foo"), or worse yet,
315 fd = open("foo", O_TRUNC)/write(fd,..)/close(fd).
316 If auto_da_alloc is enabled, ext4 will detect
317 the replace-via-rename and replace-via-truncate
318 patterns and force that any delayed allocation
319 blocks are allocated such that at the next
320 journal commit, in the default data=ordered
321 mode, the data blocks of the new file are forced
322 to disk before the rename() operation is
323 committed. This provides roughly the same level
324 of guarantees as ext3, and avoids the
325 "zero-length" problem that can happen when a
326 system crashes before the delayed allocation
327 blocks are forced to disk.
328
329noinit_itable Do not initialize any uninitialized inode table
330 blocks in the background. This feature may be
331 used by installation CD's so that the install
332 process can complete as quickly as possible; the
333 inode table initialization process would then be
334 deferred until the next time the file system
335 is unmounted.
336
337init_itable=n The lazy itable init code will wait n times the
338 number of milliseconds it took to zero out the
339 previous block group's inode table. This
340 minimizes the impact on the system performance
341 while file system's inode table is being initialized.
342
343discard Controls whether ext4 should issue discard/TRIM
344nodiscard(*) commands to the underlying block device when
345 blocks are freed. This is useful for SSD devices
346 and sparse/thinly-provisioned LUNs, but it is off
347 by default until sufficient testing has been done.
348
349nouid32 Disables 32-bit UIDs and GIDs. This is for
350 interoperability with older kernels which only
351 store and expect 16-bit values.
352
353block_validity(*) These options enable or disable the in-kernel
354noblock_validity facility for tracking filesystem metadata blocks
355 within internal data structures. This allows multi-
356 block allocator and other routines to notice
357 bugs or corrupted allocation bitmaps which cause
358 blocks to be allocated which overlap with
359 filesystem metadata blocks.
360
361dioread_lock Controls whether or not ext4 should use the DIO read
362dioread_nolock locking. If the dioread_nolock option is specified
363 ext4 will allocate uninitialized extent before buffer
364 write and convert the extent to initialized after IO
365 completes. This approach allows ext4 code to avoid
366 using inode mutex, which improves scalability on high
367 speed storages. However this does not work with
368 data journaling and dioread_nolock option will be
369 ignored with kernel warning. Note that dioread_nolock
370 code path is only used for extent-based files.
371 Because of the restrictions this options comprises
372 it is off by default (e.g. dioread_lock).
373
374max_dir_size_kb=n This limits the size of directories so that any
375 attempt to expand them beyond the specified
376 limit in kilobytes will cause an ENOSPC error.
377 This is useful in memory constrained
378 environments, where a very large directory can
379 cause severe performance problems or even
380 provoke the Out Of Memory killer. (For example,
381 if there is only 512mb memory available, a 176mb
382 directory may seriously cramp the system's style.)
383
384i_version Enable 64-bit inode version support. This option is
385 off by default.
386
387dax Use direct access (no page cache). See
388 Documentation/filesystems/dax.txt. Note that
389 this option is incompatible with data=journal.
390
391Data Mode
392=========
393There are 3 different data modes:
394
395* writeback mode
396In data=writeback mode, ext4 does not journal data at all. This mode provides
397a similar level of journaling as that of XFS, JFS, and ReiserFS in its default
398mode - metadata journaling. A crash+recovery can cause incorrect data to
399appear in files which were written shortly before the crash. This mode will
400typically provide the best ext4 performance.
401
402* ordered mode
403In data=ordered mode, ext4 only officially journals metadata, but it logically
404groups metadata information related to data changes with the data blocks into a
405single unit called a transaction. When it's time to write the new metadata
406out to disk, the associated data blocks are written first. In general,
407this mode performs slightly slower than writeback but significantly faster than journal mode.
408
409* journal mode
410data=journal mode provides full data and metadata journaling. All new data is
411written to the journal first, and then to its final location.
412In the event of a crash, the journal can be replayed, bringing both data and
413metadata into a consistent state. This mode is the slowest except when data
414needs to be read from and written to disk at the same time where it
415outperforms all others modes. Enabling this mode will disable delayed
416allocation and O_DIRECT support.
417
418/proc entries
419=============
420
421Information about mounted ext4 file systems can be found in
422/proc/fs/ext4. Each mounted filesystem will have a directory in
423/proc/fs/ext4 based on its device name (i.e., /proc/fs/ext4/hdc or
424/proc/fs/ext4/dm-0). The files in each per-device directory are shown
425in table below.
426
427Files in /proc/fs/ext4/<devname>
428..............................................................................
429 File Content
430 mb_groups details of multiblock allocator buddy cache of free blocks
431..............................................................................
432
433/sys entries
434============
435
436Information about mounted ext4 file systems can be found in
437/sys/fs/ext4. Each mounted filesystem will have a directory in
438/sys/fs/ext4 based on its device name (i.e., /sys/fs/ext4/hdc or
439/sys/fs/ext4/dm-0). The files in each per-device directory are shown
440in table below.
441
442Files in /sys/fs/ext4/<devname>
443(see also Documentation/ABI/testing/sysfs-fs-ext4)
444..............................................................................
445 File Content
446
447 delayed_allocation_blocks This file is read-only and shows the number of
448 blocks that are dirty in the page cache, but
449 which do not have their location in the
450 filesystem allocated yet.
451
452 inode_goal Tuning parameter which (if non-zero) controls
453 the goal inode used by the inode allocator in
454 preference to all other allocation heuristics.
455 This is intended for debugging use only, and
456 should be 0 on production systems.
457
458 inode_readahead_blks Tuning parameter which controls the maximum
459 number of inode table blocks that ext4's inode
460 table readahead algorithm will pre-read into
461 the buffer cache
462
463 lifetime_write_kbytes This file is read-only and shows the number of
464 kilobytes of data that have been written to this
465 filesystem since it was created.
466
467 max_writeback_mb_bump The maximum number of megabytes the writeback
468 code will try to write out before move on to
469 another inode.
470
471 mb_group_prealloc The multiblock allocator will round up allocation
472 requests to a multiple of this tuning parameter if
473 the stripe size is not set in the ext4 superblock
474
475 mb_max_to_scan The maximum number of extents the multiblock
476 allocator will search to find the best extent
477
478 mb_min_to_scan The minimum number of extents the multiblock
479 allocator will search to find the best extent
480
481 mb_order2_req Tuning parameter which controls the minimum size
482 for requests (as a power of 2) where the buddy
483 cache is used
484
485 mb_stats Controls whether the multiblock allocator should
486 collect statistics, which are shown during the
487 unmount. 1 means to collect statistics, 0 means
488 not to collect statistics
489
490 mb_stream_req Files which have fewer blocks than this tunable
491 parameter will have their blocks allocated out
492 of a block group specific preallocation pool, so
493 that small files are packed closely together.
494 Each large file will have its blocks allocated
495 out of its own unique preallocation pool.
496
497 session_write_kbytes This file is read-only and shows the number of
498 kilobytes of data that have been written to this
499 filesystem since it was mounted.
500
501 reserved_clusters This is RW file and contains number of reserved
502 clusters in the file system which will be used
503 in the specific situations to avoid costly
504 zeroout, unexpected ENOSPC, or possible data
505 loss. The default is 2% or 4096 clusters,
506 whichever is smaller and this can be changed
507 however it can never exceed number of clusters
508 in the file system. If there is not enough space
509 for the reserved space when mounting the file
510 mount will _not_ fail.
511..............................................................................
512
513Ioctls
514======
515
516There is some Ext4 specific functionality which can be accessed by applications
517through the system call interfaces. The list of all Ext4 specific ioctls are
518shown in the table below.
519
520Table of Ext4 specific ioctls
521..............................................................................
522 Ioctl Description
523 EXT4_IOC_GETFLAGS Get additional attributes associated with inode.
524 The ioctl argument is an integer bitfield, with
525 bit values described in ext4.h. This ioctl is an
526 alias for FS_IOC_GETFLAGS.
527
528 EXT4_IOC_SETFLAGS Set additional attributes associated with inode.
529 The ioctl argument is an integer bitfield, with
530 bit values described in ext4.h. This ioctl is an
531 alias for FS_IOC_SETFLAGS.
532
533 EXT4_IOC_GETVERSION
534 EXT4_IOC_GETVERSION_OLD
535 Get the inode i_generation number stored for
536 each inode. The i_generation number is normally
537 changed only when new inode is created and it is
538 particularly useful for network filesystems. The
539 '_OLD' version of this ioctl is an alias for
540 FS_IOC_GETVERSION.
541
542 EXT4_IOC_SETVERSION
543 EXT4_IOC_SETVERSION_OLD
544 Set the inode i_generation number stored for
545 each inode. The '_OLD' version of this ioctl
546 is an alias for FS_IOC_SETVERSION.
547
548 EXT4_IOC_GROUP_EXTEND This ioctl has the same purpose as the resize
549 mount option. It allows to resize filesystem
550 to the end of the last existing block group,
551 further resize has to be done with resize2fs,
552 either online, or offline. The argument points
553 to the unsigned logn number representing the
554 filesystem new block count.
555
556 EXT4_IOC_MOVE_EXT Move the block extents from orig_fd (the one
557 this ioctl is pointing to) to the donor_fd (the
558 one specified in move_extent structure passed
559 as an argument to this ioctl). Then, exchange
560 inode metadata between orig_fd and donor_fd.
561 This is especially useful for online
562 defragmentation, because the allocator has the
563 opportunity to allocate moved blocks better,
564 ideally into one contiguous extent.
565
566 EXT4_IOC_GROUP_ADD Add a new group descriptor to an existing or
567 new group descriptor block. The new group
568 descriptor is described by ext4_new_group_input
569 structure, which is passed as an argument to
570 this ioctl. This is especially useful in
571 conjunction with EXT4_IOC_GROUP_EXTEND,
572 which allows online resize of the filesystem
573 to the end of the last existing block group.
574 Those two ioctls combined is used in userspace
575 online resize tool (e.g. resize2fs).
576
577 EXT4_IOC_MIGRATE This ioctl operates on the filesystem itself.
578 It converts (migrates) ext3 indirect block mapped
579 inode to ext4 extent mapped inode by walking
580 through indirect block mapping of the original
581 inode and converting contiguous block ranges
582 into ext4 extents of the temporary inode. Then,
583 inodes are swapped. This ioctl might help, when
584 migrating from ext3 to ext4 filesystem, however
585 suggestion is to create fresh ext4 filesystem
586 and copy data from the backup. Note, that
587 filesystem has to support extents for this ioctl
588 to work.
589
590 EXT4_IOC_ALLOC_DA_BLKS Force all of the delay allocated blocks to be
591 allocated to preserve application-expected ext3
592 behaviour. Note that this will also start
593 triggering a write of the data blocks, but this
594 behaviour may change in the future as it is
595 not necessary and has been done this way only
596 for sake of simplicity.
597
598 EXT4_IOC_RESIZE_FS Resize the filesystem to a new size. The number
599 of blocks of resized filesystem is passed in via
600 64 bit integer argument. The kernel allocates
601 bitmaps and inode table, the userspace tool thus
602 just passes the new number of blocks.
603
604 EXT4_IOC_SWAP_BOOT Swap i_blocks and associated attributes
605 (like i_blocks, i_size, i_flags, ...) from
606 the specified inode with inode
607 EXT4_BOOT_LOADER_INO (#5). This is typically
608 used to store a boot loader in a secure part of
609 the filesystem, where it can't be changed by a
610 normal user by accident.
611 The data blocks of the previous boot loader
612 will be associated with the given inode.
613
614..............................................................................
615
616References
617==========
618
619kernel source: <file:fs/ext4/>
620 <file:fs/jbd2/>
621
622programs: http://e2fsprogs.sourceforge.net/
623
624useful links: http://fedoraproject.org/wiki/ext3-devel
625 http://www.bullopensource.org/ext4/
626 http://ext4.wiki.kernel.org/index.php/Main_Page
627 http://fedoraproject.org/wiki/Features/Ext4