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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 ftp://ftp.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 and http://acl.bestbits.at/
206 for more information about 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 and http://acl.bestbits.at/ for more information
213 about acl.
214
215bsddf (*) Make 'df' act like BSD.
216minixdf Make 'df' act like Minix.
217
218debug Extra debugging information is sent to syslog.
219
220abort Simulate the effects of calling ext4_abort() for
221 debugging purposes. This is normally used while
222 remounting a filesystem which is already mounted.
223
224errors=remount-ro Remount the filesystem read-only on an error.
225errors=continue Keep going on a filesystem error.
226errors=panic Panic and halt the machine if an error occurs.
227 (These mount options override the errors behavior
228 specified in the superblock, which can be configured
229 using tune2fs)
230
231data_err=ignore(*) Just print an error message if an error occurs
232 in a file data buffer in ordered mode.
233data_err=abort Abort the journal if an error occurs in a file
234 data buffer in ordered mode.
235
236grpid Give objects the same group ID as their creator.
237bsdgroups
238
239nogrpid (*) New objects have the group ID of their creator.
240sysvgroups
241
242resgid=n The group ID which may use the reserved blocks.
243
244resuid=n The user ID which may use the reserved blocks.
245
246sb=n Use alternate superblock at this location.
247
248quota These options are ignored by the filesystem. They
249noquota are used only by quota tools to recognize volumes
250grpquota where quota should be turned on. See documentation
251usrquota in the quota-tools package for more details
252 (http://sourceforge.net/projects/linuxquota).
253
254jqfmt=<quota type> These options tell filesystem details about quota
255usrjquota=<file> so that quota information can be properly updated
256grpjquota=<file> during journal replay. They replace the above
257 quota options. See documentation in the quota-tools
258 package for more details
259 (http://sourceforge.net/projects/linuxquota).
260
261stripe=n Number of filesystem blocks that mballoc will try
262 to use for allocation size and alignment. For RAID5/6
263 systems this should be the number of data
264 disks * RAID chunk size in file system blocks.
265
266delalloc (*) Defer block allocation until just before ext4
267 writes out the block(s) in question. This
268 allows ext4 to better allocation decisions
269 more efficiently.
270nodelalloc Disable delayed allocation. Blocks are allocated
271 when the data is copied from userspace to the
272 page cache, either via the write(2) system call
273 or when an mmap'ed page which was previously
274 unallocated is written for the first time.
275
276max_batch_time=usec Maximum amount of time ext4 should wait for
277 additional filesystem operations to be batch
278 together with a synchronous write operation.
279 Since a synchronous write operation is going to
280 force a commit and then a wait for the I/O
281 complete, it doesn't cost much, and can be a
282 huge throughput win, we wait for a small amount
283 of time to see if any other transactions can
284 piggyback on the synchronous write. The
285 algorithm used is designed to automatically tune
286 for the speed of the disk, by measuring the
287 amount of time (on average) that it takes to
288 finish committing a transaction. Call this time
289 the "commit time". If the time that the
290 transaction has been running is less than the
291 commit time, ext4 will try sleeping for the
292 commit time to see if other operations will join
293 the transaction. The commit time is capped by
294 the max_batch_time, which defaults to 15000us
295 (15ms). This optimization can be turned off
296 entirely by setting max_batch_time to 0.
297
298min_batch_time=usec This parameter sets the commit time (as
299 described above) to be at least min_batch_time.
300 It defaults to zero microseconds. Increasing
301 this parameter may improve the throughput of
302 multi-threaded, synchronous workloads on very
303 fast disks, at the cost of increasing latency.
304
305journal_ioprio=prio The I/O priority (from 0 to 7, where 0 is the
306 highest priority) which should be used for I/O
307 operations submitted by kjournald2 during a
308 commit operation. This defaults to 3, which is
309 a slightly higher priority than the default I/O
310 priority.
311
312auto_da_alloc(*) Many broken applications don't use fsync() when
313noauto_da_alloc replacing existing files via patterns such as
314 fd = open("foo.new")/write(fd,..)/close(fd)/
315 rename("foo.new", "foo"), or worse yet,
316 fd = open("foo", O_TRUNC)/write(fd,..)/close(fd).
317 If auto_da_alloc is enabled, ext4 will detect
318 the replace-via-rename and replace-via-truncate
319 patterns and force that any delayed allocation
320 blocks are allocated such that at the next
321 journal commit, in the default data=ordered
322 mode, the data blocks of the new file are forced
323 to disk before the rename() operation is
324 committed. This provides roughly the same level
325 of guarantees as ext3, and avoids the
326 "zero-length" problem that can happen when a
327 system crashes before the delayed allocation
328 blocks are forced to disk.
329
330noinit_itable Do not initialize any uninitialized inode table
331 blocks in the background. This feature may be
332 used by installation CD's so that the install
333 process can complete as quickly as possible; the
334 inode table initialization process would then be
335 deferred until the next time the file system
336 is unmounted.
337
338init_itable=n The lazy itable init code will wait n times the
339 number of milliseconds it took to zero out the
340 previous block group's inode table. This
341 minimizes the impact on the system performance
342 while file system's inode table is being initialized.
343
344discard Controls whether ext4 should issue discard/TRIM
345nodiscard(*) commands to the underlying block device when
346 blocks are freed. This is useful for SSD devices
347 and sparse/thinly-provisioned LUNs, but it is off
348 by default until sufficient testing has been done.
349
350nouid32 Disables 32-bit UIDs and GIDs. This is for
351 interoperability with older kernels which only
352 store and expect 16-bit values.
353
354block_validity This options allows to enables/disables the in-kernel
355noblock_validity facility for tracking filesystem metadata blocks
356 within internal data structures. This allows multi-
357 block allocator and other routines to quickly locate
358 extents which might overlap with filesystem metadata
359 blocks. This option is intended for debugging
360 purposes and since it negatively affects the
361 performance, it is off by default.
362
363dioread_lock Controls whether or not ext4 should use the DIO read
364dioread_nolock locking. If the dioread_nolock option is specified
365 ext4 will allocate uninitialized extent before buffer
366 write and convert the extent to initialized after IO
367 completes. This approach allows ext4 code to avoid
368 using inode mutex, which improves scalability on high
369 speed storages. However this does not work with
370 data journaling and dioread_nolock option will be
371 ignored with kernel warning. Note that dioread_nolock
372 code path is only used for extent-based files.
373 Because of the restrictions this options comprises
374 it is off by default (e.g. dioread_lock).
375
376max_dir_size_kb=n This limits the size of directories so that any
377 attempt to expand them beyond the specified
378 limit in kilobytes will cause an ENOSPC error.
379 This is useful in memory constrained
380 environments, where a very large directory can
381 cause severe performance problems or even
382 provoke the Out Of Memory killer. (For example,
383 if there is only 512mb memory available, a 176mb
384 directory may seriously cramp the system's style.)
385
386i_version Enable 64-bit inode version support. This option is
387 off by default.
388
389dax Use direct access (no page cache). See
390 Documentation/filesystems/dax.txt. Note that
391 this option is incompatible with data=journal.
392
393Data Mode
394=========
395There are 3 different data modes:
396
397* writeback mode
398In data=writeback mode, ext4 does not journal data at all. This mode provides
399a similar level of journaling as that of XFS, JFS, and ReiserFS in its default
400mode - metadata journaling. A crash+recovery can cause incorrect data to
401appear in files which were written shortly before the crash. This mode will
402typically provide the best ext4 performance.
403
404* ordered mode
405In data=ordered mode, ext4 only officially journals metadata, but it logically
406groups metadata information related to data changes with the data blocks into a
407single unit called a transaction. When it's time to write the new metadata
408out to disk, the associated data blocks are written first. In general,
409this mode performs slightly slower than writeback but significantly faster than journal mode.
410
411* journal mode
412data=journal mode provides full data and metadata journaling. All new data is
413written to the journal first, and then to its final location.
414In the event of a crash, the journal can be replayed, bringing both data and
415metadata into a consistent state. This mode is the slowest except when data
416needs to be read from and written to disk at the same time where it
417outperforms all others modes. Enabling this mode will disable delayed
418allocation and O_DIRECT support.
419
420/proc entries
421=============
422
423Information about mounted ext4 file systems can be found in
424/proc/fs/ext4. Each mounted filesystem will have a directory in
425/proc/fs/ext4 based on its device name (i.e., /proc/fs/ext4/hdc or
426/proc/fs/ext4/dm-0). The files in each per-device directory are shown
427in table below.
428
429Files in /proc/fs/ext4/<devname>
430..............................................................................
431 File Content
432 mb_groups details of multiblock allocator buddy cache of free blocks
433..............................................................................
434
435/sys entries
436============
437
438Information about mounted ext4 file systems can be found in
439/sys/fs/ext4. Each mounted filesystem will have a directory in
440/sys/fs/ext4 based on its device name (i.e., /sys/fs/ext4/hdc or
441/sys/fs/ext4/dm-0). The files in each per-device directory are shown
442in table below.
443
444Files in /sys/fs/ext4/<devname>
445(see also Documentation/ABI/testing/sysfs-fs-ext4)
446..............................................................................
447 File Content
448
449 delayed_allocation_blocks This file is read-only and shows the number of
450 blocks that are dirty in the page cache, but
451 which do not have their location in the
452 filesystem allocated yet.
453
454 inode_goal Tuning parameter which (if non-zero) controls
455 the goal inode used by the inode allocator in
456 preference to all other allocation heuristics.
457 This is intended for debugging use only, and
458 should be 0 on production systems.
459
460 inode_readahead_blks Tuning parameter which controls the maximum
461 number of inode table blocks that ext4's inode
462 table readahead algorithm will pre-read into
463 the buffer cache
464
465 lifetime_write_kbytes This file is read-only and shows the number of
466 kilobytes of data that have been written to this
467 filesystem since it was created.
468
469 max_writeback_mb_bump The maximum number of megabytes the writeback
470 code will try to write out before move on to
471 another inode.
472
473 mb_group_prealloc The multiblock allocator will round up allocation
474 requests to a multiple of this tuning parameter if
475 the stripe size is not set in the ext4 superblock
476
477 mb_max_to_scan The maximum number of extents the multiblock
478 allocator will search to find the best extent
479
480 mb_min_to_scan The minimum number of extents the multiblock
481 allocator will search to find the best extent
482
483 mb_order2_req Tuning parameter which controls the minimum size
484 for requests (as a power of 2) where the buddy
485 cache is used
486
487 mb_stats Controls whether the multiblock allocator should
488 collect statistics, which are shown during the
489 unmount. 1 means to collect statistics, 0 means
490 not to collect statistics
491
492 mb_stream_req Files which have fewer blocks than this tunable
493 parameter will have their blocks allocated out
494 of a block group specific preallocation pool, so
495 that small files are packed closely together.
496 Each large file will have its blocks allocated
497 out of its own unique preallocation pool.
498
499 session_write_kbytes This file is read-only and shows the number of
500 kilobytes of data that have been written to this
501 filesystem since it was mounted.
502
503 reserved_clusters This is RW file and contains number of reserved
504 clusters in the file system which will be used
505 in the specific situations to avoid costly
506 zeroout, unexpected ENOSPC, or possible data
507 loss. The default is 2% or 4096 clusters,
508 whichever is smaller and this can be changed
509 however it can never exceed number of clusters
510 in the file system. If there is not enough space
511 for the reserved space when mounting the file
512 mount will _not_ fail.
513..............................................................................
514
515Ioctls
516======
517
518There is some Ext4 specific functionality which can be accessed by applications
519through the system call interfaces. The list of all Ext4 specific ioctls are
520shown in the table below.
521
522Table of Ext4 specific ioctls
523..............................................................................
524 Ioctl Description
525 EXT4_IOC_GETFLAGS Get additional attributes associated with inode.
526 The ioctl argument is an integer bitfield, with
527 bit values described in ext4.h. This ioctl is an
528 alias for FS_IOC_GETFLAGS.
529
530 EXT4_IOC_SETFLAGS Set additional attributes associated with inode.
531 The ioctl argument is an integer bitfield, with
532 bit values described in ext4.h. This ioctl is an
533 alias for FS_IOC_SETFLAGS.
534
535 EXT4_IOC_GETVERSION
536 EXT4_IOC_GETVERSION_OLD
537 Get the inode i_generation number stored for
538 each inode. The i_generation number is normally
539 changed only when new inode is created and it is
540 particularly useful for network filesystems. The
541 '_OLD' version of this ioctl is an alias for
542 FS_IOC_GETVERSION.
543
544 EXT4_IOC_SETVERSION
545 EXT4_IOC_SETVERSION_OLD
546 Set the inode i_generation number stored for
547 each inode. The '_OLD' version of this ioctl
548 is an alias for FS_IOC_SETVERSION.
549
550 EXT4_IOC_GROUP_EXTEND This ioctl has the same purpose as the resize
551 mount option. It allows to resize filesystem
552 to the end of the last existing block group,
553 further resize has to be done with resize2fs,
554 either online, or offline. The argument points
555 to the unsigned logn number representing the
556 filesystem new block count.
557
558 EXT4_IOC_MOVE_EXT Move the block extents from orig_fd (the one
559 this ioctl is pointing to) to the donor_fd (the
560 one specified in move_extent structure passed
561 as an argument to this ioctl). Then, exchange
562 inode metadata between orig_fd and donor_fd.
563 This is especially useful for online
564 defragmentation, because the allocator has the
565 opportunity to allocate moved blocks better,
566 ideally into one contiguous extent.
567
568 EXT4_IOC_GROUP_ADD Add a new group descriptor to an existing or
569 new group descriptor block. The new group
570 descriptor is described by ext4_new_group_input
571 structure, which is passed as an argument to
572 this ioctl. This is especially useful in
573 conjunction with EXT4_IOC_GROUP_EXTEND,
574 which allows online resize of the filesystem
575 to the end of the last existing block group.
576 Those two ioctls combined is used in userspace
577 online resize tool (e.g. resize2fs).
578
579 EXT4_IOC_MIGRATE This ioctl operates on the filesystem itself.
580 It converts (migrates) ext3 indirect block mapped
581 inode to ext4 extent mapped inode by walking
582 through indirect block mapping of the original
583 inode and converting contiguous block ranges
584 into ext4 extents of the temporary inode. Then,
585 inodes are swapped. This ioctl might help, when
586 migrating from ext3 to ext4 filesystem, however
587 suggestion is to create fresh ext4 filesystem
588 and copy data from the backup. Note, that
589 filesystem has to support extents for this ioctl
590 to work.
591
592 EXT4_IOC_ALLOC_DA_BLKS Force all of the delay allocated blocks to be
593 allocated to preserve application-expected ext3
594 behaviour. Note that this will also start
595 triggering a write of the data blocks, but this
596 behaviour may change in the future as it is
597 not necessary and has been done this way only
598 for sake of simplicity.
599
600 EXT4_IOC_RESIZE_FS Resize the filesystem to a new size. The number
601 of blocks of resized filesystem is passed in via
602 64 bit integer argument. The kernel allocates
603 bitmaps and inode table, the userspace tool thus
604 just passes the new number of blocks.
605
606EXT4_IOC_SWAP_BOOT Swap i_blocks and associated attributes
607 (like i_blocks, i_size, i_flags, ...) from
608 the specified inode with inode
609 EXT4_BOOT_LOADER_INO (#5). This is typically
610 used to store a boot loader in a secure part of
611 the filesystem, where it can't be changed by a
612 normal user by accident.
613 The data blocks of the previous boot loader
614 will be associated with the given inode.
615
616..............................................................................
617
618References
619==========
620
621kernel source: <file:fs/ext4/>
622 <file:fs/jbd2/>
623
624programs: http://e2fsprogs.sourceforge.net/
625
626useful links: http://fedoraproject.org/wiki/ext3-devel
627 http://www.bullopensource.org/ext4/
628 http://ext4.wiki.kernel.org/index.php/Main_Page
629 http://fedoraproject.org/wiki/Features/Ext4
1
2Ext4 Filesystem
3===============
4
5Ext4 is an 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 ftp://ftp.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_dev=devnum When the external journal device's major/minor numbers
148 have changed, this option allows the user to specify
149 the new journal location. The journal device is
150 identified through its new major/minor numbers encoded
151 in devnum.
152
153norecovery Don't load the journal on mounting. Note that
154noload if the filesystem was not unmounted cleanly,
155 skipping the journal replay will lead to the
156 filesystem containing inconsistencies that can
157 lead to any number of problems.
158
159data=journal All data are committed into the journal prior to being
160 written into the main file system. Enabling
161 this mode will disable delayed allocation and
162 O_DIRECT support.
163
164data=ordered (*) All data are forced directly out to the main file
165 system prior to its metadata being committed to the
166 journal.
167
168data=writeback Data ordering is not preserved, data may be written
169 into the main file system after its metadata has been
170 committed to the journal.
171
172commit=nrsec (*) Ext4 can be told to sync all its data and metadata
173 every 'nrsec' seconds. The default value is 5 seconds.
174 This means that if you lose your power, you will lose
175 as much as the latest 5 seconds of work (your
176 filesystem will not be damaged though, thanks to the
177 journaling). This default value (or any low value)
178 will hurt performance, but it's good for data-safety.
179 Setting it to 0 will have the same effect as leaving
180 it at the default (5 seconds).
181 Setting it to very large values will improve
182 performance.
183
184barrier=<0|1(*)> This enables/disables the use of write barriers in
185barrier(*) the jbd code. barrier=0 disables, barrier=1 enables.
186nobarrier This also requires an IO stack which can support
187 barriers, and if jbd gets an error on a barrier
188 write, it will disable again with a warning.
189 Write barriers enforce proper on-disk ordering
190 of journal commits, making volatile disk write caches
191 safe to use, at some performance penalty. If
192 your disks are battery-backed in one way or another,
193 disabling barriers may safely improve performance.
194 The mount options "barrier" and "nobarrier" can
195 also be used to enable or disable barriers, for
196 consistency with other ext4 mount options.
197
198inode_readahead_blks=n This tuning parameter controls the maximum
199 number of inode table blocks that ext4's inode
200 table readahead algorithm will pre-read into
201 the buffer cache. The default value is 32 blocks.
202
203nouser_xattr Disables Extended User Attributes. If you have extended
204 attribute support enabled in the kernel configuration
205 (CONFIG_EXT4_FS_XATTR), extended attribute support
206 is enabled by default on mount. See the attr(5) manual
207 page and http://acl.bestbits.at/ for more information
208 about extended attributes.
209
210noacl This option disables POSIX Access Control List
211 support. If ACL support is enabled in the kernel
212 configuration (CONFIG_EXT4_FS_POSIX_ACL), ACL is
213 enabled by default on mount. See the acl(5) manual
214 page and http://acl.bestbits.at/ for more information
215 about acl.
216
217bsddf (*) Make 'df' act like BSD.
218minixdf Make 'df' act like Minix.
219
220debug Extra debugging information is sent to syslog.
221
222abort Simulate the effects of calling ext4_abort() for
223 debugging purposes. This is normally used while
224 remounting a filesystem which is already mounted.
225
226errors=remount-ro Remount the filesystem read-only on an error.
227errors=continue Keep going on a filesystem error.
228errors=panic Panic and halt the machine if an error occurs.
229 (These mount options override the errors behavior
230 specified in the superblock, which can be configured
231 using tune2fs)
232
233data_err=ignore(*) Just print an error message if an error occurs
234 in a file data buffer in ordered mode.
235data_err=abort Abort the journal if an error occurs in a file
236 data buffer in ordered mode.
237
238grpid Give objects the same group ID as their creator.
239bsdgroups
240
241nogrpid (*) New objects have the group ID of their creator.
242sysvgroups
243
244resgid=n The group ID which may use the reserved blocks.
245
246resuid=n The user ID which may use the reserved blocks.
247
248sb=n Use alternate superblock at this location.
249
250quota These options are ignored by the filesystem. They
251noquota are used only by quota tools to recognize volumes
252grpquota where quota should be turned on. See documentation
253usrquota in the quota-tools package for more details
254 (http://sourceforge.net/projects/linuxquota).
255
256jqfmt=<quota type> These options tell filesystem details about quota
257usrjquota=<file> so that quota information can be properly updated
258grpjquota=<file> during journal replay. They replace the above
259 quota options. See documentation in the quota-tools
260 package for more details
261 (http://sourceforge.net/projects/linuxquota).
262
263stripe=n Number of filesystem blocks that mballoc will try
264 to use for allocation size and alignment. For RAID5/6
265 systems this should be the number of data
266 disks * RAID chunk size in file system blocks.
267
268delalloc (*) Defer block allocation until just before ext4
269 writes out the block(s) in question. This
270 allows ext4 to better allocation decisions
271 more efficiently.
272nodelalloc Disable delayed allocation. Blocks are allocated
273 when the data is copied from userspace to the
274 page cache, either via the write(2) system call
275 or when an mmap'ed page which was previously
276 unallocated is written for the first time.
277
278max_batch_time=usec Maximum amount of time ext4 should wait for
279 additional filesystem operations to be batch
280 together with a synchronous write operation.
281 Since a synchronous write operation is going to
282 force a commit and then a wait for the I/O
283 complete, it doesn't cost much, and can be a
284 huge throughput win, we wait for a small amount
285 of time to see if any other transactions can
286 piggyback on the synchronous write. The
287 algorithm used is designed to automatically tune
288 for the speed of the disk, by measuring the
289 amount of time (on average) that it takes to
290 finish committing a transaction. Call this time
291 the "commit time". If the time that the
292 transaction has been running is less than the
293 commit time, ext4 will try sleeping for the
294 commit time to see if other operations will join
295 the transaction. The commit time is capped by
296 the max_batch_time, which defaults to 15000us
297 (15ms). This optimization can be turned off
298 entirely by setting max_batch_time to 0.
299
300min_batch_time=usec This parameter sets the commit time (as
301 described above) to be at least min_batch_time.
302 It defaults to zero microseconds. Increasing
303 this parameter may improve the throughput of
304 multi-threaded, synchronous workloads on very
305 fast disks, at the cost of increasing latency.
306
307journal_ioprio=prio The I/O priority (from 0 to 7, where 0 is the
308 highest priority) which should be used for I/O
309 operations submitted by kjournald2 during a
310 commit operation. This defaults to 3, which is
311 a slightly higher priority than the default I/O
312 priority.
313
314auto_da_alloc(*) Many broken applications don't use fsync() when
315noauto_da_alloc replacing existing files via patterns such as
316 fd = open("foo.new")/write(fd,..)/close(fd)/
317 rename("foo.new", "foo"), or worse yet,
318 fd = open("foo", O_TRUNC)/write(fd,..)/close(fd).
319 If auto_da_alloc is enabled, ext4 will detect
320 the replace-via-rename and replace-via-truncate
321 patterns and force that any delayed allocation
322 blocks are allocated such that at the next
323 journal commit, in the default data=ordered
324 mode, the data blocks of the new file are forced
325 to disk before the rename() operation is
326 committed. This provides roughly the same level
327 of guarantees as ext3, and avoids the
328 "zero-length" problem that can happen when a
329 system crashes before the delayed allocation
330 blocks are forced to disk.
331
332noinit_itable Do not initialize any uninitialized inode table
333 blocks in the background. This feature may be
334 used by installation CD's so that the install
335 process can complete as quickly as possible; the
336 inode table initialization process would then be
337 deferred until the next time the file system
338 is unmounted.
339
340init_itable=n The lazy itable init code will wait n times the
341 number of milliseconds it took to zero out the
342 previous block group's inode table. This
343 minimizes the impact on the system performance
344 while file system's inode table is being initialized.
345
346discard Controls whether ext4 should issue discard/TRIM
347nodiscard(*) commands to the underlying block device when
348 blocks are freed. This is useful for SSD devices
349 and sparse/thinly-provisioned LUNs, but it is off
350 by default until sufficient testing has been done.
351
352nouid32 Disables 32-bit UIDs and GIDs. This is for
353 interoperability with older kernels which only
354 store and expect 16-bit values.
355
356block_validity This options allows to enables/disables the in-kernel
357noblock_validity facility for tracking filesystem metadata blocks
358 within internal data structures. This allows multi-
359 block allocator and other routines to quickly locate
360 extents which might overlap with filesystem metadata
361 blocks. This option is intended for debugging
362 purposes and since it negatively affects the
363 performance, it is off by default.
364
365dioread_lock Controls whether or not ext4 should use the DIO read
366dioread_nolock locking. If the dioread_nolock option is specified
367 ext4 will allocate uninitialized extent before buffer
368 write and convert the extent to initialized after IO
369 completes. This approach allows ext4 code to avoid
370 using inode mutex, which improves scalability on high
371 speed storages. However this does not work with
372 data journaling and dioread_nolock option will be
373 ignored with kernel warning. Note that dioread_nolock
374 code path is only used for extent-based files.
375 Because of the restrictions this options comprises
376 it is off by default (e.g. dioread_lock).
377
378i_version Enable 64-bit inode version support. This option is
379 off by default.
380
381Data Mode
382=========
383There are 3 different data modes:
384
385* writeback mode
386In data=writeback mode, ext4 does not journal data at all. This mode provides
387a similar level of journaling as that of XFS, JFS, and ReiserFS in its default
388mode - metadata journaling. A crash+recovery can cause incorrect data to
389appear in files which were written shortly before the crash. This mode will
390typically provide the best ext4 performance.
391
392* ordered mode
393In data=ordered mode, ext4 only officially journals metadata, but it logically
394groups metadata information related to data changes with the data blocks into a
395single unit called a transaction. When it's time to write the new metadata
396out to disk, the associated data blocks are written first. In general,
397this mode performs slightly slower than writeback but significantly faster than journal mode.
398
399* journal mode
400data=journal mode provides full data and metadata journaling. All new data is
401written to the journal first, and then to its final location.
402In the event of a crash, the journal can be replayed, bringing both data and
403metadata into a consistent state. This mode is the slowest except when data
404needs to be read from and written to disk at the same time where it
405outperforms all others modes. Enabling this mode will disable delayed
406allocation and O_DIRECT support.
407
408/proc entries
409=============
410
411Information about mounted ext4 file systems can be found in
412/proc/fs/ext4. Each mounted filesystem will have a directory in
413/proc/fs/ext4 based on its device name (i.e., /proc/fs/ext4/hdc or
414/proc/fs/ext4/dm-0). The files in each per-device directory are shown
415in table below.
416
417Files in /proc/fs/ext4/<devname>
418..............................................................................
419 File Content
420 mb_groups details of multiblock allocator buddy cache of free blocks
421..............................................................................
422
423/sys entries
424============
425
426Information about mounted ext4 file systems can be found in
427/sys/fs/ext4. Each mounted filesystem will have a directory in
428/sys/fs/ext4 based on its device name (i.e., /sys/fs/ext4/hdc or
429/sys/fs/ext4/dm-0). The files in each per-device directory are shown
430in table below.
431
432Files in /sys/fs/ext4/<devname>
433(see also Documentation/ABI/testing/sysfs-fs-ext4)
434..............................................................................
435 File Content
436
437 delayed_allocation_blocks This file is read-only and shows the number of
438 blocks that are dirty in the page cache, but
439 which do not have their location in the
440 filesystem allocated yet.
441
442 inode_goal Tuning parameter which (if non-zero) controls
443 the goal inode used by the inode allocator in
444 preference to all other allocation heuristics.
445 This is intended for debugging use only, and
446 should be 0 on production systems.
447
448 inode_readahead_blks Tuning parameter which controls the maximum
449 number of inode table blocks that ext4's inode
450 table readahead algorithm will pre-read into
451 the buffer cache
452
453 lifetime_write_kbytes This file is read-only and shows the number of
454 kilobytes of data that have been written to this
455 filesystem since it was created.
456
457 max_writeback_mb_bump The maximum number of megabytes the writeback
458 code will try to write out before move on to
459 another inode.
460
461 mb_group_prealloc The multiblock allocator will round up allocation
462 requests to a multiple of this tuning parameter if
463 the stripe size is not set in the ext4 superblock
464
465 mb_max_to_scan The maximum number of extents the multiblock
466 allocator will search to find the best extent
467
468 mb_min_to_scan The minimum number of extents the multiblock
469 allocator will search to find the best extent
470
471 mb_order2_req Tuning parameter which controls the minimum size
472 for requests (as a power of 2) where the buddy
473 cache is used
474
475 mb_stats Controls whether the multiblock allocator should
476 collect statistics, which are shown during the
477 unmount. 1 means to collect statistics, 0 means
478 not to collect statistics
479
480 mb_stream_req Files which have fewer blocks than this tunable
481 parameter will have their blocks allocated out
482 of a block group specific preallocation pool, so
483 that small files are packed closely together.
484 Each large file will have its blocks allocated
485 out of its own unique preallocation pool.
486
487 session_write_kbytes This file is read-only and shows the number of
488 kilobytes of data that have been written to this
489 filesystem since it was mounted.
490..............................................................................
491
492Ioctls
493======
494
495There is some Ext4 specific functionality which can be accessed by applications
496through the system call interfaces. The list of all Ext4 specific ioctls are
497shown in the table below.
498
499Table of Ext4 specific ioctls
500..............................................................................
501 Ioctl Description
502 EXT4_IOC_GETFLAGS Get additional attributes associated with inode.
503 The ioctl argument is an integer bitfield, with
504 bit values described in ext4.h. This ioctl is an
505 alias for FS_IOC_GETFLAGS.
506
507 EXT4_IOC_SETFLAGS Set additional attributes associated with inode.
508 The ioctl argument is an integer bitfield, with
509 bit values described in ext4.h. This ioctl is an
510 alias for FS_IOC_SETFLAGS.
511
512 EXT4_IOC_GETVERSION
513 EXT4_IOC_GETVERSION_OLD
514 Get the inode i_generation number stored for
515 each inode. The i_generation number is normally
516 changed only when new inode is created and it is
517 particularly useful for network filesystems. The
518 '_OLD' version of this ioctl is an alias for
519 FS_IOC_GETVERSION.
520
521 EXT4_IOC_SETVERSION
522 EXT4_IOC_SETVERSION_OLD
523 Set the inode i_generation number stored for
524 each inode. The '_OLD' version of this ioctl
525 is an alias for FS_IOC_SETVERSION.
526
527 EXT4_IOC_GROUP_EXTEND This ioctl has the same purpose as the resize
528 mount option. It allows to resize filesystem
529 to the end of the last existing block group,
530 further resize has to be done with resize2fs,
531 either online, or offline. The argument points
532 to the unsigned logn number representing the
533 filesystem new block count.
534
535 EXT4_IOC_MOVE_EXT Move the block extents from orig_fd (the one
536 this ioctl is pointing to) to the donor_fd (the
537 one specified in move_extent structure passed
538 as an argument to this ioctl). Then, exchange
539 inode metadata between orig_fd and donor_fd.
540 This is especially useful for online
541 defragmentation, because the allocator has the
542 opportunity to allocate moved blocks better,
543 ideally into one contiguous extent.
544
545 EXT4_IOC_GROUP_ADD Add a new group descriptor to an existing or
546 new group descriptor block. The new group
547 descriptor is described by ext4_new_group_input
548 structure, which is passed as an argument to
549 this ioctl. This is especially useful in
550 conjunction with EXT4_IOC_GROUP_EXTEND,
551 which allows online resize of the filesystem
552 to the end of the last existing block group.
553 Those two ioctls combined is used in userspace
554 online resize tool (e.g. resize2fs).
555
556 EXT4_IOC_MIGRATE This ioctl operates on the filesystem itself.
557 It converts (migrates) ext3 indirect block mapped
558 inode to ext4 extent mapped inode by walking
559 through indirect block mapping of the original
560 inode and converting contiguous block ranges
561 into ext4 extents of the temporary inode. Then,
562 inodes are swapped. This ioctl might help, when
563 migrating from ext3 to ext4 filesystem, however
564 suggestion is to create fresh ext4 filesystem
565 and copy data from the backup. Note, that
566 filesystem has to support extents for this ioctl
567 to work.
568
569 EXT4_IOC_ALLOC_DA_BLKS Force all of the delay allocated blocks to be
570 allocated to preserve application-expected ext3
571 behaviour. Note that this will also start
572 triggering a write of the data blocks, but this
573 behaviour may change in the future as it is
574 not necessary and has been done this way only
575 for sake of simplicity.
576
577 EXT4_IOC_RESIZE_FS Resize the filesystem to a new size. The number
578 of blocks of resized filesystem is passed in via
579 64 bit integer argument. The kernel allocates
580 bitmaps and inode table, the userspace tool thus
581 just passes the new number of blocks.
582
583..............................................................................
584
585References
586==========
587
588kernel source: <file:fs/ext4/>
589 <file:fs/jbd2/>
590
591programs: http://e2fsprogs.sourceforge.net/
592
593useful links: http://fedoraproject.org/wiki/ext3-devel
594 http://www.bullopensource.org/ext4/
595 http://ext4.wiki.kernel.org/index.php/Main_Page
596 http://fedoraproject.org/wiki/Features/Ext4