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2 Ext4 Filesystem
3 ===============
5 Ext4 is an an advanced level of the ext3 filesystem which incorporates
6 scalability and reliability enhancements for supporting large filesystems
7 (64 bit) in keeping with increasing disk capacities and state-of-the-art
8 feature requirements.
10 Mailing list: linux-ext4@vger.kernel.org
11 Web site: http://ext4.wiki.kernel.org
14 1. Quick usage instructions:
15 ===========================
17 Note: 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
21 - Compile and install the latest version of e2fsprogs (as of this
22 writing version 1.41.3) from:
24 http://sourceforge.net/project/showfiles.php?group_id=2406
26 or
28 ftp://ftp.kernel.org/pub/linux/kernel/people/tytso/e2fsprogs/
30 or grab the latest git repository from:
32 git://git.kernel.org/pub/scm/fs/ext2/e2fsprogs.git
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.
40 - Create a new filesystem using the ext4 filesystem type:
42 # mke2fs -t ext4 /dev/hda1
44 Or to configure an existing ext3 filesystem to support extents:
46 # tune2fs -O extents /dev/hda1
48 If the filesystem was created with 128 byte inodes, it can be
49 converted to use 256 byte for greater efficiency via:
51 # tune2fs -I 256 /dev/hda1
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.)
57 - Mounting:
59 # mount -t ext4 /dev/hda1 /wherever
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.
78 2. Features
79 ===========
81 2.1 Currently available
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)
103 [1] Filesystems with a block size of 1k may see a limit imposed by the
104 directory hash tree having a maximum depth of two.
106 2.2 Candidate features for future inclusion
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)
114 There are several others under discussion, whether they all make it in is
115 partly a function of how much time everyone has to work on them. Features like
116 metadata checksumming have been discussed and planned for a bit but no patches
117 exist yet so I'm not sure they're in the near-term roadmap.
119 The big performance win will come with mballoc, delalloc and flex_bg
120 grouping of bitmaps and inode tables. Some test results available here:
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
125 3. Options
126 ==========
128 When mounting an ext4 filesystem, the following option are accepted:
129 (*) == default
131 ro 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.
137 journal_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.
142 journal_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.
147 journal=update Update the ext4 file system's journal to the current
148 format.
150 journal_dev=devnum When the external journal device's major/minor numbers
151 have changed, this option allows the user to specify
152 the new journal location. The journal device is
153 identified through its new major/minor numbers encoded
154 in devnum.
156 norecovery Don't load the journal on mounting. Note that
157 noload if the filesystem was not unmounted cleanly,
158 skipping the journal replay will lead to the
159 filesystem containing inconsistencies that can
160 lead to any number of problems.
162 data=journal All data are committed into the journal prior to being
163 written into the main file system. Enabling
164 this mode will disable delayed allocation and
165 O_DIRECT support.
167 data=ordered (*) All data are forced directly out to the main file
168 system prior to its metadata being committed to the
169 journal.
171 data=writeback Data ordering is not preserved, data may be written
172 into the main file system after its metadata has been
173 committed to the journal.
175 commit=nrsec (*) Ext4 can be told to sync all its data and metadata
176 every 'nrsec' seconds. The default value is 5 seconds.
177 This means that if you lose your power, you will lose
178 as much as the latest 5 seconds of work (your
179 filesystem will not be damaged though, thanks to the
180 journaling). This default value (or any low value)
181 will hurt performance, but it's good for data-safety.
182 Setting it to 0 will have the same effect as leaving
183 it at the default (5 seconds).
184 Setting it to very large values will improve
185 performance.
187 barrier=<0|1(*)> This enables/disables the use of write barriers in
188 barrier(*) the jbd code. barrier=0 disables, barrier=1 enables.
189 nobarrier This also requires an IO stack which can support
190 barriers, and if jbd gets an error on a barrier
191 write, it will disable again with a warning.
192 Write barriers enforce proper on-disk ordering
193 of journal commits, making volatile disk write caches
194 safe to use, at some performance penalty. If
195 your disks are battery-backed in one way or another,
196 disabling barriers may safely improve performance.
197 The mount options "barrier" and "nobarrier" can
198 also be used to enable or disable barriers, for
199 consistency with other ext4 mount options.
201 inode_readahead_blks=n This tuning parameter controls the maximum
202 number of inode table blocks that ext4's inode
203 table readahead algorithm will pre-read into
204 the buffer cache. The default value is 32 blocks.
206 nouser_xattr Disables Extended User Attributes. If you have extended
207 attribute support enabled in the kernel configuration
208 (CONFIG_EXT4_FS_XATTR), extended attribute support
209 is enabled by default on mount. See the attr(5) manual
210 page and http://acl.bestbits.at/ for more information
211 about extended attributes.
213 noacl This option disables POSIX Access Control List
214 support. If ACL support is enabled in the kernel
215 configuration (CONFIG_EXT4_FS_POSIX_ACL), ACL is
216 enabled by default on mount. See the acl(5) manual
217 page and http://acl.bestbits.at/ for more information
218 about acl.
220 bsddf (*) Make 'df' act like BSD.
221 minixdf Make 'df' act like Minix.
223 debug Extra debugging information is sent to syslog.
225 abort Simulate the effects of calling ext4_abort() for
226 debugging purposes. This is normally used while
227 remounting a filesystem which is already mounted.
229 errors=remount-ro Remount the filesystem read-only on an error.
230 errors=continue Keep going on a filesystem error.
231 errors=panic Panic and halt the machine if an error occurs.
232 (These mount options override the errors behavior
233 specified in the superblock, which can be configured
234 using tune2fs)
236 data_err=ignore(*) Just print an error message if an error occurs
237 in a file data buffer in ordered mode.
238 data_err=abort Abort the journal if an error occurs in a file
239 data buffer in ordered mode.
241 grpid Give objects the same group ID as their creator.
242 bsdgroups
244 nogrpid (*) New objects have the group ID of their creator.
245 sysvgroups
247 resgid=n The group ID which may use the reserved blocks.
249 resuid=n The user ID which may use the reserved blocks.
251 sb=n Use alternate superblock at this location.
253 quota These options are ignored by the filesystem. They
254 noquota are used only by quota tools to recognize volumes
255 grpquota where quota should be turned on. See documentation
256 usrquota in the quota-tools package for more details
257 (http://sourceforge.net/projects/linuxquota).
259 jqfmt=<quota type> These options tell filesystem details about quota
260 usrjquota=<file> so that quota information can be properly updated
261 grpjquota=<file> during journal replay. They replace the above
262 quota options. See documentation in the quota-tools
263 package for more details
264 (http://sourceforge.net/projects/linuxquota).
266 stripe=n Number of filesystem blocks that mballoc will try
267 to use for allocation size and alignment. For RAID5/6
268 systems this should be the number of data
269 disks * RAID chunk size in file system blocks.
271 delalloc (*) Defer block allocation until just before ext4
272 writes out the block(s) in question. This
273 allows ext4 to better allocation decisions
274 more efficiently.
275 nodelalloc Disable delayed allocation. Blocks are allocated
276 when the data is copied from userspace to the
277 page cache, either via the write(2) system call
278 or when an mmap'ed page which was previously
279 unallocated is written for the first time.
281 max_batch_time=usec Maximum amount of time ext4 should wait for
282 additional filesystem operations to be batch
283 together with a synchronous write operation.
284 Since a synchronous write operation is going to
285 force a commit and then a wait for the I/O
286 complete, it doesn't cost much, and can be a
287 huge throughput win, we wait for a small amount
288 of time to see if any other transactions can
289 piggyback on the synchronous write. The
290 algorithm used is designed to automatically tune
291 for the speed of the disk, by measuring the
292 amount of time (on average) that it takes to
293 finish committing a transaction. Call this time
294 the "commit time". If the time that the
295 transaction has been running is less than the
296 commit time, ext4 will try sleeping for the
297 commit time to see if other operations will join
298 the transaction. The commit time is capped by
299 the max_batch_time, which defaults to 15000us
300 (15ms). This optimization can be turned off
301 entirely by setting max_batch_time to 0.
303 min_batch_time=usec This parameter sets the commit time (as
304 described above) to be at least min_batch_time.
305 It defaults to zero microseconds. Increasing
306 this parameter may improve the throughput of
307 multi-threaded, synchronous workloads on very
308 fast disks, at the cost of increasing latency.
310 journal_ioprio=prio The I/O priority (from 0 to 7, where 0 is the
311 highest priorty) which should be used for I/O
312 operations submitted by kjournald2 during a
313 commit operation. This defaults to 3, which is
314 a slightly higher priority than the default I/O
315 priority.
317 auto_da_alloc(*) Many broken applications don't use fsync() when
318 noauto_da_alloc replacing existing files via patterns such as
319 fd = open("foo.new")/write(fd,..)/close(fd)/
320 rename("foo.new", "foo"), or worse yet,
321 fd = open("foo", O_TRUNC)/write(fd,..)/close(fd).
322 If auto_da_alloc is enabled, ext4 will detect
323 the replace-via-rename and replace-via-truncate
324 patterns and force that any delayed allocation
325 blocks are allocated such that at the next
326 journal commit, in the default data=ordered
327 mode, the data blocks of the new file are forced
328 to disk before the rename() operation is
329 committed. This provides roughly the same level
330 of guarantees as ext3, and avoids the
331 "zero-length" problem that can happen when a
332 system crashes before the delayed allocation
333 blocks are forced to disk.
335 noinit_itable Do not initialize any uninitialized inode table
336 blocks in the background. This feature may be
337 used by installation CD's so that the install
338 process can complete as quickly as possible; the
339 inode table initialization process would then be
340 deferred until the next time the file system
341 is unmounted.
343 init_itable=n The lazy itable init code will wait n times the
344 number of milliseconds it took to zero out the
345 previous block group's inode table. This
346 minimizes the impact on the systme performance
347 while file system's inode table is being initialized.
349 discard Controls whether ext4 should issue discard/TRIM
350 nodiscard(*) commands to the underlying block device when
351 blocks are freed. This is useful for SSD devices
352 and sparse/thinly-provisioned LUNs, but it is off
353 by default until sufficient testing has been done.
355 nouid32 Disables 32-bit UIDs and GIDs. This is for
356 interoperability with older kernels which only
357 store and expect 16-bit values.
359 resize Allows to resize filesystem to the end of the last
360 existing block group, further resize has to be done
361 with resize2fs either online, or offline. It can be
362 used only with conjunction with remount.
364 block_validity This options allows to enables/disables the in-kernel
365 noblock_validity facility for tracking filesystem metadata blocks
366 within internal data structures. This allows multi-
367 block allocator and other routines to quickly locate
368 extents which might overlap with filesystem metadata
369 blocks. This option is intended for debugging
370 purposes and since it negatively affects the
371 performance, it is off by default.
373 dioread_lock Controls whether or not ext4 should use the DIO read
374 dioread_nolock locking. If the dioread_nolock option is specified
375 ext4 will allocate uninitialized extent before buffer
376 write and convert the extent to initialized after IO
377 completes. This approach allows ext4 code to avoid
378 using inode mutex, which improves scalability on high
379 speed storages. However this does not work with
380 data journaling and dioread_nolock option will be
381 ignored with kernel warning. Note that dioread_nolock
382 code path is only used for extent-based files.
383 Because of the restrictions this options comprises
384 it is off by default (e.g. dioread_lock).
386 i_version Enable 64-bit inode version support. This option is
387 off by default.
389 Data Mode
390 =========
391 There are 3 different data modes:
393 * writeback mode
394 In data=writeback mode, ext4 does not journal data at all. This mode provides
395 a similar level of journaling as that of XFS, JFS, and ReiserFS in its default
396 mode - metadata journaling. A crash+recovery can cause incorrect data to
397 appear in files which were written shortly before the crash. This mode will
398 typically provide the best ext4 performance.
400 * ordered mode
401 In data=ordered mode, ext4 only officially journals metadata, but it logically
402 groups metadata information related to data changes with the data blocks into a
403 single unit called a transaction. When it's time to write the new metadata
404 out to disk, the associated data blocks are written first. In general,
405 this mode performs slightly slower than writeback but significantly faster than journal mode.
407 * journal mode
408 data=journal mode provides full data and metadata journaling. All new data is
409 written to the journal first, and then to its final location.
410 In the event of a crash, the journal can be replayed, bringing both data and
411 metadata into a consistent state. This mode is the slowest except when data
412 needs to be read from and written to disk at the same time where it
413 outperforms all others modes. Enabling this mode will disable delayed
414 allocation and O_DIRECT support.
416 /proc entries
417 =============
419 Information about mounted ext4 file systems can be found in
420 /proc/fs/ext4. Each mounted filesystem will have a directory in
421 /proc/fs/ext4 based on its device name (i.e., /proc/fs/ext4/hdc or
422 /proc/fs/ext4/dm-0). The files in each per-device directory are shown
423 in table below.
425 Files in /proc/fs/ext4/<devname>
426 ..............................................................................
427 File Content
428 mb_groups details of multiblock allocator buddy cache of free blocks
429 ..............................................................................
431 /sys entries
432 ============
434 Information about mounted ext4 file systems can be found in
435 /sys/fs/ext4. Each mounted filesystem will have a directory in
436 /sys/fs/ext4 based on its device name (i.e., /sys/fs/ext4/hdc or
437 /sys/fs/ext4/dm-0). The files in each per-device directory are shown
438 in table below.
440 Files in /sys/fs/ext4/<devname>
441 (see also Documentation/ABI/testing/sysfs-fs-ext4)
442 ..............................................................................
443 File Content
445 delayed_allocation_blocks This file is read-only and shows the number of
446 blocks that are dirty in the page cache, but
447 which do not have their location in the
448 filesystem allocated yet.
450 inode_goal Tuning parameter which (if non-zero) controls
451 the goal inode used by the inode allocator in
452 preference to all other allocation heuristics.
453 This is intended for debugging use only, and
454 should be 0 on production systems.
456 inode_readahead_blks Tuning parameter which controls the maximum
457 number of inode table blocks that ext4's inode
458 table readahead algorithm will pre-read into
459 the buffer cache
461 lifetime_write_kbytes This file is read-only and shows the number of
462 kilobytes of data that have been written to this
463 filesystem since it was created.
465 max_writeback_mb_bump The maximum number of megabytes the writeback
466 code will try to write out before move on to
467 another inode.
469 mb_group_prealloc The multiblock allocator will round up allocation
470 requests to a multiple of this tuning parameter if
471 the stripe size is not set in the ext4 superblock
473 mb_max_to_scan The maximum number of extents the multiblock
474 allocator will search to find the best extent
476 mb_min_to_scan The minimum number of extents the multiblock
477 allocator will search to find the best extent
479 mb_order2_req Tuning parameter which controls the minimum size
480 for requests (as a power of 2) where the buddy
481 cache is used
483 mb_stats Controls whether the multiblock allocator should
484 collect statistics, which are shown during the
485 unmount. 1 means to collect statistics, 0 means
486 not to collect statistics
488 mb_stream_req Files which have fewer blocks than this tunable
489 parameter will have their blocks allocated out
490 of a block group specific preallocation pool, so
491 that small files are packed closely together.
492 Each large file will have its blocks allocated
493 out of its own unique preallocation pool.
495 session_write_kbytes This file is read-only and shows the number of
496 kilobytes of data that have been written to this
497 filesystem since it was mounted.
498 ..............................................................................
500 Ioctls
501 ======
503 There is some Ext4 specific functionality which can be accessed by applications
504 through the system call interfaces. The list of all Ext4 specific ioctls are
505 shown in the table below.
507 Table of Ext4 specific ioctls
508 ..............................................................................
509 Ioctl Description
510 EXT4_IOC_GETFLAGS Get additional attributes associated with inode.
511 The ioctl argument is an integer bitfield, with
512 bit values described in ext4.h. This ioctl is an
513 alias for FS_IOC_GETFLAGS.
515 EXT4_IOC_SETFLAGS Set additional attributes associated with inode.
516 The ioctl argument is an integer bitfield, with
517 bit values described in ext4.h. This ioctl is an
518 alias for FS_IOC_SETFLAGS.
520 EXT4_IOC_GETVERSION
521 EXT4_IOC_GETVERSION_OLD
522 Get the inode i_generation number stored for
523 each inode. The i_generation number is normally
524 changed only when new inode is created and it is
525 particularly useful for network filesystems. The
526 '_OLD' version of this ioctl is an alias for
527 FS_IOC_GETVERSION.
529 EXT4_IOC_SETVERSION
530 EXT4_IOC_SETVERSION_OLD
531 Set the inode i_generation number stored for
532 each inode. The '_OLD' version of this ioctl
533 is an alias for FS_IOC_SETVERSION.
535 EXT4_IOC_GROUP_EXTEND This ioctl has the same purpose as the resize
536 mount option. It allows to resize filesystem
537 to the end of the last existing block group,
538 further resize has to be done with resize2fs,
539 either online, or offline. The argument points
540 to the unsigned logn number representing the
541 filesystem new block count.
543 EXT4_IOC_MOVE_EXT Move the block extents from orig_fd (the one
544 this ioctl is pointing to) to the donor_fd (the
545 one specified in move_extent structure passed
546 as an argument to this ioctl). Then, exchange
547 inode metadata between orig_fd and donor_fd.
548 This is especially useful for online
549 defragmentation, because the allocator has the
550 opportunity to allocate moved blocks better,
551 ideally into one contiguous extent.
553 EXT4_IOC_GROUP_ADD Add a new group descriptor to an existing or
554 new group descriptor block. The new group
555 descriptor is described by ext4_new_group_input
556 structure, which is passed as an argument to
557 this ioctl. This is especially useful in
558 conjunction with EXT4_IOC_GROUP_EXTEND,
559 which allows online resize of the filesystem
560 to the end of the last existing block group.
561 Those two ioctls combined is used in userspace
562 online resize tool (e.g. resize2fs).
564 EXT4_IOC_MIGRATE This ioctl operates on the filesystem itself.
565 It converts (migrates) ext3 indirect block mapped
566 inode to ext4 extent mapped inode by walking
567 through indirect block mapping of the original
568 inode and converting contiguous block ranges
569 into ext4 extents of the temporary inode. Then,
570 inodes are swapped. This ioctl might help, when
571 migrating from ext3 to ext4 filesystem, however
572 suggestion is to create fresh ext4 filesystem
573 and copy data from the backup. Note, that
574 filesystem has to support extents for this ioctl
575 to work.
577 EXT4_IOC_ALLOC_DA_BLKS Force all of the delay allocated blocks to be
578 allocated to preserve application-expected ext3
579 behaviour. Note that this will also start
580 triggering a write of the data blocks, but this
581 behaviour may change in the future as it is
582 not necessary and has been done this way only
583 for sake of simplicity.
585 EXT4_IOC_RESIZE_FS Resize the filesystem to a new size. The number
586 of blocks of resized filesystem is passed in via
587 64 bit integer argument. The kernel allocates
588 bitmaps and inode table, the userspace tool thus
589 just passes the new number of blocks.
591 ..............................................................................
593 References
594 ==========
596 kernel source: <file:fs/ext4/>
597 <file:fs/jbd2/>
599 programs: http://e2fsprogs.sourceforge.net/
601 useful links: http://fedoraproject.org/wiki/ext3-devel
602 http://www.bullopensource.org/ext4/
603 http://ext4.wiki.kernel.org/index.php/Main_Page
604 http://fedoraproject.org/wiki/Features/Ext4