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
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
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
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
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.
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
97 * Inode allocation using large virtual block groups via flex_bg
99 * large block (up to pagesize) support
100 * efficient new ordered mode in JBD2 and ext4(avoid using buffer head to force
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
128 When mounting an ext4 filesystem, the following option are accepted:
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'
147 journal_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
153 norecovery Don't load the journal on mounting. Note that
154 noload 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.
159 data=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
164 data=ordered (*) All data are forced directly out to the main file
165 system prior to its metadata being committed to the
168 data=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.
172 commit=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
184 barrier=<0|1(*)> This enables/disables the use of write barriers in
185 barrier(*) the jbd code. barrier=0 disables, barrier=1 enables.
186 nobarrier 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.
198 inode_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.
203 nouser_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.
210 noacl 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
217 bsddf (*) Make 'df' act like BSD.
218 minixdf Make 'df' act like Minix.
220 debug Extra debugging information is sent to syslog.
222 abort Simulate the effects of calling ext4_abort() for
223 debugging purposes. This is normally used while
224 remounting a filesystem which is already mounted.
226 errors=remount-ro Remount the filesystem read-only on an error.
227 errors=continue Keep going on a filesystem error.
228 errors=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
233 data_err=ignore(*) Just print an error message if an error occurs
234 in a file data buffer in ordered mode.
235 data_err=abort Abort the journal if an error occurs in a file
236 data buffer in ordered mode.
238 grpid Give objects the same group ID as their creator.
241 nogrpid (*) New objects have the group ID of their creator.
244 resgid=n The group ID which may use the reserved blocks.
246 resuid=n The user ID which may use the reserved blocks.
248 sb=n Use alternate superblock at this location.
250 quota These options are ignored by the filesystem. They
251 noquota are used only by quota tools to recognize volumes
252 grpquota where quota should be turned on. See documentation
253 usrquota in the quota-tools package for more details
254 (http://sourceforge.net/projects/linuxquota).
256 jqfmt=<quota type> These options tell filesystem details about quota
257 usrjquota=<file> so that quota information can be properly updated
258 grpjquota=<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).
263 stripe=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.
268 delalloc (*) Defer block allocation until just before ext4
269 writes out the block(s) in question. This
270 allows ext4 to better allocation decisions
272 nodelalloc 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.
278 max_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.
300 min_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.
307 journal_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
314 auto_da_alloc(*) Many broken applications don't use fsync() when
315 noauto_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.
332 noinit_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
340 init_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.
346 discard Controls whether ext4 should issue discard/TRIM
347 nodiscard(*) 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.
352 nouid32 Disables 32-bit UIDs and GIDs. This is for
353 interoperability with older kernels which only
354 store and expect 16-bit values.
356 block_validity This options allows to enables/disables the in-kernel
357 noblock_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.
365 dioread_lock Controls whether or not ext4 should use the DIO read
366 dioread_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).
378 i_version Enable 64-bit inode version support. This option is
383 There are 3 different data modes:
386 In data=writeback mode, ext4 does not journal data at all. This mode provides
387 a similar level of journaling as that of XFS, JFS, and ReiserFS in its default
388 mode - metadata journaling. A crash+recovery can cause incorrect data to
389 appear in files which were written shortly before the crash. This mode will
390 typically provide the best ext4 performance.
393 In data=ordered mode, ext4 only officially journals metadata, but it logically
394 groups metadata information related to data changes with the data blocks into a
395 single unit called a transaction. When it's time to write the new metadata
396 out to disk, the associated data blocks are written first. In general,
397 this mode performs slightly slower than writeback but significantly faster than journal mode.
400 data=journal mode provides full data and metadata journaling. All new data is
401 written to the journal first, and then to its final location.
402 In the event of a crash, the journal can be replayed, bringing both data and
403 metadata into a consistent state. This mode is the slowest except when data
404 needs to be read from and written to disk at the same time where it
405 outperforms all others modes. Enabling this mode will disable delayed
406 allocation and O_DIRECT support.
411 Information 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
417 Files in /proc/fs/ext4/<devname>
418 ..............................................................................
420 mb_groups details of multiblock allocator buddy cache of free blocks
421 ..............................................................................
426 Information 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
432 Files in /sys/fs/ext4/<devname>
433 (see also Documentation/ABI/testing/sysfs-fs-ext4)
434 ..............................................................................
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.
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.
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
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.
457 max_writeback_mb_bump The maximum number of megabytes the writeback
458 code will try to write out before move on to
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
465 mb_max_to_scan The maximum number of extents the multiblock
466 allocator will search to find the best extent
468 mb_min_to_scan The minimum number of extents the multiblock
469 allocator will search to find the best extent
471 mb_order2_req Tuning parameter which controls the minimum size
472 for requests (as a power of 2) where the buddy
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
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.
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 ..............................................................................
495 There is some Ext4 specific functionality which can be accessed by applications
496 through the system call interfaces. The list of all Ext4 specific ioctls are
497 shown in the table below.
499 Table of Ext4 specific ioctls
500 ..............................................................................
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.
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.
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
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.
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.
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.
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).
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
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.
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.
583 ..............................................................................
588 kernel source: <file:fs/ext4/>
591 programs: http://e2fsprogs.sourceforge.net/
593 useful 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
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