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Dave Kleikampfc513a32006-10-11 01:21:25 -07001
2Ext4 Filesystem
3===============
4
Diego Calleja22359f52008-10-17 09:15:14 -04005Ext4 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.
Dave Kleikampfc513a32006-10-11 01:21:25 -07009
Diego Calleja22359f52008-10-17 09:15:14 -040010Mailing list: linux-ext4@vger.kernel.org
11Web site: http://ext4.wiki.kernel.org
Dave Kleikampfc513a32006-10-11 01:21:25 -070012
13
141. Quick usage instructions:
15===========================
16
Diego Calleja22359f52008-10-17 09:15:14 -040017Note: 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
Jose R. Santos93e32702008-07-11 19:27:31 -040021 - Compile and install the latest version of e2fsprogs (as of this
Diego Calleja22359f52008-10-17 09:15:14 -040022 writing version 1.41.3) from:
Jose R. Santos93e32702008-07-11 19:27:31 -040023
24 http://sourceforge.net/project/showfiles.php?group_id=2406
25
26 or
27
Dave Kleikampfc513a32006-10-11 01:21:25 -070028 ftp://ftp.kernel.org/pub/linux/kernel/people/tytso/e2fsprogs/
29
Jose R. Santos93e32702008-07-11 19:27:31 -040030 or grab the latest git repository from:
Dave Kleikampfc513a32006-10-11 01:21:25 -070031
Jose R. Santos93e32702008-07-11 19:27:31 -040032 git://git.kernel.org/pub/scm/fs/ext2/e2fsprogs.git
Dave Kleikampfc513a32006-10-11 01:21:25 -070033
Theodore Ts'o45373982008-07-27 19:59:21 -040034 - 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
Theodore Ts'o03010a32008-10-10 20:02:48 -040040 - Create a new filesystem using the ext4 filesystem type:
Dave Kleikampfc513a32006-10-11 01:21:25 -070041
Theodore Ts'o03010a32008-10-10 20:02:48 -040042 # mke2fs -t ext4 /dev/hda1
Dave Kleikampfc513a32006-10-11 01:21:25 -070043
Diego Calleja22359f52008-10-17 09:15:14 -040044 Or to configure an existing ext3 filesystem to support extents:
Dave Kleikampfc513a32006-10-11 01:21:25 -070045
Diego Calleja22359f52008-10-17 09:15:14 -040046 # tune2fs -O extents /dev/hda1
Jose R. Santos93e32702008-07-11 19:27:31 -040047
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
Theodore Ts'o03010a32008-10-10 20:02:48 -040053 (Note: we currently do not have tools to convert an ext4
Jose R. Santos93e32702008-07-11 19:27:31 -040054 filesystem back to ext3; so please do not do try this on production
55 filesystems.)
56
57 - Mounting:
58
Theodore Ts'o03010a32008-10-10 20:02:48 -040059 # mount -t ext4 /dev/hda1 /wherever
Dave Kleikampfc513a32006-10-11 01:21:25 -070060
Theodore Ts'o8e1a4852009-01-06 14:53:06 -050061 - 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
Lukas Czernerad434012011-06-07 12:27:05 +020071 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.
Dave Kleikampfc513a32006-10-11 01:21:25 -070077
782. Features
79===========
80
812.1 Currently available
82
Jose R. Santos93e32702008-07-11 19:27:31 -040083* ability to use filesystems > 16TB (e2fsprogs support not available yet)
Dave Kleikampfc513a32006-10-11 01:21:25 -070084* extent format reduces metadata overhead (RAM, IO for access, transactions)
85* extent format more robust in face of on-disk corruption due to magics,
Theodore Ts'o8e1a4852009-01-06 14:53:06 -050086* internal redundancy in tree
Mingming Cao49f14872008-07-11 19:27:31 -040087* improved file allocation (multi-block alloc)
Theodore Ts'o722bde62009-02-23 00:51:57 -050088* lift 32000 subdirectory limit imposed by i_links_count[1]
Jose R. Santos93e32702008-07-11 19:27:31 -040089* 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
Mingming Cao49f14872008-07-11 19:27:31 -040098* delayed allocation
99* large block (up to pagesize) support
Lucas De Marchi25985ed2011-03-30 22:57:33 -0300100* efficient new ordered mode in JBD2 and ext4(avoid using buffer head to force
Mingming Cao49f14872008-07-11 19:27:31 -0400101 the ordering)
Dave Kleikampfc513a32006-10-11 01:21:25 -0700102
Theodore Ts'o722bde62009-02-23 00:51:57 -0500103[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
Dave Kleikampfc513a32006-10-11 01:21:25 -07001062.2 Candidate features for future inclusion
107
Jose R. Santos93e32702008-07-11 19:27:31 -0400108* Online defrag (patches available but not well tested)
Lucas De Marchi25985ed2011-03-30 22:57:33 -0300109* reduced mke2fs time via lazy itable initialization in conjunction with
Jose R. Santos93e32702008-07-11 19:27:31 -0400110 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)
Dave Kleikampfc513a32006-10-11 01:21:25 -0700113
Jose R. Santos93e32702008-07-11 19:27:31 -0400114There 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.
Dave Kleikampfc513a32006-10-11 01:21:25 -0700118
Jose R. Santos93e32702008-07-11 19:27:31 -0400119The big performance win will come with mballoc, delalloc and flex_bg
120grouping of bitmaps and inode tables. Some test results available here:
Dave Kleikampfc513a32006-10-11 01:21:25 -0700121
Diego Calleja22359f52008-10-17 09:15:14 -0400122 - 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
Dave Kleikampfc513a32006-10-11 01:21:25 -0700124
1253. Options
126==========
127
128When mounting an ext4 filesystem, the following option are accepted:
129(*) == default
130
Theodore Ts'o8e1a4852009-01-06 14:53:06 -0500131ro 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
Linus Torvaldsd4da6c92009-11-02 10:15:27 -0800137journal_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
Girish Shilamkar818d2762008-01-28 23:58:27 -0500142journal_async_commit Commit block can be written to disk without waiting
143 for descriptor blocks. If enabled older kernels cannot
Linus Torvaldsd4da6c92009-11-02 10:15:27 -0800144 mount the device. This will enable 'journal_checksum'
145 internally.
Girish Shilamkar818d2762008-01-28 23:58:27 -0500146
Dave Kleikampfc513a32006-10-11 01:21:25 -0700147journal_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
Eric Sandeene3bb52a2009-11-19 14:28:50 -0500153norecovery Don't load the journal on mounting. Note that
154noload if the filesystem was not unmounted cleanly,
Theodore Ts'o8e1a4852009-01-06 14:53:06 -0500155 skipping the journal replay will lead to the
156 filesystem containing inconsistencies that can
157 lead to any number of problems.
Dave Kleikampfc513a32006-10-11 01:21:25 -0700158
159data=journal All data are committed into the journal prior to being
Theodore Ts'o56889782011-09-03 18:22:38 -0400160 written into the main file system. Enabling
161 this mode will disable delayed allocation and
162 O_DIRECT support.
Dave Kleikampfc513a32006-10-11 01:21:25 -0700163
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
Eric Sandeen571640c2008-05-26 12:29:46 -0400184barrier=<0|1(*)> This enables/disables the use of write barriers in
Theodore Ts'o06705bf2009-03-28 10:59:57 -0400185barrier(*) the jbd code. barrier=0 disables, barrier=1 enables.
186nobarrier This also requires an IO stack which can support
Eric Sandeen571640c2008-05-26 12:29:46 -0400187 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.
Theodore Ts'o06705bf2009-03-28 10:59:57 -0400194 The mount options "barrier" and "nobarrier" can
195 also be used to enable or disable barriers, for
196 consistency with other ext4 mount options.
Dave Kleikampfc513a32006-10-11 01:21:25 -0700197
Fang Wenqi6d3b82f2009-12-24 17:51:42 -0500198inode_readahead_blks=n This tuning parameter controls the maximum
Theodore Ts'o240799c2008-10-09 23:53:47 -0400199 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
Theodore Ts'oaf909a52011-10-08 14:01:08 -0400203nouser_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.
Dave Kleikampfc513a32006-10-11 01:21:25 -0700209
210noacl This option disables POSIX Access Control List
Theodore Ts'oaf909a52011-10-08 14:01:08 -0400211 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.
Dave Kleikampfc513a32006-10-11 01:21:25 -0700216
Dave Kleikampfc513a32006-10-11 01:21:25 -0700217bsddf (*) Make 'df' act like BSD.
218minixdf Make 'df' act like Minix.
219
Dave Kleikampfc513a32006-10-11 01:21:25 -0700220debug Extra debugging information is sent to syslog.
221
Theodore Ts'o8a8a2052009-06-13 10:08:59 -0400222abort 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
Theodore Ts'o8e1a4852009-01-06 14:53:06 -0500226errors=remount-ro Remount the filesystem read-only on an error.
Dave Kleikampfc513a32006-10-11 01:21:25 -0700227errors=continue Keep going on a filesystem error.
228errors=panic Panic and halt the machine if an error occurs.
Theodore Ts'o8e1a4852009-01-06 14:53:06 -0500229 (These mount options override the errors behavior
230 specified in the superblock, which can be configured
231 using tune2fs)
Dave Kleikampfc513a32006-10-11 01:21:25 -0700232
Hidehiro Kawai5bf56832008-10-10 22:12:43 -0400233data_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
Dave Kleikampfc513a32006-10-11 01:21:25 -0700238grpid 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
Jan Kara13588702009-09-18 12:22:29 -0400250quota 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).
Dave Kleikampfc513a32006-10-11 01:21:25 -0700262
Alex Tomasc9de5602008-01-29 00:19:52 -0500263stripe=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.
Jan Kara83653882009-09-29 15:59:34 -0400267
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.
Theodore Ts'o240799c2008-10-09 23:53:47 -0400277
Theodore Ts'o30773842009-01-03 20:27:38 -0500278max_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
Matt LaPlante19f59462009-04-27 15:06:31 +0200292 transaction has been running is less than the
Theodore Ts'o30773842009-01-03 20:27:38 -0500293 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
Theodore Ts'ob3881f72009-01-05 22:46:26 -0500307journal_ioprio=prio The I/O priority (from 0 to 7, where 0 is the
Masanari Iida40e47122012-03-04 23:16:11 +0900308 highest priority) which should be used for I/O
Theodore Ts'ob3881f72009-01-05 22:46:26 -0500309 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
Theodore Ts'o06705bf2009-03-28 10:59:57 -0400314auto_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
Matt LaPlante19f59462009-04-27 15:06:31 +0200326 committed. This provides roughly the same level
Theodore Ts'o06705bf2009-03-28 10:59:57 -0400327 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
Lukas Czernerbfff6872010-10-27 21:30:05 -0400332noinit_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
Masanari Iida40e47122012-03-04 23:16:11 +0900343 minimizes the impact on the system performance
Lukas Czernerbfff6872010-10-27 21:30:05 -0400344 while file system's inode table is being initialized.
345
Lukas Czerner6f9524e2011-02-21 20:16:21 -0500346discard Controls whether ext4 should issue discard/TRIM
Eric Sandeen5328e632009-11-19 14:25:42 -0500347nodiscard(*) 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
Lukas Czerner6f9524e2011-02-21 20:16:21 -0500352nouid32 Disables 32-bit UIDs and GIDs. This is for
353 interoperability with older kernels which only
354 store and expect 16-bit values.
355
Lukas Czerner6f9524e2011-02-21 20:16:21 -0500356block_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
Lukas Czernerad434012011-06-07 12:27:05 +0200371 speed storages. However this does not work with
Lukas Czerner6f9524e2011-02-21 20:16:21 -0500372 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
Dave Kleikampfc513a32006-10-11 01:21:25 -0700381Data Mode
Jose R. Santos93e32702008-07-11 19:27:31 -0400382=========
Dave Kleikampfc513a32006-10-11 01:21:25 -0700383There 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
Mingming Cao49f14872008-07-11 19:27:31 -0400394groups 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.
Dave Kleikampfc513a32006-10-11 01:21:25 -0700398
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
Theodore Ts'o56889782011-09-03 18:22:38 -0400405outperforms all others modes. Enabling this mode will disable delayed
406allocation and O_DIRECT support.
Dave Kleikampfc513a32006-10-11 01:21:25 -0700407
Lukas Czerner6f9524e2011-02-21 20:16:21 -0500408/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.
Yongqiang Yang19c52462012-01-04 17:09:44 -0500576
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
Lukas Czerner6f9524e2011-02-21 20:16:21 -0500583..............................................................................
584
Dave Kleikampfc513a32006-10-11 01:21:25 -0700585References
586==========
587
588kernel source: <file:fs/ext4/>
589 <file:fs/jbd2/>
590
591programs: http://e2fsprogs.sourceforge.net/
Dave Kleikampfc513a32006-10-11 01:21:25 -0700592
593useful links: http://fedoraproject.org/wiki/ext3-devel
594 http://www.bullopensource.org/ext4/
Jose R. Santos93e32702008-07-11 19:27:31 -0400595 http://ext4.wiki.kernel.org/index.php/Main_Page
596 http://fedoraproject.org/wiki/Features/Ext4