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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
71 data=writeback,nobh' can be faster for some workloads. (Note
72 however that running mounted with data=writeback can potentially
73 leave stale data exposed in recently written files in case of an
74 unclean shutdown, which could be a security exposure in some
75 situations.) Configuring the filesystem with a large journal can
76 also be helpful for 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=update Update the ext4 file system's journal to the current
148 format.
149
Dave Kleikampfc513a32006-10-11 01:21:25 -0700150journal_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.
155
Eric Sandeene3bb52a2009-11-19 14:28:50 -0500156norecovery Don't load the journal on mounting. Note that
157noload if the filesystem was not unmounted cleanly,
Theodore Ts'o8e1a4852009-01-06 14:53:06 -0500158 skipping the journal replay will lead to the
159 filesystem containing inconsistencies that can
160 lead to any number of problems.
Dave Kleikampfc513a32006-10-11 01:21:25 -0700161
162data=journal All data are committed into the journal prior to being
163 written into the main file system.
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
Eric Sandeen571640c2008-05-26 12:29:46 -0400185barrier=<0|1(*)> This enables/disables the use of write barriers in
Theodore Ts'o06705bf2009-03-28 10:59:57 -0400186barrier(*) the jbd code. barrier=0 disables, barrier=1 enables.
187nobarrier This also requires an IO stack which can support
Eric Sandeen571640c2008-05-26 12:29:46 -0400188 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.
Theodore Ts'o06705bf2009-03-28 10:59:57 -0400195 The mount options "barrier" and "nobarrier" can
196 also be used to enable or disable barriers, for
197 consistency with other ext4 mount options.
Dave Kleikampfc513a32006-10-11 01:21:25 -0700198
Fang Wenqi6d3b82f2009-12-24 17:51:42 -0500199inode_readahead_blks=n This tuning parameter controls the maximum
Theodore Ts'o240799c2008-10-09 23:53:47 -0400200 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
Dave Kleikampfc513a32006-10-11 01:21:25 -0700204orlov (*) This enables the new Orlov block allocator. It is
205 enabled by default.
206
207oldalloc This disables the Orlov block allocator and enables
208 the old block allocator. Orlov should have better
209 performance - we'd like to get some feedback if it's
210 the contrary for you.
211
212user_xattr Enables Extended User Attributes. Additionally, you
213 need to have extended attribute support enabled in the
214 kernel configuration (CONFIG_EXT4_FS_XATTR). See the
215 attr(5) manual page and http://acl.bestbits.at/ to
216 learn more about extended attributes.
217
218nouser_xattr Disables Extended User Attributes.
219
220acl Enables POSIX Access Control Lists support.
221 Additionally, you need to have ACL support enabled in
222 the kernel configuration (CONFIG_EXT4_FS_POSIX_ACL).
223 See the acl(5) manual page and http://acl.bestbits.at/
224 for more information.
225
226noacl This option disables POSIX Access Control List
227 support.
228
Dave Kleikampfc513a32006-10-11 01:21:25 -0700229bsddf (*) Make 'df' act like BSD.
230minixdf Make 'df' act like Minix.
231
Dave Kleikampfc513a32006-10-11 01:21:25 -0700232debug Extra debugging information is sent to syslog.
233
Theodore Ts'o8a8a2052009-06-13 10:08:59 -0400234abort Simulate the effects of calling ext4_abort() for
235 debugging purposes. This is normally used while
236 remounting a filesystem which is already mounted.
237
Theodore Ts'o8e1a4852009-01-06 14:53:06 -0500238errors=remount-ro Remount the filesystem read-only on an error.
Dave Kleikampfc513a32006-10-11 01:21:25 -0700239errors=continue Keep going on a filesystem error.
240errors=panic Panic and halt the machine if an error occurs.
Theodore Ts'o8e1a4852009-01-06 14:53:06 -0500241 (These mount options override the errors behavior
242 specified in the superblock, which can be configured
243 using tune2fs)
Dave Kleikampfc513a32006-10-11 01:21:25 -0700244
Hidehiro Kawai5bf56832008-10-10 22:12:43 -0400245data_err=ignore(*) Just print an error message if an error occurs
246 in a file data buffer in ordered mode.
247data_err=abort Abort the journal if an error occurs in a file
248 data buffer in ordered mode.
249
Dave Kleikampfc513a32006-10-11 01:21:25 -0700250grpid Give objects the same group ID as their creator.
251bsdgroups
252
253nogrpid (*) New objects have the group ID of their creator.
254sysvgroups
255
256resgid=n The group ID which may use the reserved blocks.
257
258resuid=n The user ID which may use the reserved blocks.
259
260sb=n Use alternate superblock at this location.
261
Jan Kara13588702009-09-18 12:22:29 -0400262quota These options are ignored by the filesystem. They
263noquota are used only by quota tools to recognize volumes
264grpquota where quota should be turned on. See documentation
265usrquota in the quota-tools package for more details
266 (http://sourceforge.net/projects/linuxquota).
267
268jqfmt=<quota type> These options tell filesystem details about quota
269usrjquota=<file> so that quota information can be properly updated
270grpjquota=<file> during journal replay. They replace the above
271 quota options. See documentation in the quota-tools
272 package for more details
273 (http://sourceforge.net/projects/linuxquota).
Dave Kleikampfc513a32006-10-11 01:21:25 -0700274
275bh (*) ext4 associates buffer heads to data pages to
276nobh (a) cache disk block mapping information
277 (b) link pages into transaction to provide
278 ordering guarantees.
279 "bh" option forces use of buffer heads.
280 "nobh" option tries to avoid associating buffer
281 heads (supported only for "writeback" mode).
282
Alex Tomasc9de5602008-01-29 00:19:52 -0500283stripe=n Number of filesystem blocks that mballoc will try
284 to use for allocation size and alignment. For RAID5/6
285 systems this should be the number of data
286 disks * RAID chunk size in file system blocks.
Jan Kara83653882009-09-29 15:59:34 -0400287
288delalloc (*) Defer block allocation until just before ext4
289 writes out the block(s) in question. This
290 allows ext4 to better allocation decisions
291 more efficiently.
292nodelalloc Disable delayed allocation. Blocks are allocated
293 when the data is copied from userspace to the
294 page cache, either via the write(2) system call
295 or when an mmap'ed page which was previously
296 unallocated is written for the first time.
Theodore Ts'o240799c2008-10-09 23:53:47 -0400297
Theodore Ts'o30773842009-01-03 20:27:38 -0500298max_batch_time=usec Maximum amount of time ext4 should wait for
299 additional filesystem operations to be batch
300 together with a synchronous write operation.
301 Since a synchronous write operation is going to
302 force a commit and then a wait for the I/O
303 complete, it doesn't cost much, and can be a
304 huge throughput win, we wait for a small amount
305 of time to see if any other transactions can
306 piggyback on the synchronous write. The
307 algorithm used is designed to automatically tune
308 for the speed of the disk, by measuring the
309 amount of time (on average) that it takes to
310 finish committing a transaction. Call this time
311 the "commit time". If the time that the
Matt LaPlante19f59462009-04-27 15:06:31 +0200312 transaction has been running is less than the
Theodore Ts'o30773842009-01-03 20:27:38 -0500313 commit time, ext4 will try sleeping for the
314 commit time to see if other operations will join
315 the transaction. The commit time is capped by
316 the max_batch_time, which defaults to 15000us
317 (15ms). This optimization can be turned off
318 entirely by setting max_batch_time to 0.
319
320min_batch_time=usec This parameter sets the commit time (as
321 described above) to be at least min_batch_time.
322 It defaults to zero microseconds. Increasing
323 this parameter may improve the throughput of
324 multi-threaded, synchronous workloads on very
325 fast disks, at the cost of increasing latency.
326
Theodore Ts'ob3881f72009-01-05 22:46:26 -0500327journal_ioprio=prio The I/O priority (from 0 to 7, where 0 is the
328 highest priorty) which should be used for I/O
329 operations submitted by kjournald2 during a
330 commit operation. This defaults to 3, which is
331 a slightly higher priority than the default I/O
332 priority.
333
Theodore Ts'o06705bf2009-03-28 10:59:57 -0400334auto_da_alloc(*) Many broken applications don't use fsync() when
335noauto_da_alloc replacing existing files via patterns such as
336 fd = open("foo.new")/write(fd,..)/close(fd)/
337 rename("foo.new", "foo"), or worse yet,
338 fd = open("foo", O_TRUNC)/write(fd,..)/close(fd).
339 If auto_da_alloc is enabled, ext4 will detect
340 the replace-via-rename and replace-via-truncate
341 patterns and force that any delayed allocation
342 blocks are allocated such that at the next
343 journal commit, in the default data=ordered
344 mode, the data blocks of the new file are forced
345 to disk before the rename() operation is
Matt LaPlante19f59462009-04-27 15:06:31 +0200346 committed. This provides roughly the same level
Theodore Ts'o06705bf2009-03-28 10:59:57 -0400347 of guarantees as ext3, and avoids the
348 "zero-length" problem that can happen when a
349 system crashes before the delayed allocation
350 blocks are forced to disk.
351
Lukas Czernerbfff6872010-10-27 21:30:05 -0400352noinit_itable Do not initialize any uninitialized inode table
353 blocks in the background. This feature may be
354 used by installation CD's so that the install
355 process can complete as quickly as possible; the
356 inode table initialization process would then be
357 deferred until the next time the file system
358 is unmounted.
359
360init_itable=n The lazy itable init code will wait n times the
361 number of milliseconds it took to zero out the
362 previous block group's inode table. This
363 minimizes the impact on the systme performance
364 while file system's inode table is being initialized.
365
Lukas Czerner6f9524e2011-02-21 20:16:21 -0500366discard Controls whether ext4 should issue discard/TRIM
Eric Sandeen5328e632009-11-19 14:25:42 -0500367nodiscard(*) commands to the underlying block device when
368 blocks are freed. This is useful for SSD devices
369 and sparse/thinly-provisioned LUNs, but it is off
370 by default until sufficient testing has been done.
371
Lukas Czerner6f9524e2011-02-21 20:16:21 -0500372nouid32 Disables 32-bit UIDs and GIDs. This is for
373 interoperability with older kernels which only
374 store and expect 16-bit values.
375
376resize Allows to resize filesystem to the end of the last
377 existing block group, further resize has to be done
378 with resize2fs either online, or offline. It can be
379 used only with conjunction with remount.
380
381block_validity This options allows to enables/disables the in-kernel
382noblock_validity facility for tracking filesystem metadata blocks
383 within internal data structures. This allows multi-
384 block allocator and other routines to quickly locate
385 extents which might overlap with filesystem metadata
386 blocks. This option is intended for debugging
387 purposes and since it negatively affects the
388 performance, it is off by default.
389
390dioread_lock Controls whether or not ext4 should use the DIO read
391dioread_nolock locking. If the dioread_nolock option is specified
392 ext4 will allocate uninitialized extent before buffer
393 write and convert the extent to initialized after IO
394 completes. This approach allows ext4 code to avoid
395 using inode mutex, which improves scalability on high
396 speed storages. However this does not work with nobh
397 option and the mount will fail. Nor does it work with
398 data journaling and dioread_nolock option will be
399 ignored with kernel warning. Note that dioread_nolock
400 code path is only used for extent-based files.
401 Because of the restrictions this options comprises
402 it is off by default (e.g. dioread_lock).
403
404i_version Enable 64-bit inode version support. This option is
405 off by default.
406
Dave Kleikampfc513a32006-10-11 01:21:25 -0700407Data Mode
Jose R. Santos93e32702008-07-11 19:27:31 -0400408=========
Dave Kleikampfc513a32006-10-11 01:21:25 -0700409There are 3 different data modes:
410
411* writeback mode
412In data=writeback mode, ext4 does not journal data at all. This mode provides
413a similar level of journaling as that of XFS, JFS, and ReiserFS in its default
414mode - metadata journaling. A crash+recovery can cause incorrect data to
415appear in files which were written shortly before the crash. This mode will
416typically provide the best ext4 performance.
417
418* ordered mode
419In data=ordered mode, ext4 only officially journals metadata, but it logically
Mingming Cao49f14872008-07-11 19:27:31 -0400420groups metadata information related to data changes with the data blocks into a
421single unit called a transaction. When it's time to write the new metadata
422out to disk, the associated data blocks are written first. In general,
423this mode performs slightly slower than writeback but significantly faster than journal mode.
Dave Kleikampfc513a32006-10-11 01:21:25 -0700424
425* journal mode
426data=journal mode provides full data and metadata journaling. All new data is
427written to the journal first, and then to its final location.
428In the event of a crash, the journal can be replayed, bringing both data and
429metadata into a consistent state. This mode is the slowest except when data
430needs to be read from and written to disk at the same time where it
Matt LaPlante19f59462009-04-27 15:06:31 +0200431outperforms all others modes. Currently ext4 does not have delayed
Mingming Cao49f14872008-07-11 19:27:31 -0400432allocation support if this data journalling mode is selected.
Dave Kleikampfc513a32006-10-11 01:21:25 -0700433
Lukas Czerner6f9524e2011-02-21 20:16:21 -0500434/proc entries
435=============
436
437Information about mounted ext4 file systems can be found in
438/proc/fs/ext4. Each mounted filesystem will have a directory in
439/proc/fs/ext4 based on its device name (i.e., /proc/fs/ext4/hdc or
440/proc/fs/ext4/dm-0). The files in each per-device directory are shown
441in table below.
442
443Files in /proc/fs/ext4/<devname>
444..............................................................................
445 File Content
446 mb_groups details of multiblock allocator buddy cache of free blocks
447..............................................................................
448
449/sys entries
450============
451
452Information about mounted ext4 file systems can be found in
453/sys/fs/ext4. Each mounted filesystem will have a directory in
454/sys/fs/ext4 based on its device name (i.e., /sys/fs/ext4/hdc or
455/sys/fs/ext4/dm-0). The files in each per-device directory are shown
456in table below.
457
458Files in /sys/fs/ext4/<devname>
459(see also Documentation/ABI/testing/sysfs-fs-ext4)
460..............................................................................
461 File Content
462
463 delayed_allocation_blocks This file is read-only and shows the number of
464 blocks that are dirty in the page cache, but
465 which do not have their location in the
466 filesystem allocated yet.
467
468 inode_goal Tuning parameter which (if non-zero) controls
469 the goal inode used by the inode allocator in
470 preference to all other allocation heuristics.
471 This is intended for debugging use only, and
472 should be 0 on production systems.
473
474 inode_readahead_blks Tuning parameter which controls the maximum
475 number of inode table blocks that ext4's inode
476 table readahead algorithm will pre-read into
477 the buffer cache
478
479 lifetime_write_kbytes This file is read-only and shows the number of
480 kilobytes of data that have been written to this
481 filesystem since it was created.
482
483 max_writeback_mb_bump The maximum number of megabytes the writeback
484 code will try to write out before move on to
485 another inode.
486
487 mb_group_prealloc The multiblock allocator will round up allocation
488 requests to a multiple of this tuning parameter if
489 the stripe size is not set in the ext4 superblock
490
491 mb_max_to_scan The maximum number of extents the multiblock
492 allocator will search to find the best extent
493
494 mb_min_to_scan The minimum number of extents the multiblock
495 allocator will search to find the best extent
496
497 mb_order2_req Tuning parameter which controls the minimum size
498 for requests (as a power of 2) where the buddy
499 cache is used
500
501 mb_stats Controls whether the multiblock allocator should
502 collect statistics, which are shown during the
503 unmount. 1 means to collect statistics, 0 means
504 not to collect statistics
505
506 mb_stream_req Files which have fewer blocks than this tunable
507 parameter will have their blocks allocated out
508 of a block group specific preallocation pool, so
509 that small files are packed closely together.
510 Each large file will have its blocks allocated
511 out of its own unique preallocation pool.
512
513 session_write_kbytes This file is read-only and shows the number of
514 kilobytes of data that have been written to this
515 filesystem since it was mounted.
516..............................................................................
517
518Ioctls
519======
520
521There is some Ext4 specific functionality which can be accessed by applications
522through the system call interfaces. The list of all Ext4 specific ioctls are
523shown in the table below.
524
525Table of Ext4 specific ioctls
526..............................................................................
527 Ioctl Description
528 EXT4_IOC_GETFLAGS Get additional attributes associated with inode.
529 The ioctl argument is an integer bitfield, with
530 bit values described in ext4.h. This ioctl is an
531 alias for FS_IOC_GETFLAGS.
532
533 EXT4_IOC_SETFLAGS Set additional attributes associated with inode.
534 The ioctl argument is an integer bitfield, with
535 bit values described in ext4.h. This ioctl is an
536 alias for FS_IOC_SETFLAGS.
537
538 EXT4_IOC_GETVERSION
539 EXT4_IOC_GETVERSION_OLD
540 Get the inode i_generation number stored for
541 each inode. The i_generation number is normally
542 changed only when new inode is created and it is
543 particularly useful for network filesystems. The
544 '_OLD' version of this ioctl is an alias for
545 FS_IOC_GETVERSION.
546
547 EXT4_IOC_SETVERSION
548 EXT4_IOC_SETVERSION_OLD
549 Set the inode i_generation number stored for
550 each inode. The '_OLD' version of this ioctl
551 is an alias for FS_IOC_SETVERSION.
552
553 EXT4_IOC_GROUP_EXTEND This ioctl has the same purpose as the resize
554 mount option. It allows to resize filesystem
555 to the end of the last existing block group,
556 further resize has to be done with resize2fs,
557 either online, or offline. The argument points
558 to the unsigned logn number representing the
559 filesystem new block count.
560
561 EXT4_IOC_MOVE_EXT Move the block extents from orig_fd (the one
562 this ioctl is pointing to) to the donor_fd (the
563 one specified in move_extent structure passed
564 as an argument to this ioctl). Then, exchange
565 inode metadata between orig_fd and donor_fd.
566 This is especially useful for online
567 defragmentation, because the allocator has the
568 opportunity to allocate moved blocks better,
569 ideally into one contiguous extent.
570
571 EXT4_IOC_GROUP_ADD Add a new group descriptor to an existing or
572 new group descriptor block. The new group
573 descriptor is described by ext4_new_group_input
574 structure, which is passed as an argument to
575 this ioctl. This is especially useful in
576 conjunction with EXT4_IOC_GROUP_EXTEND,
577 which allows online resize of the filesystem
578 to the end of the last existing block group.
579 Those two ioctls combined is used in userspace
580 online resize tool (e.g. resize2fs).
581
582 EXT4_IOC_MIGRATE This ioctl operates on the filesystem itself.
583 It converts (migrates) ext3 indirect block mapped
584 inode to ext4 extent mapped inode by walking
585 through indirect block mapping of the original
586 inode and converting contiguous block ranges
587 into ext4 extents of the temporary inode. Then,
588 inodes are swapped. This ioctl might help, when
589 migrating from ext3 to ext4 filesystem, however
590 suggestion is to create fresh ext4 filesystem
591 and copy data from the backup. Note, that
592 filesystem has to support extents for this ioctl
593 to work.
594
595 EXT4_IOC_ALLOC_DA_BLKS Force all of the delay allocated blocks to be
596 allocated to preserve application-expected ext3
597 behaviour. Note that this will also start
598 triggering a write of the data blocks, but this
599 behaviour may change in the future as it is
600 not necessary and has been done this way only
601 for sake of simplicity.
602..............................................................................
603
Dave Kleikampfc513a32006-10-11 01:21:25 -0700604References
605==========
606
607kernel source: <file:fs/ext4/>
608 <file:fs/jbd2/>
609
610programs: http://e2fsprogs.sourceforge.net/
Dave Kleikampfc513a32006-10-11 01:21:25 -0700611
612useful links: http://fedoraproject.org/wiki/ext3-devel
613 http://www.bullopensource.org/ext4/
Jose R. Santos93e32702008-07-11 19:27:31 -0400614 http://ext4.wiki.kernel.org/index.php/Main_Page
615 http://fedoraproject.org/wiki/Features/Ext4