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,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.
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 * efficent 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 conjuction 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=update Update the ext4 file system's journal to the current
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
156 noload Don't load the journal on mounting. Note that
157 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.
165 data=ordered (*) All data are forced directly out to the main file
166 system prior to its metadata being committed to the
169 data=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.
173 commit=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
185 barrier=<0|1(*)> This enables/disables the use of write barriers in
186 barrier(*) the jbd code. barrier=0 disables, barrier=1 enables.
187 nobarrier This also requires an IO stack which can support
188 barriers, and if jbd gets an error on a barrier
189 write, it will disable again with a warning.
190 Write barriers enforce proper on-disk ordering
191 of journal commits, making volatile disk write caches
192 safe to use, at some performance penalty. If
193 your disks are battery-backed in one way or another,
194 disabling barriers may safely improve performance.
195 The mount options "barrier" and "nobarrier" can
196 also be used to enable or disable barriers, for
197 consistency with other ext4 mount options.
199 inode_readahead=n This tuning parameter controls the maximum
200 number of inode table blocks that ext4's inode
201 table readahead algorithm will pre-read into
202 the buffer cache. The default value is 32 blocks.
204 orlov (*) This enables the new Orlov block allocator. It is
207 oldalloc 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.
212 user_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.
218 nouser_xattr Disables Extended User Attributes.
220 acl 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.
226 noacl This option disables POSIX Access Control List
233 bsddf (*) Make 'df' act like BSD.
234 minixdf Make 'df' act like Minix.
236 debug Extra debugging information is sent to syslog.
238 abort Simulate the effects of calling ext4_abort() for
239 debugging purposes. This is normally used while
240 remounting a filesystem which is already mounted.
242 errors=remount-ro Remount the filesystem read-only on an error.
243 errors=continue Keep going on a filesystem error.
244 errors=panic Panic and halt the machine if an error occurs.
245 (These mount options override the errors behavior
246 specified in the superblock, which can be configured
249 data_err=ignore(*) Just print an error message if an error occurs
250 in a file data buffer in ordered mode.
251 data_err=abort Abort the journal if an error occurs in a file
252 data buffer in ordered mode.
254 grpid Give objects the same group ID as their creator.
257 nogrpid (*) New objects have the group ID of their creator.
260 resgid=n The group ID which may use the reserved blocks.
262 resuid=n The user ID which may use the reserved blocks.
264 sb=n Use alternate superblock at this location.
271 bh (*) ext4 associates buffer heads to data pages to
272 nobh (a) cache disk block mapping information
273 (b) link pages into transaction to provide
275 "bh" option forces use of buffer heads.
276 "nobh" option tries to avoid associating buffer
277 heads (supported only for "writeback" mode).
279 stripe=n Number of filesystem blocks that mballoc will try
280 to use for allocation size and alignment. For RAID5/6
281 systems this should be the number of data
282 disks * RAID chunk size in file system blocks.
283 delalloc (*) Deferring block allocation until write-out time.
284 nodelalloc Disable delayed allocation. Blocks are allocation
285 when data is copied from user to page cache.
287 max_batch_time=usec Maximum amount of time ext4 should wait for
288 additional filesystem operations to be batch
289 together with a synchronous write operation.
290 Since a synchronous write operation is going to
291 force a commit and then a wait for the I/O
292 complete, it doesn't cost much, and can be a
293 huge throughput win, we wait for a small amount
294 of time to see if any other transactions can
295 piggyback on the synchronous write. The
296 algorithm used is designed to automatically tune
297 for the speed of the disk, by measuring the
298 amount of time (on average) that it takes to
299 finish committing a transaction. Call this time
300 the "commit time". If the time that the
301 transaction has been running is less than the
302 commit time, ext4 will try sleeping for the
303 commit time to see if other operations will join
304 the transaction. The commit time is capped by
305 the max_batch_time, which defaults to 15000us
306 (15ms). This optimization can be turned off
307 entirely by setting max_batch_time to 0.
309 min_batch_time=usec This parameter sets the commit time (as
310 described above) to be at least min_batch_time.
311 It defaults to zero microseconds. Increasing
312 this parameter may improve the throughput of
313 multi-threaded, synchronous workloads on very
314 fast disks, at the cost of increasing latency.
316 journal_ioprio=prio The I/O priority (from 0 to 7, where 0 is the
317 highest priorty) which should be used for I/O
318 operations submitted by kjournald2 during a
319 commit operation. This defaults to 3, which is
320 a slightly higher priority than the default I/O
323 auto_da_alloc(*) Many broken applications don't use fsync() when
324 noauto_da_alloc replacing existing files via patterns such as
325 fd = open("foo.new")/write(fd,..)/close(fd)/
326 rename("foo.new", "foo"), or worse yet,
327 fd = open("foo", O_TRUNC)/write(fd,..)/close(fd).
328 If auto_da_alloc is enabled, ext4 will detect
329 the replace-via-rename and replace-via-truncate
330 patterns and force that any delayed allocation
331 blocks are allocated such that at the next
332 journal commit, in the default data=ordered
333 mode, the data blocks of the new file are forced
334 to disk before the rename() operation is
335 committed. This provides roughly the same level
336 of guarantees as ext3, and avoids the
337 "zero-length" problem that can happen when a
338 system crashes before the delayed allocation
339 blocks are forced to disk.
343 There are 3 different data modes:
346 In data=writeback mode, ext4 does not journal data at all. This mode provides
347 a similar level of journaling as that of XFS, JFS, and ReiserFS in its default
348 mode - metadata journaling. A crash+recovery can cause incorrect data to
349 appear in files which were written shortly before the crash. This mode will
350 typically provide the best ext4 performance.
353 In data=ordered mode, ext4 only officially journals metadata, but it logically
354 groups metadata information related to data changes with the data blocks into a
355 single unit called a transaction. When it's time to write the new metadata
356 out to disk, the associated data blocks are written first. In general,
357 this mode performs slightly slower than writeback but significantly faster than journal mode.
360 data=journal mode provides full data and metadata journaling. All new data is
361 written to the journal first, and then to its final location.
362 In the event of a crash, the journal can be replayed, bringing both data and
363 metadata into a consistent state. This mode is the slowest except when data
364 needs to be read from and written to disk at the same time where it
365 outperforms all others modes. Currently ext4 does not have delayed
366 allocation support if this data journalling mode is selected.
371 kernel source: <file:fs/ext4/>
374 programs: http://e2fsprogs.sourceforge.net/
376 useful links: http://fedoraproject.org/wiki/ext3-devel
377 http://www.bullopensource.org/ext4/
378 http://ext4.wiki.kernel.org/index.php/Main_Page
379 http://fedoraproject.org/wiki/Features/Ext4