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fc513a33 DK |
1 | |
2 | Ext4 Filesystem | |
3 | =============== | |
4 | ||
22359f57 DC |
5 | Ext4 is an an advanced level of the ext3 filesystem which incorporates |
6 | scalability and reliability enhancements for supporting large filesystems | |
7 | (64 bit) in keeping with increasing disk capacities and state-of-the-art | |
8 | feature requirements. | |
fc513a33 | 9 | |
22359f57 DC |
10 | Mailing list: linux-ext4@vger.kernel.org |
11 | Web site: http://ext4.wiki.kernel.org | |
fc513a33 DK |
12 | |
13 | ||
14 | 1. Quick usage instructions: | |
15 | =========================== | |
16 | ||
22359f57 DC |
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 | |
20 | ||
93e3270c | 21 | - Compile and install the latest version of e2fsprogs (as of this |
22359f57 | 22 | writing version 1.41.3) from: |
93e3270c JS |
23 | |
24 | http://sourceforge.net/project/showfiles.php?group_id=2406 | |
25 | ||
26 | or | |
27 | ||
fc513a33 DK |
28 | ftp://ftp.kernel.org/pub/linux/kernel/people/tytso/e2fsprogs/ |
29 | ||
93e3270c JS |
30 | or grab the latest git repository from: |
31 | ||
32 | git://git.kernel.org/pub/scm/fs/ext2/e2fsprogs.git | |
33 | ||
4537398d TT |
34 | - Note that it is highly important to install the mke2fs.conf file |
35 | that comes with the e2fsprogs 1.41.x sources in /etc/mke2fs.conf. If | |
36 | you have edited the /etc/mke2fs.conf file installed on your system, | |
37 | you will need to merge your changes with the version from e2fsprogs | |
38 | 1.41.x. | |
39 | ||
03010a33 | 40 | - Create a new filesystem using the ext4 filesystem type: |
93e3270c | 41 | |
03010a33 | 42 | # mke2fs -t ext4 /dev/hda1 |
93e3270c | 43 | |
22359f57 | 44 | Or to configure an existing ext3 filesystem to support extents: |
fc513a33 | 45 | |
22359f57 | 46 | # tune2fs -O extents /dev/hda1 |
fc513a33 | 47 | |
93e3270c JS |
48 | If the filesystem was created with 128 byte inodes, it can be |
49 | converted to use 256 byte for greater efficiency via: | |
fc513a33 | 50 | |
93e3270c | 51 | # tune2fs -I 256 /dev/hda1 |
fc513a33 | 52 | |
03010a33 | 53 | (Note: we currently do not have tools to convert an ext4 |
93e3270c JS |
54 | filesystem back to ext3; so please do not do try this on production |
55 | filesystems.) | |
fc513a33 | 56 | |
93e3270c JS |
57 | - Mounting: |
58 | ||
03010a33 | 59 | # mount -t ext4 /dev/hda1 /wherever |
fc513a33 | 60 | |
8e1a4857 TT |
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. | |
fc513a33 DK |
77 | |
78 | 2. Features | |
79 | =========== | |
80 | ||
81 | 2.1 Currently available | |
82 | ||
93e3270c | 83 | * ability to use filesystems > 16TB (e2fsprogs support not available yet) |
fc513a33 DK |
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, | |
8e1a4857 | 86 | * internal redundancy in tree |
49f1487b | 87 | * improved file allocation (multi-block alloc) |
722bde68 | 88 | * lift 32000 subdirectory limit imposed by i_links_count[1] |
93e3270c JS |
89 | * nsec timestamps for mtime, atime, ctime, create time |
90 | * inode version field on disk (NFSv4, Lustre) | |
91 | * reduced e2fsck time via uninit_bg feature | |
92 | * journal checksumming for robustness, performance | |
93 | * persistent file preallocation (e.g for streaming media, databases) | |
94 | * ability to pack bitmaps and inode tables into larger virtual groups via the | |
95 | flex_bg feature | |
96 | * large file support | |
97 | * Inode allocation using large virtual block groups via flex_bg | |
49f1487b MC |
98 | * delayed allocation |
99 | * large block (up to pagesize) support | |
100 | * efficent new ordered mode in JBD2 and ext4(avoid using buffer head to force | |
101 | the ordering) | |
fc513a33 | 102 | |
722bde68 TT |
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. | |
105 | ||
fc513a33 DK |
106 | 2.2 Candidate features for future inclusion |
107 | ||
93e3270c JS |
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) | |
fc513a33 | 113 | |
93e3270c JS |
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. | |
fc513a33 | 118 | |
93e3270c JS |
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: | |
fc513a33 | 121 | |
22359f57 DC |
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 | |
fc513a33 DK |
124 | |
125 | 3. Options | |
126 | ========== | |
127 | ||
128 | When mounting an ext4 filesystem, the following option are accepted: | |
129 | (*) == default | |
130 | ||
8e1a4857 TT |
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. | |
136 | ||
d4da6c9c LT |
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. | |
141 | ||
818d276c GS |
142 | journal_async_commit Commit block can be written to disk without waiting |
143 | for descriptor blocks. If enabled older kernels cannot | |
d4da6c9c LT |
144 | mount the device. This will enable 'journal_checksum' |
145 | internally. | |
818d276c | 146 | |
fc513a33 DK |
147 | journal=update Update the ext4 file system's journal to the current |
148 | format. | |
149 | ||
fc513a33 DK |
150 | journal_dev=devnum When the external journal device's major/minor numbers |
151 | have changed, this option allows the user to specify | |
152 | the new journal location. The journal device is | |
153 | identified through its new major/minor numbers encoded | |
154 | in devnum. | |
155 | ||
e3bb52ae ES |
156 | norecovery Don't load the journal on mounting. Note that |
157 | noload if the filesystem was not unmounted cleanly, | |
8e1a4857 TT |
158 | skipping the journal replay will lead to the |
159 | filesystem containing inconsistencies that can | |
160 | lead to any number of problems. | |
fc513a33 DK |
161 | |
162 | data=journal All data are committed into the journal prior to being | |
163 | written into the main file system. | |
164 | ||
165 | data=ordered (*) All data are forced directly out to the main file | |
166 | system prior to its metadata being committed to the | |
167 | journal. | |
168 | ||
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. | |
172 | ||
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 | |
183 | performance. | |
184 | ||
571640ca | 185 | barrier=<0|1(*)> This enables/disables the use of write barriers in |
06705bff TT |
186 | barrier(*) the jbd code. barrier=0 disables, barrier=1 enables. |
187 | nobarrier This also requires an IO stack which can support | |
571640ca ES |
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. | |
06705bff TT |
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. | |
fc513a33 | 198 | |
6d3b82f2 | 199 | inode_readahead_blks=n This tuning parameter controls the maximum |
240799cd TT |
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. | |
203 | ||
fc513a33 DK |
204 | orlov (*) This enables the new Orlov block allocator. It is |
205 | enabled by default. | |
206 | ||
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. | |
211 | ||
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. | |
217 | ||
218 | nouser_xattr Disables Extended User Attributes. | |
219 | ||
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. | |
225 | ||
226 | noacl This option disables POSIX Access Control List | |
227 | support. | |
228 | ||
229 | reservation | |
230 | ||
231 | noreservation | |
232 | ||
233 | bsddf (*) Make 'df' act like BSD. | |
234 | minixdf Make 'df' act like Minix. | |
235 | ||
fc513a33 DK |
236 | debug Extra debugging information is sent to syslog. |
237 | ||
8a8a2050 TT |
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. | |
241 | ||
8e1a4857 | 242 | errors=remount-ro Remount the filesystem read-only on an error. |
fc513a33 DK |
243 | errors=continue Keep going on a filesystem error. |
244 | errors=panic Panic and halt the machine if an error occurs. | |
8e1a4857 TT |
245 | (These mount options override the errors behavior |
246 | specified in the superblock, which can be configured | |
247 | using tune2fs) | |
fc513a33 | 248 | |
5bf5683a HK |
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. | |
253 | ||
fc513a33 DK |
254 | grpid Give objects the same group ID as their creator. |
255 | bsdgroups | |
256 | ||
257 | nogrpid (*) New objects have the group ID of their creator. | |
258 | sysvgroups | |
259 | ||
260 | resgid=n The group ID which may use the reserved blocks. | |
261 | ||
262 | resuid=n The user ID which may use the reserved blocks. | |
263 | ||
264 | sb=n Use alternate superblock at this location. | |
265 | ||
1358870d JK |
266 | quota These options are ignored by the filesystem. They |
267 | noquota are used only by quota tools to recognize volumes | |
268 | grpquota where quota should be turned on. See documentation | |
269 | usrquota in the quota-tools package for more details | |
270 | (http://sourceforge.net/projects/linuxquota). | |
271 | ||
272 | jqfmt=<quota type> These options tell filesystem details about quota | |
273 | usrjquota=<file> so that quota information can be properly updated | |
274 | grpjquota=<file> during journal replay. They replace the above | |
275 | quota options. See documentation in the quota-tools | |
276 | package for more details | |
277 | (http://sourceforge.net/projects/linuxquota). | |
fc513a33 DK |
278 | |
279 | bh (*) ext4 associates buffer heads to data pages to | |
280 | nobh (a) cache disk block mapping information | |
281 | (b) link pages into transaction to provide | |
282 | ordering guarantees. | |
283 | "bh" option forces use of buffer heads. | |
284 | "nobh" option tries to avoid associating buffer | |
285 | heads (supported only for "writeback" mode). | |
286 | ||
c9de560d AT |
287 | stripe=n Number of filesystem blocks that mballoc will try |
288 | to use for allocation size and alignment. For RAID5/6 | |
289 | systems this should be the number of data | |
290 | disks * RAID chunk size in file system blocks. | |
83653888 JK |
291 | |
292 | delalloc (*) Defer block allocation until just before ext4 | |
293 | writes out the block(s) in question. This | |
294 | allows ext4 to better allocation decisions | |
295 | more efficiently. | |
296 | nodelalloc Disable delayed allocation. Blocks are allocated | |
297 | when the data is copied from userspace to the | |
298 | page cache, either via the write(2) system call | |
299 | or when an mmap'ed page which was previously | |
300 | unallocated is written for the first time. | |
240799cd | 301 | |
30773840 TT |
302 | max_batch_time=usec Maximum amount of time ext4 should wait for |
303 | additional filesystem operations to be batch | |
304 | together with a synchronous write operation. | |
305 | Since a synchronous write operation is going to | |
306 | force a commit and then a wait for the I/O | |
307 | complete, it doesn't cost much, and can be a | |
308 | huge throughput win, we wait for a small amount | |
309 | of time to see if any other transactions can | |
310 | piggyback on the synchronous write. The | |
311 | algorithm used is designed to automatically tune | |
312 | for the speed of the disk, by measuring the | |
313 | amount of time (on average) that it takes to | |
314 | finish committing a transaction. Call this time | |
315 | the "commit time". If the time that the | |
19f59460 | 316 | transaction has been running is less than the |
30773840 TT |
317 | commit time, ext4 will try sleeping for the |
318 | commit time to see if other operations will join | |
319 | the transaction. The commit time is capped by | |
320 | the max_batch_time, which defaults to 15000us | |
321 | (15ms). This optimization can be turned off | |
322 | entirely by setting max_batch_time to 0. | |
323 | ||
324 | min_batch_time=usec This parameter sets the commit time (as | |
325 | described above) to be at least min_batch_time. | |
326 | It defaults to zero microseconds. Increasing | |
327 | this parameter may improve the throughput of | |
328 | multi-threaded, synchronous workloads on very | |
329 | fast disks, at the cost of increasing latency. | |
330 | ||
b3881f74 TT |
331 | journal_ioprio=prio The I/O priority (from 0 to 7, where 0 is the |
332 | highest priorty) which should be used for I/O | |
333 | operations submitted by kjournald2 during a | |
334 | commit operation. This defaults to 3, which is | |
335 | a slightly higher priority than the default I/O | |
336 | priority. | |
337 | ||
06705bff TT |
338 | auto_da_alloc(*) Many broken applications don't use fsync() when |
339 | noauto_da_alloc replacing existing files via patterns such as | |
340 | fd = open("foo.new")/write(fd,..)/close(fd)/ | |
341 | rename("foo.new", "foo"), or worse yet, | |
342 | fd = open("foo", O_TRUNC)/write(fd,..)/close(fd). | |
343 | If auto_da_alloc is enabled, ext4 will detect | |
344 | the replace-via-rename and replace-via-truncate | |
345 | patterns and force that any delayed allocation | |
346 | blocks are allocated such that at the next | |
347 | journal commit, in the default data=ordered | |
348 | mode, the data blocks of the new file are forced | |
349 | to disk before the rename() operation is | |
19f59460 | 350 | committed. This provides roughly the same level |
06705bff TT |
351 | of guarantees as ext3, and avoids the |
352 | "zero-length" problem that can happen when a | |
353 | system crashes before the delayed allocation | |
354 | blocks are forced to disk. | |
355 | ||
5328e635 ES |
356 | discard Controls whether ext4 should issue discard/TRIM |
357 | nodiscard(*) commands to the underlying block device when | |
358 | blocks are freed. This is useful for SSD devices | |
359 | and sparse/thinly-provisioned LUNs, but it is off | |
360 | by default until sufficient testing has been done. | |
361 | ||
fc513a33 | 362 | Data Mode |
93e3270c | 363 | ========= |
fc513a33 DK |
364 | There are 3 different data modes: |
365 | ||
366 | * writeback mode | |
367 | In data=writeback mode, ext4 does not journal data at all. This mode provides | |
368 | a similar level of journaling as that of XFS, JFS, and ReiserFS in its default | |
369 | mode - metadata journaling. A crash+recovery can cause incorrect data to | |
370 | appear in files which were written shortly before the crash. This mode will | |
371 | typically provide the best ext4 performance. | |
372 | ||
373 | * ordered mode | |
374 | In data=ordered mode, ext4 only officially journals metadata, but it logically | |
49f1487b MC |
375 | groups metadata information related to data changes with the data blocks into a |
376 | single unit called a transaction. When it's time to write the new metadata | |
377 | out to disk, the associated data blocks are written first. In general, | |
378 | this mode performs slightly slower than writeback but significantly faster than journal mode. | |
fc513a33 DK |
379 | |
380 | * journal mode | |
381 | data=journal mode provides full data and metadata journaling. All new data is | |
382 | written to the journal first, and then to its final location. | |
383 | In the event of a crash, the journal can be replayed, bringing both data and | |
384 | metadata into a consistent state. This mode is the slowest except when data | |
385 | needs to be read from and written to disk at the same time where it | |
19f59460 | 386 | outperforms all others modes. Currently ext4 does not have delayed |
49f1487b | 387 | allocation support if this data journalling mode is selected. |
fc513a33 DK |
388 | |
389 | References | |
390 | ========== | |
391 | ||
392 | kernel source: <file:fs/ext4/> | |
393 | <file:fs/jbd2/> | |
394 | ||
395 | programs: http://e2fsprogs.sourceforge.net/ | |
fc513a33 DK |
396 | |
397 | useful links: http://fedoraproject.org/wiki/ext3-devel | |
398 | http://www.bullopensource.org/ext4/ | |
93e3270c JS |
399 | http://ext4.wiki.kernel.org/index.php/Main_Page |
400 | http://fedoraproject.org/wiki/Features/Ext4 |