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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) |
93e3270c JS |
88 | * fix 32000 subdirectory limit |
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 DK |
102 | |
103 | 2.2 Candidate features for future inclusion | |
104 | ||
93e3270c JS |
105 | * Online defrag (patches available but not well tested) |
106 | * reduced mke2fs time via lazy itable initialization in conjuction with | |
107 | the uninit_bg feature (capability to do this is available in e2fsprogs | |
108 | but a kernel thread to do lazy zeroing of unused inode table blocks | |
109 | after filesystem is first mounted is required for safety) | |
fc513a33 | 110 | |
93e3270c JS |
111 | There are several others under discussion, whether they all make it in is |
112 | partly a function of how much time everyone has to work on them. Features like | |
113 | metadata checksumming have been discussed and planned for a bit but no patches | |
114 | exist yet so I'm not sure they're in the near-term roadmap. | |
fc513a33 | 115 | |
93e3270c JS |
116 | The big performance win will come with mballoc, delalloc and flex_bg |
117 | grouping of bitmaps and inode tables. Some test results available here: | |
fc513a33 | 118 | |
22359f57 DC |
119 | - http://www.bullopensource.org/ext4/20080818-ffsb/ffsb-write-2.6.27-rc1.html |
120 | - http://www.bullopensource.org/ext4/20080818-ffsb/ffsb-readwrite-2.6.27-rc1.html | |
fc513a33 DK |
121 | |
122 | 3. Options | |
123 | ========== | |
124 | ||
125 | When mounting an ext4 filesystem, the following option are accepted: | |
126 | (*) == default | |
127 | ||
8e1a4857 TT |
128 | ro Mount filesystem read only. Note that ext4 will |
129 | replay the journal (and thus write to the | |
130 | partition) even when mounted "read only". The | |
131 | mount options "ro,noload" can be used to prevent | |
132 | writes to the filesystem. | |
133 | ||
818d276c GS |
134 | journal_checksum Enable checksumming of the journal transactions. |
135 | This will allow the recovery code in e2fsck and the | |
136 | kernel to detect corruption in the kernel. It is a | |
137 | compatible change and will be ignored by older kernels. | |
138 | ||
139 | journal_async_commit Commit block can be written to disk without waiting | |
140 | for descriptor blocks. If enabled older kernels cannot | |
141 | mount the device. This will enable 'journal_checksum' | |
142 | internally. | |
143 | ||
fc513a33 DK |
144 | journal=update Update the ext4 file system's journal to the current |
145 | format. | |
146 | ||
fc513a33 DK |
147 | journal_dev=devnum When the external journal device's major/minor numbers |
148 | have changed, this option allows the user to specify | |
149 | the new journal location. The journal device is | |
150 | identified through its new major/minor numbers encoded | |
151 | in devnum. | |
152 | ||
8e1a4857 TT |
153 | noload Don't load the journal on mounting. Note that |
154 | if the filesystem was not unmounted cleanly, | |
155 | skipping the journal replay will lead to the | |
156 | filesystem containing inconsistencies that can | |
157 | lead to any number of problems. | |
fc513a33 DK |
158 | |
159 | data=journal All data are committed into the journal prior to being | |
160 | written into the main file system. | |
161 | ||
162 | data=ordered (*) All data are forced directly out to the main file | |
163 | system prior to its metadata being committed to the | |
164 | journal. | |
165 | ||
166 | data=writeback Data ordering is not preserved, data may be written | |
167 | into the main file system after its metadata has been | |
168 | committed to the journal. | |
169 | ||
170 | commit=nrsec (*) Ext4 can be told to sync all its data and metadata | |
171 | every 'nrsec' seconds. The default value is 5 seconds. | |
172 | This means that if you lose your power, you will lose | |
173 | as much as the latest 5 seconds of work (your | |
174 | filesystem will not be damaged though, thanks to the | |
175 | journaling). This default value (or any low value) | |
176 | will hurt performance, but it's good for data-safety. | |
177 | Setting it to 0 will have the same effect as leaving | |
178 | it at the default (5 seconds). | |
179 | Setting it to very large values will improve | |
180 | performance. | |
181 | ||
571640ca ES |
182 | barrier=<0|1(*)> This enables/disables the use of write barriers in |
183 | the jbd code. barrier=0 disables, barrier=1 enables. | |
184 | This also requires an IO stack which can support | |
185 | barriers, and if jbd gets an error on a barrier | |
186 | write, it will disable again with a warning. | |
187 | Write barriers enforce proper on-disk ordering | |
188 | of journal commits, making volatile disk write caches | |
189 | safe to use, at some performance penalty. If | |
190 | your disks are battery-backed in one way or another, | |
191 | disabling barriers may safely improve performance. | |
fc513a33 | 192 | |
240799cd TT |
193 | inode_readahead=n This tuning parameter controls the maximum |
194 | number of inode table blocks that ext4's inode | |
195 | table readahead algorithm will pre-read into | |
196 | the buffer cache. The default value is 32 blocks. | |
197 | ||
fc513a33 DK |
198 | orlov (*) This enables the new Orlov block allocator. It is |
199 | enabled by default. | |
200 | ||
201 | oldalloc This disables the Orlov block allocator and enables | |
202 | the old block allocator. Orlov should have better | |
203 | performance - we'd like to get some feedback if it's | |
204 | the contrary for you. | |
205 | ||
206 | user_xattr Enables Extended User Attributes. Additionally, you | |
207 | need to have extended attribute support enabled in the | |
208 | kernel configuration (CONFIG_EXT4_FS_XATTR). See the | |
209 | attr(5) manual page and http://acl.bestbits.at/ to | |
210 | learn more about extended attributes. | |
211 | ||
212 | nouser_xattr Disables Extended User Attributes. | |
213 | ||
214 | acl Enables POSIX Access Control Lists support. | |
215 | Additionally, you need to have ACL support enabled in | |
216 | the kernel configuration (CONFIG_EXT4_FS_POSIX_ACL). | |
217 | See the acl(5) manual page and http://acl.bestbits.at/ | |
218 | for more information. | |
219 | ||
220 | noacl This option disables POSIX Access Control List | |
221 | support. | |
222 | ||
223 | reservation | |
224 | ||
225 | noreservation | |
226 | ||
227 | bsddf (*) Make 'df' act like BSD. | |
228 | minixdf Make 'df' act like Minix. | |
229 | ||
fc513a33 DK |
230 | debug Extra debugging information is sent to syslog. |
231 | ||
8e1a4857 | 232 | errors=remount-ro Remount the filesystem read-only on an error. |
fc513a33 DK |
233 | errors=continue Keep going on a filesystem error. |
234 | errors=panic Panic and halt the machine if an error occurs. | |
8e1a4857 TT |
235 | (These mount options override the errors behavior |
236 | specified in the superblock, which can be configured | |
237 | using tune2fs) | |
fc513a33 | 238 | |
5bf5683a HK |
239 | data_err=ignore(*) Just print an error message if an error occurs |
240 | in a file data buffer in ordered mode. | |
241 | data_err=abort Abort the journal if an error occurs in a file | |
242 | data buffer in ordered mode. | |
243 | ||
fc513a33 DK |
244 | grpid Give objects the same group ID as their creator. |
245 | bsdgroups | |
246 | ||
247 | nogrpid (*) New objects have the group ID of their creator. | |
248 | sysvgroups | |
249 | ||
250 | resgid=n The group ID which may use the reserved blocks. | |
251 | ||
252 | resuid=n The user ID which may use the reserved blocks. | |
253 | ||
254 | sb=n Use alternate superblock at this location. | |
255 | ||
256 | quota | |
257 | noquota | |
258 | grpquota | |
259 | usrquota | |
260 | ||
261 | bh (*) ext4 associates buffer heads to data pages to | |
262 | nobh (a) cache disk block mapping information | |
263 | (b) link pages into transaction to provide | |
264 | ordering guarantees. | |
265 | "bh" option forces use of buffer heads. | |
266 | "nobh" option tries to avoid associating buffer | |
267 | heads (supported only for "writeback" mode). | |
268 | ||
c9de560d AT |
269 | stripe=n Number of filesystem blocks that mballoc will try |
270 | to use for allocation size and alignment. For RAID5/6 | |
271 | systems this should be the number of data | |
272 | disks * RAID chunk size in file system blocks. | |
49f1487b MC |
273 | delalloc (*) Deferring block allocation until write-out time. |
274 | nodelalloc Disable delayed allocation. Blocks are allocation | |
275 | when data is copied from user to page cache. | |
240799cd | 276 | |
30773840 TT |
277 | max_batch_time=usec Maximum amount of time ext4 should wait for |
278 | additional filesystem operations to be batch | |
279 | together with a synchronous write operation. | |
280 | Since a synchronous write operation is going to | |
281 | force a commit and then a wait for the I/O | |
282 | complete, it doesn't cost much, and can be a | |
283 | huge throughput win, we wait for a small amount | |
284 | of time to see if any other transactions can | |
285 | piggyback on the synchronous write. The | |
286 | algorithm used is designed to automatically tune | |
287 | for the speed of the disk, by measuring the | |
288 | amount of time (on average) that it takes to | |
289 | finish committing a transaction. Call this time | |
290 | the "commit time". If the time that the | |
291 | transactoin has been running is less than the | |
292 | commit time, ext4 will try sleeping for the | |
293 | commit time to see if other operations will join | |
294 | the transaction. The commit time is capped by | |
295 | the max_batch_time, which defaults to 15000us | |
296 | (15ms). This optimization can be turned off | |
297 | entirely by setting max_batch_time to 0. | |
298 | ||
299 | min_batch_time=usec This parameter sets the commit time (as | |
300 | described above) to be at least min_batch_time. | |
301 | It defaults to zero microseconds. Increasing | |
302 | this parameter may improve the throughput of | |
303 | multi-threaded, synchronous workloads on very | |
304 | fast disks, at the cost of increasing latency. | |
305 | ||
b3881f74 TT |
306 | journal_ioprio=prio The I/O priority (from 0 to 7, where 0 is the |
307 | highest priorty) which should be used for I/O | |
308 | operations submitted by kjournald2 during a | |
309 | commit operation. This defaults to 3, which is | |
310 | a slightly higher priority than the default I/O | |
311 | priority. | |
312 | ||
fc513a33 | 313 | Data Mode |
93e3270c | 314 | ========= |
fc513a33 DK |
315 | There are 3 different data modes: |
316 | ||
317 | * writeback mode | |
318 | In data=writeback mode, ext4 does not journal data at all. This mode provides | |
319 | a similar level of journaling as that of XFS, JFS, and ReiserFS in its default | |
320 | mode - metadata journaling. A crash+recovery can cause incorrect data to | |
321 | appear in files which were written shortly before the crash. This mode will | |
322 | typically provide the best ext4 performance. | |
323 | ||
324 | * ordered mode | |
325 | In data=ordered mode, ext4 only officially journals metadata, but it logically | |
49f1487b MC |
326 | groups metadata information related to data changes with the data blocks into a |
327 | single unit called a transaction. When it's time to write the new metadata | |
328 | out to disk, the associated data blocks are written first. In general, | |
329 | this mode performs slightly slower than writeback but significantly faster than journal mode. | |
fc513a33 DK |
330 | |
331 | * journal mode | |
332 | data=journal mode provides full data and metadata journaling. All new data is | |
333 | written to the journal first, and then to its final location. | |
334 | In the event of a crash, the journal can be replayed, bringing both data and | |
335 | metadata into a consistent state. This mode is the slowest except when data | |
336 | needs to be read from and written to disk at the same time where it | |
49f1487b MC |
337 | outperforms all others modes. Curently ext4 does not have delayed |
338 | allocation support if this data journalling mode is selected. | |
fc513a33 DK |
339 | |
340 | References | |
341 | ========== | |
342 | ||
343 | kernel source: <file:fs/ext4/> | |
344 | <file:fs/jbd2/> | |
345 | ||
346 | programs: http://e2fsprogs.sourceforge.net/ | |
fc513a33 DK |
347 | |
348 | useful links: http://fedoraproject.org/wiki/ext3-devel | |
349 | http://www.bullopensource.org/ext4/ | |
93e3270c JS |
350 | http://ext4.wiki.kernel.org/index.php/Main_Page |
351 | http://fedoraproject.org/wiki/Features/Ext4 |