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fc513a33 DK |
1 | |
2 | Ext4 Filesystem | |
3 | =============== | |
4 | ||
c9f3f2d8 | 5 | Ext4 is an advanced level of the ext3 filesystem which incorporates |
22359f57 DC |
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 | ||
3bdadc86 | 28 | https://www.kernel.org/pub/linux/kernel/people/tytso/e2fsprogs/ |
fc513a33 | 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 | |
ad434017 LC |
71 | 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. | |
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 | |
98bfa344 | 97 | * inode allocation using large virtual block groups via flex_bg |
49f1487b MC |
98 | * delayed allocation |
99 | * large block (up to pagesize) support | |
98bfa344 | 100 | * efficient new ordered mode in JBD2 and ext4 (avoid using buffer head to force |
49f1487b | 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 | ||
98bfa344 | 108 | * online defrag (patches available but not well tested) |
25985edc | 109 | * reduced mke2fs time via lazy itable initialization in conjunction with |
93e3270c JS |
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 | |
ad4eec61 | 147 | journal_path=path |
fc513a33 | 148 | journal_dev=devnum When the external journal device's major/minor numbers |
ad4eec61 | 149 | have changed, these options allow the user to specify |
fc513a33 | 150 | the new journal location. The journal device is |
ad4eec61 ES |
151 | identified through either its new major/minor numbers |
152 | encoded in devnum, or via a path to the device. | |
fc513a33 | 153 | |
e3bb52ae ES |
154 | norecovery Don't load the journal on mounting. Note that |
155 | noload if the filesystem was not unmounted cleanly, | |
8e1a4857 TT |
156 | skipping the journal replay will lead to the |
157 | filesystem containing inconsistencies that can | |
158 | lead to any number of problems. | |
fc513a33 DK |
159 | |
160 | data=journal All data are committed into the journal prior to being | |
56889787 TT |
161 | written into the main file system. Enabling |
162 | this mode will disable delayed allocation and | |
163 | O_DIRECT support. | |
fc513a33 DK |
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 | ||
939da108 TM |
204 | nouser_xattr Disables Extended User Attributes. See the |
205 | attr(5) manual page and http://acl.bestbits.at/ | |
206 | for more information about extended attributes. | |
fc513a33 DK |
207 | |
208 | noacl This option disables POSIX Access Control List | |
af909a57 TT |
209 | support. If ACL support is enabled in the kernel |
210 | configuration (CONFIG_EXT4_FS_POSIX_ACL), ACL is | |
211 | enabled by default on mount. See the acl(5) manual | |
212 | page and http://acl.bestbits.at/ for more information | |
213 | about acl. | |
fc513a33 | 214 | |
fc513a33 DK |
215 | bsddf (*) Make 'df' act like BSD. |
216 | minixdf Make 'df' act like Minix. | |
217 | ||
fc513a33 DK |
218 | debug Extra debugging information is sent to syslog. |
219 | ||
8a8a2050 TT |
220 | abort Simulate the effects of calling ext4_abort() for |
221 | debugging purposes. This is normally used while | |
222 | remounting a filesystem which is already mounted. | |
223 | ||
8e1a4857 | 224 | errors=remount-ro Remount the filesystem read-only on an error. |
fc513a33 DK |
225 | errors=continue Keep going on a filesystem error. |
226 | errors=panic Panic and halt the machine if an error occurs. | |
8e1a4857 TT |
227 | (These mount options override the errors behavior |
228 | specified in the superblock, which can be configured | |
229 | using tune2fs) | |
fc513a33 | 230 | |
5bf5683a HK |
231 | data_err=ignore(*) Just print an error message if an error occurs |
232 | in a file data buffer in ordered mode. | |
233 | data_err=abort Abort the journal if an error occurs in a file | |
234 | data buffer in ordered mode. | |
235 | ||
d5d4b30f | 236 | grpid New objects have the group ID of their parent. |
fc513a33 DK |
237 | bsdgroups |
238 | ||
239 | nogrpid (*) New objects have the group ID of their creator. | |
240 | sysvgroups | |
241 | ||
242 | resgid=n The group ID which may use the reserved blocks. | |
243 | ||
244 | resuid=n The user ID which may use the reserved blocks. | |
245 | ||
246 | sb=n Use alternate superblock at this location. | |
247 | ||
1358870d JK |
248 | quota These options are ignored by the filesystem. They |
249 | noquota are used only by quota tools to recognize volumes | |
250 | grpquota where quota should be turned on. See documentation | |
251 | usrquota in the quota-tools package for more details | |
252 | (http://sourceforge.net/projects/linuxquota). | |
253 | ||
254 | jqfmt=<quota type> These options tell filesystem details about quota | |
255 | usrjquota=<file> so that quota information can be properly updated | |
256 | grpjquota=<file> during journal replay. They replace the above | |
257 | quota options. See documentation in the quota-tools | |
258 | package for more details | |
259 | (http://sourceforge.net/projects/linuxquota). | |
fc513a33 | 260 | |
c9de560d AT |
261 | stripe=n Number of filesystem blocks that mballoc will try |
262 | to use for allocation size and alignment. For RAID5/6 | |
263 | systems this should be the number of data | |
264 | disks * RAID chunk size in file system blocks. | |
83653888 JK |
265 | |
266 | delalloc (*) Defer block allocation until just before ext4 | |
267 | writes out the block(s) in question. This | |
268 | allows ext4 to better allocation decisions | |
269 | more efficiently. | |
270 | nodelalloc Disable delayed allocation. Blocks are allocated | |
271 | when the data is copied from userspace to the | |
272 | page cache, either via the write(2) system call | |
273 | or when an mmap'ed page which was previously | |
274 | unallocated is written for the first time. | |
240799cd | 275 | |
30773840 TT |
276 | max_batch_time=usec Maximum amount of time ext4 should wait for |
277 | additional filesystem operations to be batch | |
278 | together with a synchronous write operation. | |
279 | Since a synchronous write operation is going to | |
280 | force a commit and then a wait for the I/O | |
281 | complete, it doesn't cost much, and can be a | |
282 | huge throughput win, we wait for a small amount | |
283 | of time to see if any other transactions can | |
284 | piggyback on the synchronous write. The | |
285 | algorithm used is designed to automatically tune | |
286 | for the speed of the disk, by measuring the | |
287 | amount of time (on average) that it takes to | |
288 | finish committing a transaction. Call this time | |
289 | the "commit time". If the time that the | |
19f59460 | 290 | transaction has been running is less than the |
30773840 TT |
291 | commit time, ext4 will try sleeping for the |
292 | commit time to see if other operations will join | |
293 | the transaction. The commit time is capped by | |
294 | the max_batch_time, which defaults to 15000us | |
295 | (15ms). This optimization can be turned off | |
296 | entirely by setting max_batch_time to 0. | |
297 | ||
298 | min_batch_time=usec This parameter sets the commit time (as | |
299 | described above) to be at least min_batch_time. | |
300 | It defaults to zero microseconds. Increasing | |
301 | this parameter may improve the throughput of | |
302 | multi-threaded, synchronous workloads on very | |
303 | fast disks, at the cost of increasing latency. | |
304 | ||
b3881f74 | 305 | journal_ioprio=prio The I/O priority (from 0 to 7, where 0 is the |
40e47125 | 306 | highest priority) which should be used for I/O |
b3881f74 TT |
307 | operations submitted by kjournald2 during a |
308 | commit operation. This defaults to 3, which is | |
309 | a slightly higher priority than the default I/O | |
310 | priority. | |
311 | ||
06705bff TT |
312 | auto_da_alloc(*) Many broken applications don't use fsync() when |
313 | noauto_da_alloc replacing existing files via patterns such as | |
314 | fd = open("foo.new")/write(fd,..)/close(fd)/ | |
315 | rename("foo.new", "foo"), or worse yet, | |
316 | fd = open("foo", O_TRUNC)/write(fd,..)/close(fd). | |
317 | If auto_da_alloc is enabled, ext4 will detect | |
318 | the replace-via-rename and replace-via-truncate | |
319 | patterns and force that any delayed allocation | |
320 | blocks are allocated such that at the next | |
321 | journal commit, in the default data=ordered | |
322 | mode, the data blocks of the new file are forced | |
323 | to disk before the rename() operation is | |
19f59460 | 324 | committed. This provides roughly the same level |
06705bff TT |
325 | of guarantees as ext3, and avoids the |
326 | "zero-length" problem that can happen when a | |
327 | system crashes before the delayed allocation | |
328 | blocks are forced to disk. | |
329 | ||
bfff6873 LC |
330 | noinit_itable Do not initialize any uninitialized inode table |
331 | blocks in the background. This feature may be | |
332 | used by installation CD's so that the install | |
333 | process can complete as quickly as possible; the | |
334 | inode table initialization process would then be | |
335 | deferred until the next time the file system | |
336 | is unmounted. | |
337 | ||
338 | init_itable=n The lazy itable init code will wait n times the | |
339 | number of milliseconds it took to zero out the | |
340 | previous block group's inode table. This | |
40e47125 | 341 | minimizes the impact on the system performance |
bfff6873 LC |
342 | while file system's inode table is being initialized. |
343 | ||
6f9524e9 | 344 | discard Controls whether ext4 should issue discard/TRIM |
5328e635 ES |
345 | nodiscard(*) commands to the underlying block device when |
346 | blocks are freed. This is useful for SSD devices | |
347 | and sparse/thinly-provisioned LUNs, but it is off | |
348 | by default until sufficient testing has been done. | |
349 | ||
6f9524e9 LC |
350 | nouid32 Disables 32-bit UIDs and GIDs. This is for |
351 | interoperability with older kernels which only | |
352 | store and expect 16-bit values. | |
353 | ||
9e645ab7 | 354 | block_validity(*) These options enable or disable the in-kernel |
6f9524e9 | 355 | noblock_validity facility for tracking filesystem metadata blocks |
9e645ab7 FF |
356 | within internal data structures. This allows multi- |
357 | block allocator and other routines to notice | |
358 | bugs or corrupted allocation bitmaps which cause | |
359 | blocks to be allocated which overlap with | |
360 | filesystem metadata blocks. | |
6f9524e9 LC |
361 | |
362 | dioread_lock Controls whether or not ext4 should use the DIO read | |
363 | dioread_nolock locking. If the dioread_nolock option is specified | |
364 | ext4 will allocate uninitialized extent before buffer | |
365 | write and convert the extent to initialized after IO | |
366 | completes. This approach allows ext4 code to avoid | |
367 | using inode mutex, which improves scalability on high | |
ad434017 | 368 | speed storages. However this does not work with |
6f9524e9 LC |
369 | data journaling and dioread_nolock option will be |
370 | ignored with kernel warning. Note that dioread_nolock | |
371 | code path is only used for extent-based files. | |
372 | Because of the restrictions this options comprises | |
373 | it is off by default (e.g. dioread_lock). | |
374 | ||
df981d03 TT |
375 | max_dir_size_kb=n This limits the size of directories so that any |
376 | attempt to expand them beyond the specified | |
377 | limit in kilobytes will cause an ENOSPC error. | |
378 | This is useful in memory constrained | |
379 | environments, where a very large directory can | |
380 | cause severe performance problems or even | |
381 | provoke the Out Of Memory killer. (For example, | |
382 | if there is only 512mb memory available, a 176mb | |
383 | directory may seriously cramp the system's style.) | |
384 | ||
6f9524e9 LC |
385 | i_version Enable 64-bit inode version support. This option is |
386 | off by default. | |
387 | ||
923ae0ff RZ |
388 | dax Use direct access (no page cache). See |
389 | Documentation/filesystems/dax.txt. Note that | |
390 | this option is incompatible with data=journal. | |
391 | ||
fc513a33 | 392 | Data Mode |
93e3270c | 393 | ========= |
fc513a33 DK |
394 | There are 3 different data modes: |
395 | ||
396 | * writeback mode | |
397 | In data=writeback mode, ext4 does not journal data at all. This mode provides | |
398 | a similar level of journaling as that of XFS, JFS, and ReiserFS in its default | |
399 | mode - metadata journaling. A crash+recovery can cause incorrect data to | |
400 | appear in files which were written shortly before the crash. This mode will | |
401 | typically provide the best ext4 performance. | |
402 | ||
403 | * ordered mode | |
404 | In data=ordered mode, ext4 only officially journals metadata, but it logically | |
49f1487b MC |
405 | groups metadata information related to data changes with the data blocks into a |
406 | single unit called a transaction. When it's time to write the new metadata | |
407 | out to disk, the associated data blocks are written first. In general, | |
408 | this mode performs slightly slower than writeback but significantly faster than journal mode. | |
fc513a33 DK |
409 | |
410 | * journal mode | |
411 | data=journal mode provides full data and metadata journaling. All new data is | |
412 | written to the journal first, and then to its final location. | |
413 | In the event of a crash, the journal can be replayed, bringing both data and | |
414 | metadata into a consistent state. This mode is the slowest except when data | |
415 | needs to be read from and written to disk at the same time where it | |
56889787 TT |
416 | outperforms all others modes. Enabling this mode will disable delayed |
417 | allocation and O_DIRECT support. | |
fc513a33 | 418 | |
6f9524e9 LC |
419 | /proc entries |
420 | ============= | |
421 | ||
422 | Information about mounted ext4 file systems can be found in | |
423 | /proc/fs/ext4. Each mounted filesystem will have a directory in | |
424 | /proc/fs/ext4 based on its device name (i.e., /proc/fs/ext4/hdc or | |
425 | /proc/fs/ext4/dm-0). The files in each per-device directory are shown | |
426 | in table below. | |
427 | ||
428 | Files in /proc/fs/ext4/<devname> | |
429 | .............................................................................. | |
430 | File Content | |
431 | mb_groups details of multiblock allocator buddy cache of free blocks | |
432 | .............................................................................. | |
433 | ||
434 | /sys entries | |
435 | ============ | |
436 | ||
437 | Information about mounted ext4 file systems can be found in | |
438 | /sys/fs/ext4. Each mounted filesystem will have a directory in | |
439 | /sys/fs/ext4 based on its device name (i.e., /sys/fs/ext4/hdc or | |
440 | /sys/fs/ext4/dm-0). The files in each per-device directory are shown | |
441 | in table below. | |
442 | ||
443 | Files in /sys/fs/ext4/<devname> | |
444 | (see also Documentation/ABI/testing/sysfs-fs-ext4) | |
445 | .............................................................................. | |
446 | File Content | |
447 | ||
448 | delayed_allocation_blocks This file is read-only and shows the number of | |
449 | blocks that are dirty in the page cache, but | |
450 | which do not have their location in the | |
451 | filesystem allocated yet. | |
452 | ||
453 | inode_goal Tuning parameter which (if non-zero) controls | |
454 | the goal inode used by the inode allocator in | |
455 | preference to all other allocation heuristics. | |
456 | This is intended for debugging use only, and | |
457 | should be 0 on production systems. | |
458 | ||
459 | inode_readahead_blks Tuning parameter which controls the maximum | |
460 | number of inode table blocks that ext4's inode | |
461 | table readahead algorithm will pre-read into | |
462 | the buffer cache | |
463 | ||
464 | lifetime_write_kbytes This file is read-only and shows the number of | |
465 | kilobytes of data that have been written to this | |
466 | filesystem since it was created. | |
467 | ||
468 | max_writeback_mb_bump The maximum number of megabytes the writeback | |
469 | code will try to write out before move on to | |
470 | another inode. | |
471 | ||
472 | mb_group_prealloc The multiblock allocator will round up allocation | |
473 | requests to a multiple of this tuning parameter if | |
474 | the stripe size is not set in the ext4 superblock | |
475 | ||
476 | mb_max_to_scan The maximum number of extents the multiblock | |
477 | allocator will search to find the best extent | |
478 | ||
479 | mb_min_to_scan The minimum number of extents the multiblock | |
480 | allocator will search to find the best extent | |
481 | ||
482 | mb_order2_req Tuning parameter which controls the minimum size | |
483 | for requests (as a power of 2) where the buddy | |
484 | cache is used | |
485 | ||
486 | mb_stats Controls whether the multiblock allocator should | |
487 | collect statistics, which are shown during the | |
488 | unmount. 1 means to collect statistics, 0 means | |
489 | not to collect statistics | |
490 | ||
491 | mb_stream_req Files which have fewer blocks than this tunable | |
492 | parameter will have their blocks allocated out | |
493 | of a block group specific preallocation pool, so | |
494 | that small files are packed closely together. | |
495 | Each large file will have its blocks allocated | |
496 | out of its own unique preallocation pool. | |
497 | ||
498 | session_write_kbytes This file is read-only and shows the number of | |
499 | kilobytes of data that have been written to this | |
500 | filesystem since it was mounted. | |
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501 | |
502 | reserved_clusters This is RW file and contains number of reserved | |
503 | clusters in the file system which will be used | |
504 | in the specific situations to avoid costly | |
505 | zeroout, unexpected ENOSPC, or possible data | |
506 | loss. The default is 2% or 4096 clusters, | |
507 | whichever is smaller and this can be changed | |
508 | however it can never exceed number of clusters | |
509 | in the file system. If there is not enough space | |
510 | for the reserved space when mounting the file | |
511 | mount will _not_ fail. | |
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512 | .............................................................................. |
513 | ||
514 | Ioctls | |
515 | ====== | |
516 | ||
517 | There is some Ext4 specific functionality which can be accessed by applications | |
518 | through the system call interfaces. The list of all Ext4 specific ioctls are | |
519 | shown in the table below. | |
520 | ||
521 | Table of Ext4 specific ioctls | |
522 | .............................................................................. | |
523 | Ioctl Description | |
524 | EXT4_IOC_GETFLAGS Get additional attributes associated with inode. | |
525 | The ioctl argument is an integer bitfield, with | |
526 | bit values described in ext4.h. This ioctl is an | |
527 | alias for FS_IOC_GETFLAGS. | |
528 | ||
529 | EXT4_IOC_SETFLAGS Set additional attributes associated with inode. | |
530 | The ioctl argument is an integer bitfield, with | |
531 | bit values described in ext4.h. This ioctl is an | |
532 | alias for FS_IOC_SETFLAGS. | |
533 | ||
534 | EXT4_IOC_GETVERSION | |
535 | EXT4_IOC_GETVERSION_OLD | |
536 | Get the inode i_generation number stored for | |
537 | each inode. The i_generation number is normally | |
538 | changed only when new inode is created and it is | |
539 | particularly useful for network filesystems. The | |
540 | '_OLD' version of this ioctl is an alias for | |
541 | FS_IOC_GETVERSION. | |
542 | ||
543 | EXT4_IOC_SETVERSION | |
544 | EXT4_IOC_SETVERSION_OLD | |
545 | Set the inode i_generation number stored for | |
546 | each inode. The '_OLD' version of this ioctl | |
547 | is an alias for FS_IOC_SETVERSION. | |
548 | ||
549 | EXT4_IOC_GROUP_EXTEND This ioctl has the same purpose as the resize | |
550 | mount option. It allows to resize filesystem | |
551 | to the end of the last existing block group, | |
552 | further resize has to be done with resize2fs, | |
553 | either online, or offline. The argument points | |
554 | to the unsigned logn number representing the | |
555 | filesystem new block count. | |
556 | ||
557 | EXT4_IOC_MOVE_EXT Move the block extents from orig_fd (the one | |
558 | this ioctl is pointing to) to the donor_fd (the | |
559 | one specified in move_extent structure passed | |
560 | as an argument to this ioctl). Then, exchange | |
561 | inode metadata between orig_fd and donor_fd. | |
562 | This is especially useful for online | |
563 | defragmentation, because the allocator has the | |
564 | opportunity to allocate moved blocks better, | |
565 | ideally into one contiguous extent. | |
566 | ||
567 | EXT4_IOC_GROUP_ADD Add a new group descriptor to an existing or | |
568 | new group descriptor block. The new group | |
569 | descriptor is described by ext4_new_group_input | |
570 | structure, which is passed as an argument to | |
571 | this ioctl. This is especially useful in | |
572 | conjunction with EXT4_IOC_GROUP_EXTEND, | |
573 | which allows online resize of the filesystem | |
574 | to the end of the last existing block group. | |
575 | Those two ioctls combined is used in userspace | |
576 | online resize tool (e.g. resize2fs). | |
577 | ||
578 | EXT4_IOC_MIGRATE This ioctl operates on the filesystem itself. | |
579 | It converts (migrates) ext3 indirect block mapped | |
580 | inode to ext4 extent mapped inode by walking | |
581 | through indirect block mapping of the original | |
582 | inode and converting contiguous block ranges | |
583 | into ext4 extents of the temporary inode. Then, | |
584 | inodes are swapped. This ioctl might help, when | |
585 | migrating from ext3 to ext4 filesystem, however | |
586 | suggestion is to create fresh ext4 filesystem | |
587 | and copy data from the backup. Note, that | |
588 | filesystem has to support extents for this ioctl | |
589 | to work. | |
590 | ||
591 | EXT4_IOC_ALLOC_DA_BLKS Force all of the delay allocated blocks to be | |
592 | allocated to preserve application-expected ext3 | |
593 | behaviour. Note that this will also start | |
594 | triggering a write of the data blocks, but this | |
595 | behaviour may change in the future as it is | |
596 | not necessary and has been done this way only | |
597 | for sake of simplicity. | |
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598 | |
599 | EXT4_IOC_RESIZE_FS Resize the filesystem to a new size. The number | |
600 | of blocks of resized filesystem is passed in via | |
601 | 64 bit integer argument. The kernel allocates | |
602 | bitmaps and inode table, the userspace tool thus | |
603 | just passes the new number of blocks. | |
604 | ||
98bfa344 | 605 | EXT4_IOC_SWAP_BOOT Swap i_blocks and associated attributes |
393d1d1d DTB |
606 | (like i_blocks, i_size, i_flags, ...) from |
607 | the specified inode with inode | |
608 | EXT4_BOOT_LOADER_INO (#5). This is typically | |
609 | used to store a boot loader in a secure part of | |
610 | the filesystem, where it can't be changed by a | |
611 | normal user by accident. | |
612 | The data blocks of the previous boot loader | |
613 | will be associated with the given inode. | |
614 | ||
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615 | .............................................................................. |
616 | ||
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617 | References |
618 | ========== | |
619 | ||
620 | kernel source: <file:fs/ext4/> | |
621 | <file:fs/jbd2/> | |
622 | ||
623 | programs: http://e2fsprogs.sourceforge.net/ | |
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624 | |
625 | useful links: http://fedoraproject.org/wiki/ext3-devel | |
626 | http://www.bullopensource.org/ext4/ | |
93e3270c JS |
627 | http://ext4.wiki.kernel.org/index.php/Main_Page |
628 | http://fedoraproject.org/wiki/Features/Ext4 |