1 .. SPDX-License-Identifier: GPL-2.0 1 .. SPDX-License-Identifier: GPL-2.0
2 2
3 ======================== 3 ========================
4 ext4 General Information 4 ext4 General Information
5 ======================== 5 ========================
6 6
7 Ext4 is an advanced level of the ext3 filesyst 7 Ext4 is an advanced level of the ext3 filesystem which incorporates
8 scalability and reliability enhancements for s 8 scalability and reliability enhancements for supporting large filesystems
9 (64 bit) in keeping with increasing disk capac 9 (64 bit) in keeping with increasing disk capacities and state-of-the-art
10 feature requirements. 10 feature requirements.
11 11
12 Mailing list: linux-ext4@vger.kernel.org 12 Mailing list: linux-ext4@vger.kernel.org
13 Web site: http://ext4.wiki.kernel.org 13 Web site: http://ext4.wiki.kernel.org
14 14
15 15
16 Quick usage instructions 16 Quick usage instructions
17 ======================== 17 ========================
18 18
19 Note: More extensive information for getting s 19 Note: More extensive information for getting started with ext4 can be
20 found at the ext4 wiki site at the URL: 20 found at the ext4 wiki site at the URL:
21 http://ext4.wiki.kernel.org/index.php/Ext4_How 21 http://ext4.wiki.kernel.org/index.php/Ext4_Howto
22 22
23 - The latest version of e2fsprogs can be fou 23 - The latest version of e2fsprogs can be found at:
24 24
25 https://www.kernel.org/pub/linux/kernel/pe 25 https://www.kernel.org/pub/linux/kernel/people/tytso/e2fsprogs/
26 26
27 or 27 or
28 28
29 http://sourceforge.net/project/showfiles.p 29 http://sourceforge.net/project/showfiles.php?group_id=2406
30 30
31 or grab the latest git repository from 31 or grab the latest git repository from:
32 32
33 https://git.kernel.org/pub/scm/fs/ext2/e2fs 33 https://git.kernel.org/pub/scm/fs/ext2/e2fsprogs.git
34 34
35 - Create a new filesystem using the ext4 fil 35 - Create a new filesystem using the ext4 filesystem type:
36 36
37 # mke2fs -t ext4 /dev/hda1 37 # mke2fs -t ext4 /dev/hda1
38 38
39 Or to configure an existing ext3 filesyste 39 Or to configure an existing ext3 filesystem to support extents:
40 40
41 # tune2fs -O extents /dev/hda1 41 # tune2fs -O extents /dev/hda1
42 42
43 If the filesystem was created with 128 byt 43 If the filesystem was created with 128 byte inodes, it can be
44 converted to use 256 byte for greater effi 44 converted to use 256 byte for greater efficiency via:
45 45
46 # tune2fs -I 256 /dev/hda1 46 # tune2fs -I 256 /dev/hda1
47 47
48 - Mounting: 48 - Mounting:
49 49
50 # mount -t ext4 /dev/hda1 /wherever 50 # mount -t ext4 /dev/hda1 /wherever
51 51
52 - When comparing performance with other file 52 - When comparing performance with other filesystems, it's always
53 important to try multiple workloads; very 53 important to try multiple workloads; very often a subtle change in a
54 workload parameter can completely change t 54 workload parameter can completely change the ranking of which
55 filesystems do well compared to others. W 55 filesystems do well compared to others. When comparing versus ext3,
56 note that ext4 enables write barriers by d 56 note that ext4 enables write barriers by default, while ext3 does
57 not enable write barriers by default. So 57 not enable write barriers by default. So it is useful to use
58 explicitly specify whether barriers are en 58 explicitly specify whether barriers are enabled or not when via the
59 '-o barriers=[0|1]' mount option for both 59 '-o barriers=[0|1]' mount option for both ext3 and ext4 filesystems
60 for a fair comparison. When tuning ext3 f 60 for a fair comparison. When tuning ext3 for best benchmark numbers,
61 it is often worthwhile to try changing the 61 it is often worthwhile to try changing the data journaling mode; '-o
62 data=writeback' can be faster for some wor 62 data=writeback' can be faster for some workloads. (Note however that
63 running mounted with data=writeback can po 63 running mounted with data=writeback can potentially leave stale data
64 exposed in recently written files in case 64 exposed in recently written files in case of an unclean shutdown,
65 which could be a security exposure in some 65 which could be a security exposure in some situations.) Configuring
66 the filesystem with a large journal can al 66 the filesystem with a large journal can also be helpful for
67 metadata-intensive workloads. 67 metadata-intensive workloads.
68 68
69 Features 69 Features
70 ======== 70 ========
71 71
72 Currently Available 72 Currently Available
73 ------------------- 73 -------------------
74 74
75 * ability to use filesystems > 16TB (e2fsprogs 75 * ability to use filesystems > 16TB (e2fsprogs support not available yet)
76 * extent format reduces metadata overhead (RAM 76 * extent format reduces metadata overhead (RAM, IO for access, transactions)
77 * extent format more robust in face of on-disk 77 * extent format more robust in face of on-disk corruption due to magics,
78 * internal redundancy in tree 78 * internal redundancy in tree
79 * improved file allocation (multi-block alloc) 79 * improved file allocation (multi-block alloc)
80 * lift 32000 subdirectory limit imposed by i_l 80 * lift 32000 subdirectory limit imposed by i_links_count[1]
81 * nsec timestamps for mtime, atime, ctime, cre 81 * nsec timestamps for mtime, atime, ctime, create time
82 * inode version field on disk (NFSv4, Lustre) 82 * inode version field on disk (NFSv4, Lustre)
83 * reduced e2fsck time via uninit_bg feature 83 * reduced e2fsck time via uninit_bg feature
84 * journal checksumming for robustness, perform 84 * journal checksumming for robustness, performance
85 * persistent file preallocation (e.g for strea 85 * persistent file preallocation (e.g for streaming media, databases)
86 * ability to pack bitmaps and inode tables int 86 * ability to pack bitmaps and inode tables into larger virtual groups via the
87 flex_bg feature 87 flex_bg feature
88 * large file support 88 * large file support
89 * inode allocation using large virtual block g 89 * inode allocation using large virtual block groups via flex_bg
90 * delayed allocation 90 * delayed allocation
91 * large block (up to pagesize) support 91 * large block (up to pagesize) support
92 * efficient new ordered mode in JBD2 and ext4 92 * efficient new ordered mode in JBD2 and ext4 (avoid using buffer head to force
93 the ordering) 93 the ordering)
94 * Case-insensitive file name lookups <<
95 * file-based encryption support (fscrypt) <<
96 * file-based verity support (fsverity) <<
97 94
98 [1] Filesystems with a block size of 1k may se 95 [1] Filesystems with a block size of 1k may see a limit imposed by the
99 directory hash tree having a maximum depth of 96 directory hash tree having a maximum depth of two.
100 97
101 case-insensitive file name lookups <<
102 ============================================== <<
103 <<
104 The case-insensitive file name lookup feature <<
105 per-directory basis, allowing the user to mix <<
106 case-sensitive directories in the same filesys <<
107 flipping the +F inode attribute of an empty di <<
108 case-insensitive string match operation is onl <<
109 text in encoded in a byte sequence. For that <<
110 case-insensitive directories, the filesystem m <<
111 casefold feature, which stores the filesystem- <<
112 model used. By default, the charset adopted i <<
113 Unicode (12.1.0, by the time of this writing), <<
114 form. The comparison algorithm is implemented <<
115 strings to the Canonical decomposition form, a <<
116 followed by a byte per byte comparison. <<
117 <<
118 The case-awareness is name-preserving on the d <<
119 name provided by userspace is a byte-per-byte <<
120 written in the disk. The Unicode normalizatio <<
121 kernel is thus an internal representation, and <<
122 userspace nor to the disk, with the important <<
123 used on large case-insensitive directories wit <<
124 directories, the hash must be calculated using <<
125 the filename, meaning that the normalization f <<
126 impact on where the directory entry is stored. <<
127 <<
128 When we change from viewing filenames as opaqu <<
129 them as encoded strings we need to address wha <<
130 tries to create a file with an invalid name. <<
131 within the kernel leaves the decision of what <<
132 filesystem, which select its preferred behavio <<
133 the strict mode. When Ext4 encounters one of <<
134 filesystem did not require strict mode, it fal <<
135 entire string as an opaque byte sequence, whic <<
136 operate on that file, but the case-insensitive <<
137 <<
138 Options 98 Options
139 ======= 99 =======
140 100
141 When mounting an ext4 filesystem, the followin 101 When mounting an ext4 filesystem, the following option are accepted:
142 (*) == default 102 (*) == default
143 103
144 ro 104 ro
145 Mount filesystem read only. Note that 105 Mount filesystem read only. Note that ext4 will replay the journal (and
146 thus write to the partition) even when 106 thus write to the partition) even when mounted "read only". The mount
147 options "ro,noload" can be used to pre 107 options "ro,noload" can be used to prevent writes to the filesystem.
148 108
149 journal_checksum 109 journal_checksum
150 Enable checksumming of the journal tra 110 Enable checksumming of the journal transactions. This will allow the
151 recovery code in e2fsck and the kernel 111 recovery code in e2fsck and the kernel to detect corruption in the
152 kernel. It is a compatible change and 112 kernel. It is a compatible change and will be ignored by older
153 kernels. 113 kernels.
154 114
155 journal_async_commit 115 journal_async_commit
156 Commit block can be written to disk wi 116 Commit block can be written to disk without waiting for descriptor
157 blocks. If enabled older kernels canno 117 blocks. If enabled older kernels cannot mount the device. This will
158 enable 'journal_checksum' internally. 118 enable 'journal_checksum' internally.
159 119
160 journal_path=path, journal_dev=devnum 120 journal_path=path, journal_dev=devnum
161 When the external journal device's maj 121 When the external journal device's major/minor numbers have changed,
162 these options allow the user to specif 122 these options allow the user to specify the new journal location. The
163 journal device is identified through e 123 journal device is identified through either its new major/minor numbers
164 encoded in devnum, or via a path to th 124 encoded in devnum, or via a path to the device.
165 125
166 norecovery, noload 126 norecovery, noload
167 Don't load the journal on mounting. N 127 Don't load the journal on mounting. Note that if the filesystem was
168 not unmounted cleanly, skipping the jo 128 not unmounted cleanly, skipping the journal replay will lead to the
169 filesystem containing inconsistencies 129 filesystem containing inconsistencies that can lead to any number of
170 problems. 130 problems.
171 131
172 data=journal 132 data=journal
173 All data are committed into the journa 133 All data are committed into the journal prior to being written into the
174 main file system. Enabling this mode 134 main file system. Enabling this mode will disable delayed allocation
175 and O_DIRECT support. 135 and O_DIRECT support.
176 136
177 data=ordered (*) 137 data=ordered (*)
178 All data are forced directly out to th 138 All data are forced directly out to the main file system prior to its
179 metadata being committed to the journa 139 metadata being committed to the journal.
180 140
181 data=writeback 141 data=writeback
182 Data ordering is not preserved, data m 142 Data ordering is not preserved, data may be written into the main file
183 system after its metadata has been com 143 system after its metadata has been committed to the journal.
184 144
185 commit=nrsec (*) 145 commit=nrsec (*)
186 This setting limits the maximum age of !! 146 Ext4 can be told to sync all its data and metadata every 'nrsec'
187 'nrsec' seconds. The default value is !! 147 seconds. The default value is 5 seconds. This means that if you lose
188 you lose your power, you will lose as !! 148 your power, you will lose as much as the latest 5 seconds of work (your
189 metadata changes (your filesystem will !! 149 filesystem will not be damaged though, thanks to the journaling). This
190 to the journaling). This default value !! 150 default value (or any low value) will hurt performance, but it's good
191 performance, but it's good for data-sa !! 151 for data-safety. Setting it to 0 will have the same effect as leaving
192 the same effect as leaving it at the d !! 152 it at the default (5 seconds). Setting it to very large values will
193 to very large values will improve perf !! 153 improve performance.
194 delayed allocation even older data can <<
195 writeback of those data begins only af <<
196 /proc/sys/vm/dirty_expire_centisecs. <<
197 154
198 barrier=<0|1(*)>, barrier(*), nobarrier 155 barrier=<0|1(*)>, barrier(*), nobarrier
199 This enables/disables the use of write 156 This enables/disables the use of write barriers in the jbd code.
200 barrier=0 disables, barrier=1 enables. 157 barrier=0 disables, barrier=1 enables. This also requires an IO stack
201 which can support barriers, and if jbd 158 which can support barriers, and if jbd gets an error on a barrier
202 write, it will disable again with a wa 159 write, it will disable again with a warning. Write barriers enforce
203 proper on-disk ordering of journal com 160 proper on-disk ordering of journal commits, making volatile disk write
204 caches safe to use, at some performanc 161 caches safe to use, at some performance penalty. If your disks are
205 battery-backed in one way or another, 162 battery-backed in one way or another, disabling barriers may safely
206 improve performance. The mount option 163 improve performance. The mount options "barrier" and "nobarrier" can
207 also be used to enable or disable barr 164 also be used to enable or disable barriers, for consistency with other
208 ext4 mount options. 165 ext4 mount options.
209 166
210 inode_readahead_blks=n 167 inode_readahead_blks=n
211 This tuning parameter controls the max 168 This tuning parameter controls the maximum number of inode table blocks
212 that ext4's inode table readahead algo 169 that ext4's inode table readahead algorithm will pre-read into the
213 buffer cache. The default value is 32 170 buffer cache. The default value is 32 blocks.
214 171
>> 172 nouser_xattr
>> 173 Disables Extended User Attributes. See the attr(5) manual page for
>> 174 more information about extended attributes.
>> 175
>> 176 noacl
>> 177 This option disables POSIX Access Control List support. If ACL support
>> 178 is enabled in the kernel configuration (CONFIG_EXT4_FS_POSIX_ACL), ACL
>> 179 is enabled by default on mount. See the acl(5) manual page for more
>> 180 information about acl.
>> 181
215 bsddf (*) 182 bsddf (*)
216 Make 'df' act like BSD. 183 Make 'df' act like BSD.
217 184
218 minixdf 185 minixdf
219 Make 'df' act like Minix. 186 Make 'df' act like Minix.
220 187
221 debug 188 debug
222 Extra debugging information is sent to 189 Extra debugging information is sent to syslog.
223 190
224 abort 191 abort
225 Simulate the effects of calling ext4_a 192 Simulate the effects of calling ext4_abort() for debugging purposes.
226 This is normally used while remounting 193 This is normally used while remounting a filesystem which is already
227 mounted. 194 mounted.
228 195
229 errors=remount-ro 196 errors=remount-ro
230 Remount the filesystem read-only on an 197 Remount the filesystem read-only on an error.
231 198
232 errors=continue 199 errors=continue
233 Keep going on a filesystem error. 200 Keep going on a filesystem error.
234 201
235 errors=panic 202 errors=panic
236 Panic and halt the machine if an error 203 Panic and halt the machine if an error occurs. (These mount options
237 override the errors behavior specified 204 override the errors behavior specified in the superblock, which can be
238 configured using tune2fs) 205 configured using tune2fs)
239 206
240 data_err=ignore(*) 207 data_err=ignore(*)
241 Just print an error message if an erro 208 Just print an error message if an error occurs in a file data buffer in
242 ordered mode. 209 ordered mode.
243 data_err=abort 210 data_err=abort
244 Abort the journal if an error occurs i 211 Abort the journal if an error occurs in a file data buffer in ordered
245 mode. 212 mode.
246 213
247 grpid | bsdgroups 214 grpid | bsdgroups
248 New objects have the group ID of their 215 New objects have the group ID of their parent.
249 216
250 nogrpid (*) | sysvgroups 217 nogrpid (*) | sysvgroups
251 New objects have the group ID of their 218 New objects have the group ID of their creator.
252 219
253 resgid=n 220 resgid=n
254 The group ID which may use the reserve 221 The group ID which may use the reserved blocks.
255 222
256 resuid=n 223 resuid=n
257 The user ID which may use the reserved 224 The user ID which may use the reserved blocks.
258 225
259 sb= 226 sb=
260 Use alternate superblock at this locat 227 Use alternate superblock at this location.
261 228
262 quota, noquota, grpquota, usrquota 229 quota, noquota, grpquota, usrquota
263 These options are ignored by the files 230 These options are ignored by the filesystem. They are used only by
264 quota tools to recognize volumes where 231 quota tools to recognize volumes where quota should be turned on. See
265 documentation in the quota-tools packa 232 documentation in the quota-tools package for more details
266 (http://sourceforge.net/projects/linux 233 (http://sourceforge.net/projects/linuxquota).
267 234
268 jqfmt=<quota type>, usrjquota=<file>, grpjqu 235 jqfmt=<quota type>, usrjquota=<file>, grpjquota=<file>
269 These options tell filesystem details 236 These options tell filesystem details about quota so that quota
270 information can be properly updated du 237 information can be properly updated during journal replay. They replace
271 the above quota options. See documenta 238 the above quota options. See documentation in the quota-tools package
272 for more details (http://sourceforge.n 239 for more details (http://sourceforge.net/projects/linuxquota).
273 240
274 stripe=n 241 stripe=n
275 Number of filesystem blocks that mball 242 Number of filesystem blocks that mballoc will try to use for allocation
276 size and alignment. For RAID5/6 system 243 size and alignment. For RAID5/6 systems this should be the number of
277 data disks * RAID chunk size in file 244 data disks * RAID chunk size in file system blocks.
278 245
279 delalloc (*) 246 delalloc (*)
280 Defer block allocation until just befo 247 Defer block allocation until just before ext4 writes out the block(s)
281 in question. This allows ext4 to bett 248 in question. This allows ext4 to better allocation decisions more
282 efficiently. 249 efficiently.
283 250
284 nodelalloc 251 nodelalloc
285 Disable delayed allocation. Blocks ar 252 Disable delayed allocation. Blocks are allocated when the data is
286 copied from userspace to the page cach 253 copied from userspace to the page cache, either via the write(2) system
287 call or when an mmap'ed page which was 254 call or when an mmap'ed page which was previously unallocated is
288 written for the first time. 255 written for the first time.
289 256
290 max_batch_time=usec 257 max_batch_time=usec
291 Maximum amount of time ext4 should wai 258 Maximum amount of time ext4 should wait for additional filesystem
292 operations to be batch together with a 259 operations to be batch together with a synchronous write operation.
293 Since a synchronous write operation is 260 Since a synchronous write operation is going to force a commit and then
294 a wait for the I/O complete, it doesn' 261 a wait for the I/O complete, it doesn't cost much, and can be a huge
295 throughput win, we wait for a small am 262 throughput win, we wait for a small amount of time to see if any other
296 transactions can piggyback on the sync 263 transactions can piggyback on the synchronous write. The algorithm
297 used is designed to automatically tune 264 used is designed to automatically tune for the speed of the disk, by
298 measuring the amount of time (on avera 265 measuring the amount of time (on average) that it takes to finish
299 committing a transaction. Call this t 266 committing a transaction. Call this time the "commit time". If the
300 time that the transaction has been run 267 time that the transaction has been running is less than the commit
301 time, ext4 will try sleeping for the c 268 time, ext4 will try sleeping for the commit time to see if other
302 operations will join the transaction. 269 operations will join the transaction. The commit time is capped by
303 the max_batch_time, which defaults to 270 the max_batch_time, which defaults to 15000us (15ms). This
304 optimization can be turned off entirel 271 optimization can be turned off entirely by setting max_batch_time to 0.
305 272
306 min_batch_time=usec 273 min_batch_time=usec
307 This parameter sets the commit time (a 274 This parameter sets the commit time (as described above) to be at least
308 min_batch_time. It defaults to zero m 275 min_batch_time. It defaults to zero microseconds. Increasing this
309 parameter may improve the throughput o 276 parameter may improve the throughput of multi-threaded, synchronous
310 workloads on very fast disks, at the c 277 workloads on very fast disks, at the cost of increasing latency.
311 278
312 journal_ioprio=prio 279 journal_ioprio=prio
313 The I/O priority (from 0 to 7, where 0 280 The I/O priority (from 0 to 7, where 0 is the highest priority) which
314 should be used for I/O operations subm 281 should be used for I/O operations submitted by kjournald2 during a
315 commit operation. This defaults to 3, 282 commit operation. This defaults to 3, which is a slightly higher
316 priority than the default I/O priority 283 priority than the default I/O priority.
317 284
318 auto_da_alloc(*), noauto_da_alloc 285 auto_da_alloc(*), noauto_da_alloc
319 Many broken applications don't use fsy 286 Many broken applications don't use fsync() when replacing existing
320 files via patterns such as fd = open(" 287 files via patterns such as fd = open("foo.new")/write(fd,..)/close(fd)/
321 rename("foo.new", "foo"), or worse yet 288 rename("foo.new", "foo"), or worse yet, fd = open("foo",
322 O_TRUNC)/write(fd,..)/close(fd). If a 289 O_TRUNC)/write(fd,..)/close(fd). If auto_da_alloc is enabled, ext4
323 will detect the replace-via-rename and 290 will detect the replace-via-rename and replace-via-truncate patterns
324 and force that any delayed allocation 291 and force that any delayed allocation blocks are allocated such that at
325 the next journal commit, in the defaul 292 the next journal commit, in the default data=ordered mode, the data
326 blocks of the new file are forced to d 293 blocks of the new file are forced to disk before the rename() operation
327 is committed. This provides roughly t 294 is committed. This provides roughly the same level of guarantees as
328 ext3, and avoids the "zero-length" pro 295 ext3, and avoids the "zero-length" problem that can happen when a
329 system crashes before the delayed allo 296 system crashes before the delayed allocation blocks are forced to disk.
330 297
331 noinit_itable 298 noinit_itable
332 Do not initialize any uninitialized in 299 Do not initialize any uninitialized inode table blocks in the
333 background. This feature may be used 300 background. This feature may be used by installation CD's so that the
334 install process can complete as quickl 301 install process can complete as quickly as possible; the inode table
335 initialization process would then be d 302 initialization process would then be deferred until the next time the
336 file system is unmounted. 303 file system is unmounted.
337 304
338 init_itable=n 305 init_itable=n
339 The lazy itable init code will wait n 306 The lazy itable init code will wait n times the number of milliseconds
340 it took to zero out the previous block 307 it took to zero out the previous block group's inode table. This
341 minimizes the impact on the system per 308 minimizes the impact on the system performance while file system's
342 inode table is being initialized. 309 inode table is being initialized.
343 310
344 discard, nodiscard(*) 311 discard, nodiscard(*)
345 Controls whether ext4 should issue dis 312 Controls whether ext4 should issue discard/TRIM commands to the
346 underlying block device when blocks ar 313 underlying block device when blocks are freed. This is useful for SSD
347 devices and sparse/thinly-provisioned 314 devices and sparse/thinly-provisioned LUNs, but it is off by default
348 until sufficient testing has been done 315 until sufficient testing has been done.
349 316
350 nouid32 317 nouid32
351 Disables 32-bit UIDs and GIDs. This i 318 Disables 32-bit UIDs and GIDs. This is for interoperability with
352 older kernels which only store and exp 319 older kernels which only store and expect 16-bit values.
353 320
354 block_validity(*), noblock_validity 321 block_validity(*), noblock_validity
355 These options enable or disable the in 322 These options enable or disable the in-kernel facility for tracking
356 filesystem metadata blocks within inte 323 filesystem metadata blocks within internal data structures. This
357 allows multi- block allocator and othe 324 allows multi- block allocator and other routines to notice bugs or
358 corrupted allocation bitmaps which cau 325 corrupted allocation bitmaps which cause blocks to be allocated which
359 overlap with filesystem metadata block 326 overlap with filesystem metadata blocks.
360 327
361 dioread_lock, dioread_nolock 328 dioread_lock, dioread_nolock
362 Controls whether or not ext4 should us 329 Controls whether or not ext4 should use the DIO read locking. If the
363 dioread_nolock option is specified ext 330 dioread_nolock option is specified ext4 will allocate uninitialized
364 extent before buffer write and convert 331 extent before buffer write and convert the extent to initialized after
365 IO completes. This approach allows ext 332 IO completes. This approach allows ext4 code to avoid using inode
366 mutex, which improves scalability on h 333 mutex, which improves scalability on high speed storages. However this
367 does not work with data journaling and 334 does not work with data journaling and dioread_nolock option will be
368 ignored with kernel warning. Note that 335 ignored with kernel warning. Note that dioread_nolock code path is only
369 used for extent-based files. Because 336 used for extent-based files. Because of the restrictions this options
370 comprises it is off by default (e.g. d 337 comprises it is off by default (e.g. dioread_lock).
371 338
372 max_dir_size_kb=n 339 max_dir_size_kb=n
373 This limits the size of directories so 340 This limits the size of directories so that any attempt to expand them
374 beyond the specified limit in kilobyte 341 beyond the specified limit in kilobytes will cause an ENOSPC error.
375 This is useful in memory constrained e 342 This is useful in memory constrained environments, where a very large
376 directory can cause severe performance 343 directory can cause severe performance problems or even provoke the Out
377 Of Memory killer. (For example, if th 344 Of Memory killer. (For example, if there is only 512mb memory
378 available, a 176mb directory may serio 345 available, a 176mb directory may seriously cramp the system's style.)
379 346
380 i_version 347 i_version
381 Enable 64-bit inode version support. T 348 Enable 64-bit inode version support. This option is off by default.
382 349
383 dax 350 dax
384 Use direct access (no page cache). Se 351 Use direct access (no page cache). See
385 Documentation/filesystems/dax.rst. No !! 352 Documentation/filesystems/dax.txt. Note that this option is
386 incompatible with data=journal. 353 incompatible with data=journal.
387 354
388 inlinecrypt <<
389 When possible, encrypt/decrypt the con <<
390 blk-crypto framework rather than files <<
391 allows the use of inline encryption ha <<
392 unaffected. For more details, see <<
393 Documentation/block/inline-encryption. <<
394 <<
395 Data Mode 355 Data Mode
396 ========= 356 =========
397 There are 3 different data modes: 357 There are 3 different data modes:
398 358
399 * writeback mode 359 * writeback mode
400 360
401 In data=writeback mode, ext4 does not journa 361 In data=writeback mode, ext4 does not journal data at all. This mode provides
402 a similar level of journaling as that of XFS 362 a similar level of journaling as that of XFS, JFS, and ReiserFS in its default
403 mode - metadata journaling. A crash+recover 363 mode - metadata journaling. A crash+recovery can cause incorrect data to
404 appear in files which were written shortly b 364 appear in files which were written shortly before the crash. This mode will
405 typically provide the best ext4 performance. 365 typically provide the best ext4 performance.
406 366
407 * ordered mode 367 * ordered mode
408 368
409 In data=ordered mode, ext4 only officially j 369 In data=ordered mode, ext4 only officially journals metadata, but it logically
410 groups metadata information related to data 370 groups metadata information related to data changes with the data blocks into
411 a single unit called a transaction. When it 371 a single unit called a transaction. When it's time to write the new metadata
412 out to disk, the associated data blocks are 372 out to disk, the associated data blocks are written first. In general, this
413 mode performs slightly slower than writeback 373 mode performs slightly slower than writeback but significantly faster than
414 journal mode. 374 journal mode.
415 375
416 * journal mode 376 * journal mode
417 377
418 data=journal mode provides full data and met 378 data=journal mode provides full data and metadata journaling. All new data is
419 written to the journal first, and then to it 379 written to the journal first, and then to its final location. In the event of
420 a crash, the journal can be replayed, bringi 380 a crash, the journal can be replayed, bringing both data and metadata into a
421 consistent state. This mode is the slowest 381 consistent state. This mode is the slowest except when data needs to be read
422 from and written to disk at the same time wh 382 from and written to disk at the same time where it outperforms all others
423 modes. Enabling this mode will disable dela 383 modes. Enabling this mode will disable delayed allocation and O_DIRECT
424 support. 384 support.
425 385
426 /proc entries 386 /proc entries
427 ============= 387 =============
428 388
429 Information about mounted ext4 file systems ca 389 Information about mounted ext4 file systems can be found in
430 /proc/fs/ext4. Each mounted filesystem will h 390 /proc/fs/ext4. Each mounted filesystem will have a directory in
431 /proc/fs/ext4 based on its device name (i.e., 391 /proc/fs/ext4 based on its device name (i.e., /proc/fs/ext4/hdc or
432 /proc/fs/ext4/dm-0). The files in each per-d 392 /proc/fs/ext4/dm-0). The files in each per-device directory are shown
433 in table below. 393 in table below.
434 394
435 Files in /proc/fs/ext4/<devname> 395 Files in /proc/fs/ext4/<devname>
436 396
437 mb_groups 397 mb_groups
438 details of multiblock allocator buddy 398 details of multiblock allocator buddy cache of free blocks
439 399
440 /sys entries 400 /sys entries
441 ============ 401 ============
442 402
443 Information about mounted ext4 file systems ca 403 Information about mounted ext4 file systems can be found in
444 /sys/fs/ext4. Each mounted filesystem will ha 404 /sys/fs/ext4. Each mounted filesystem will have a directory in
445 /sys/fs/ext4 based on its device name (i.e., / 405 /sys/fs/ext4 based on its device name (i.e., /sys/fs/ext4/hdc or
446 /sys/fs/ext4/dm-0). The files in each per-de 406 /sys/fs/ext4/dm-0). The files in each per-device directory are shown
447 in table below. 407 in table below.
448 408
449 Files in /sys/fs/ext4/<devname>: 409 Files in /sys/fs/ext4/<devname>:
450 410
451 (see also Documentation/ABI/testing/sysfs-fs-e 411 (see also Documentation/ABI/testing/sysfs-fs-ext4)
452 412
453 delayed_allocation_blocks 413 delayed_allocation_blocks
454 This file is read-only and shows the n 414 This file is read-only and shows the number of blocks that are dirty in
455 the page cache, but which do not have 415 the page cache, but which do not have their location in the filesystem
456 allocated yet. 416 allocated yet.
457 417
458 inode_goal 418 inode_goal
459 Tuning parameter which (if non-zero) c 419 Tuning parameter which (if non-zero) controls the goal inode used by
460 the inode allocator in preference to a 420 the inode allocator in preference to all other allocation heuristics.
461 This is intended for debugging use onl 421 This is intended for debugging use only, and should be 0 on production
462 systems. 422 systems.
463 423
464 inode_readahead_blks 424 inode_readahead_blks
465 Tuning parameter which controls the ma 425 Tuning parameter which controls the maximum number of inode table
466 blocks that ext4's inode table readahe 426 blocks that ext4's inode table readahead algorithm will pre-read into
467 the buffer cache. 427 the buffer cache.
468 428
469 lifetime_write_kbytes 429 lifetime_write_kbytes
470 This file is read-only and shows the n 430 This file is read-only and shows the number of kilobytes of data that
471 have been written to this filesystem s 431 have been written to this filesystem since it was created.
472 432
473 max_writeback_mb_bump 433 max_writeback_mb_bump
474 The maximum number of megabytes the wr 434 The maximum number of megabytes the writeback code will try to write
475 out before move on to another inode. 435 out before move on to another inode.
476 436
477 mb_group_prealloc 437 mb_group_prealloc
478 The multiblock allocator will round up 438 The multiblock allocator will round up allocation requests to a
479 multiple of this tuning parameter if t 439 multiple of this tuning parameter if the stripe size is not set in the
480 ext4 superblock 440 ext4 superblock
481 441
482 mb_max_to_scan 442 mb_max_to_scan
483 The maximum number of extents the mult 443 The maximum number of extents the multiblock allocator will search to
484 find the best extent. 444 find the best extent.
485 445
486 mb_min_to_scan 446 mb_min_to_scan
487 The minimum number of extents the mult 447 The minimum number of extents the multiblock allocator will search to
488 find the best extent. 448 find the best extent.
489 449
490 mb_order2_req 450 mb_order2_req
491 Tuning parameter which controls the mi 451 Tuning parameter which controls the minimum size for requests (as a
492 power of 2) where the buddy cache is u 452 power of 2) where the buddy cache is used.
493 453
494 mb_stats 454 mb_stats
495 Controls whether the multiblock alloca 455 Controls whether the multiblock allocator should collect statistics,
496 which are shown during the unmount. 1 456 which are shown during the unmount. 1 means to collect statistics, 0
497 means not to collect statistics. 457 means not to collect statistics.
498 458
499 mb_stream_req 459 mb_stream_req
500 Files which have fewer blocks than thi 460 Files which have fewer blocks than this tunable parameter will have
501 their blocks allocated out of a block 461 their blocks allocated out of a block group specific preallocation
502 pool, so that small files are packed c 462 pool, so that small files are packed closely together. Each large file
503 will have its blocks allocated out of 463 will have its blocks allocated out of its own unique preallocation
504 pool. 464 pool.
505 465
506 session_write_kbytes 466 session_write_kbytes
507 This file is read-only and shows the n 467 This file is read-only and shows the number of kilobytes of data that
508 have been written to this filesystem s 468 have been written to this filesystem since it was mounted.
509 469
510 reserved_clusters 470 reserved_clusters
511 This is RW file and contains number of 471 This is RW file and contains number of reserved clusters in the file
512 system which will be used in the speci 472 system which will be used in the specific situations to avoid costly
513 zeroout, unexpected ENOSPC, or possibl 473 zeroout, unexpected ENOSPC, or possible data loss. The default is 2% or
514 4096 clusters, whichever is smaller an 474 4096 clusters, whichever is smaller and this can be changed however it
515 can never exceed number of clusters in 475 can never exceed number of clusters in the file system. If there is not
516 enough space for the reserved space wh 476 enough space for the reserved space when mounting the file mount will
517 _not_ fail. 477 _not_ fail.
518 478
519 Ioctls 479 Ioctls
520 ====== 480 ======
521 481
522 Ext4 implements various ioctls which can be us !! 482 There is some Ext4 specific functionality which can be accessed by applications
523 ext4-specific functionality. An incomplete lis !! 483 through the system call interfaces. The list of all Ext4 specific ioctls are
524 table below. This list includes truly ext4-spe !! 484 shown in the table below.
525 well as ioctls that may have been ext4-specifi <<
526 by some other filesystem(s) too (``FS_IOC_*``) <<
527 485
528 Table of Ext4 ioctls !! 486 Table of Ext4 specific ioctls
529 487
530 FS_IOC_GETFLAGS !! 488 EXT4_IOC_GETFLAGS
531 Get additional attributes associated w 489 Get additional attributes associated with inode. The ioctl argument is
532 an integer bitfield, with bit values d !! 490 an integer bitfield, with bit values described in ext4.h. This ioctl is
>> 491 an alias for FS_IOC_GETFLAGS.
533 492
534 FS_IOC_SETFLAGS !! 493 EXT4_IOC_SETFLAGS
535 Set additional attributes associated w 494 Set additional attributes associated with inode. The ioctl argument is
536 an integer bitfield, with bit values d !! 495 an integer bitfield, with bit values described in ext4.h. This ioctl is
>> 496 an alias for FS_IOC_SETFLAGS.
537 497
538 EXT4_IOC_GETVERSION, EXT4_IOC_GETVERSION_OLD 498 EXT4_IOC_GETVERSION, EXT4_IOC_GETVERSION_OLD
539 Get the inode i_generation number stor 499 Get the inode i_generation number stored for each inode. The
540 i_generation number is normally change 500 i_generation number is normally changed only when new inode is created
541 and it is particularly useful for netw 501 and it is particularly useful for network filesystems. The '_OLD'
542 version of this ioctl is an alias for 502 version of this ioctl is an alias for FS_IOC_GETVERSION.
543 503
544 EXT4_IOC_SETVERSION, EXT4_IOC_SETVERSION_OLD 504 EXT4_IOC_SETVERSION, EXT4_IOC_SETVERSION_OLD
545 Set the inode i_generation number stor 505 Set the inode i_generation number stored for each inode. The '_OLD'
546 version of this ioctl is an alias for 506 version of this ioctl is an alias for FS_IOC_SETVERSION.
547 507
548 EXT4_IOC_GROUP_EXTEND 508 EXT4_IOC_GROUP_EXTEND
549 This ioctl has the same purpose as the 509 This ioctl has the same purpose as the resize mount option. It allows
550 to resize filesystem to the end of the 510 to resize filesystem to the end of the last existing block group,
551 further resize has to be done with res 511 further resize has to be done with resize2fs, either online, or
552 offline. The argument points to the un 512 offline. The argument points to the unsigned logn number representing
553 the filesystem new block count. 513 the filesystem new block count.
554 514
555 EXT4_IOC_MOVE_EXT 515 EXT4_IOC_MOVE_EXT
556 Move the block extents from orig_fd (t 516 Move the block extents from orig_fd (the one this ioctl is pointing to)
557 to the donor_fd (the one specified in 517 to the donor_fd (the one specified in move_extent structure passed as
558 an argument to this ioctl). Then, exch 518 an argument to this ioctl). Then, exchange inode metadata between
559 orig_fd and donor_fd. This is especia 519 orig_fd and donor_fd. This is especially useful for online
560 defragmentation, because the allocator 520 defragmentation, because the allocator has the opportunity to allocate
561 moved blocks better, ideally into one 521 moved blocks better, ideally into one contiguous extent.
562 522
563 EXT4_IOC_GROUP_ADD 523 EXT4_IOC_GROUP_ADD
564 Add a new group descriptor to an exist 524 Add a new group descriptor to an existing or new group descriptor
565 block. The new group descriptor is des 525 block. The new group descriptor is described by ext4_new_group_input
566 structure, which is passed as an argum 526 structure, which is passed as an argument to this ioctl. This is
567 especially useful in conjunction with 527 especially useful in conjunction with EXT4_IOC_GROUP_EXTEND, which
568 allows online resize of the filesystem 528 allows online resize of the filesystem to the end of the last existing
569 block group. Those two ioctls combine 529 block group. Those two ioctls combined is used in userspace online
570 resize tool (e.g. resize2fs). 530 resize tool (e.g. resize2fs).
571 531
572 EXT4_IOC_MIGRATE 532 EXT4_IOC_MIGRATE
573 This ioctl operates on the filesystem 533 This ioctl operates on the filesystem itself. It converts (migrates)
574 ext3 indirect block mapped inode to ex 534 ext3 indirect block mapped inode to ext4 extent mapped inode by walking
575 through indirect block mapping of the 535 through indirect block mapping of the original inode and converting
576 contiguous block ranges into ext4 exte 536 contiguous block ranges into ext4 extents of the temporary inode. Then,
577 inodes are swapped. This ioctl might h 537 inodes are swapped. This ioctl might help, when migrating from ext3 to
578 ext4 filesystem, however suggestion is 538 ext4 filesystem, however suggestion is to create fresh ext4 filesystem
579 and copy data from the backup. Note, t 539 and copy data from the backup. Note, that filesystem has to support
580 extents for this ioctl to work. 540 extents for this ioctl to work.
581 541
582 EXT4_IOC_ALLOC_DA_BLKS 542 EXT4_IOC_ALLOC_DA_BLKS
583 Force all of the delay allocated block 543 Force all of the delay allocated blocks to be allocated to preserve
584 application-expected ext3 behaviour. N 544 application-expected ext3 behaviour. Note that this will also start
585 triggering a write of the data blocks, 545 triggering a write of the data blocks, but this behaviour may change in
586 the future as it is not necessary and 546 the future as it is not necessary and has been done this way only for
587 sake of simplicity. 547 sake of simplicity.
588 548
589 EXT4_IOC_RESIZE_FS 549 EXT4_IOC_RESIZE_FS
590 Resize the filesystem to a new size. 550 Resize the filesystem to a new size. The number of blocks of resized
591 filesystem is passed in via 64 bit int 551 filesystem is passed in via 64 bit integer argument. The kernel
592 allocates bitmaps and inode table, the 552 allocates bitmaps and inode table, the userspace tool thus just passes
593 the new number of blocks. 553 the new number of blocks.
594 554
595 EXT4_IOC_SWAP_BOOT 555 EXT4_IOC_SWAP_BOOT
596 Swap i_blocks and associated attribute 556 Swap i_blocks and associated attributes (like i_blocks, i_size,
597 i_flags, ...) from the specified inode 557 i_flags, ...) from the specified inode with inode EXT4_BOOT_LOADER_INO
598 (#5). This is typically used to store 558 (#5). This is typically used to store a boot loader in a secure part of
599 the filesystem, where it can't be chan 559 the filesystem, where it can't be changed by a normal user by accident.
600 The data blocks of the previous boot l 560 The data blocks of the previous boot loader will be associated with the
601 given inode. 561 given inode.
602 562
603 References 563 References
604 ========== 564 ==========
605 565
606 kernel source: <file:fs/ext4/> 566 kernel source: <file:fs/ext4/>
607 <file:fs/jbd2/> 567 <file:fs/jbd2/>
608 568
609 programs: http://e2fsprogs.sourceforge.n 569 programs: http://e2fsprogs.sourceforge.net/
610 570
611 useful links: https://fedoraproject.org/wiki !! 571 useful links: http://fedoraproject.org/wiki/ext3-devel
612 http://www.bullopensource.org/ 572 http://www.bullopensource.org/ext4/
613 http://ext4.wiki.kernel.org/in 573 http://ext4.wiki.kernel.org/index.php/Main_Page
614 https://fedoraproject.org/wiki !! 574 http://fedoraproject.org/wiki/Features/Ext4
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