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 94 * Case-insensitive file name lookups
95 * file-based encryption support (fscrypt) 95 * file-based encryption support (fscrypt)
96 * file-based verity support (fsverity) 96 * file-based verity support (fsverity)
97 97
98 [1] Filesystems with a block size of 1k may se 98 [1] Filesystems with a block size of 1k may see a limit imposed by the
99 directory hash tree having a maximum depth of 99 directory hash tree having a maximum depth of two.
100 100
101 case-insensitive file name lookups 101 case-insensitive file name lookups
102 ============================================== 102 ======================================================
103 103
104 The case-insensitive file name lookup feature 104 The case-insensitive file name lookup feature is supported on a
105 per-directory basis, allowing the user to mix 105 per-directory basis, allowing the user to mix case-insensitive and
106 case-sensitive directories in the same filesys 106 case-sensitive directories in the same filesystem. It is enabled by
107 flipping the +F inode attribute of an empty di 107 flipping the +F inode attribute of an empty directory. The
108 case-insensitive string match operation is onl 108 case-insensitive string match operation is only defined when we know how
109 text in encoded in a byte sequence. For that 109 text in encoded in a byte sequence. For that reason, in order to enable
110 case-insensitive directories, the filesystem m 110 case-insensitive directories, the filesystem must have the
111 casefold feature, which stores the filesystem- 111 casefold feature, which stores the filesystem-wide encoding
112 model used. By default, the charset adopted i 112 model used. By default, the charset adopted is the latest version of
113 Unicode (12.1.0, by the time of this writing), 113 Unicode (12.1.0, by the time of this writing), encoded in the UTF-8
114 form. The comparison algorithm is implemented 114 form. The comparison algorithm is implemented by normalizing the
115 strings to the Canonical decomposition form, a 115 strings to the Canonical decomposition form, as defined by Unicode,
116 followed by a byte per byte comparison. 116 followed by a byte per byte comparison.
117 117
118 The case-awareness is name-preserving on the d 118 The case-awareness is name-preserving on the disk, meaning that the file
119 name provided by userspace is a byte-per-byte 119 name provided by userspace is a byte-per-byte match to what is actually
120 written in the disk. The Unicode normalizatio 120 written in the disk. The Unicode normalization format used by the
121 kernel is thus an internal representation, and 121 kernel is thus an internal representation, and not exposed to the
122 userspace nor to the disk, with the important 122 userspace nor to the disk, with the important exception of disk hashes,
123 used on large case-insensitive directories wit 123 used on large case-insensitive directories with DX feature. On DX
124 directories, the hash must be calculated using 124 directories, the hash must be calculated using the casefolded version of
125 the filename, meaning that the normalization f 125 the filename, meaning that the normalization format used actually has an
126 impact on where the directory entry is stored. 126 impact on where the directory entry is stored.
127 127
128 When we change from viewing filenames as opaqu 128 When we change from viewing filenames as opaque byte sequences to seeing
129 them as encoded strings we need to address wha 129 them as encoded strings we need to address what happens when a program
130 tries to create a file with an invalid name. 130 tries to create a file with an invalid name. The Unicode subsystem
131 within the kernel leaves the decision of what 131 within the kernel leaves the decision of what to do in this case to the
132 filesystem, which select its preferred behavio 132 filesystem, which select its preferred behavior by enabling/disabling
133 the strict mode. When Ext4 encounters one of 133 the strict mode. When Ext4 encounters one of those strings and the
134 filesystem did not require strict mode, it fal 134 filesystem did not require strict mode, it falls back to considering the
135 entire string as an opaque byte sequence, whic 135 entire string as an opaque byte sequence, which still allows the user to
136 operate on that file, but the case-insensitive 136 operate on that file, but the case-insensitive lookups won't work.
137 137
138 Options 138 Options
139 ======= 139 =======
140 140
141 When mounting an ext4 filesystem, the followin 141 When mounting an ext4 filesystem, the following option are accepted:
142 (*) == default 142 (*) == default
143 143
144 ro 144 ro
145 Mount filesystem read only. Note that 145 Mount filesystem read only. Note that ext4 will replay the journal (and
146 thus write to the partition) even when 146 thus write to the partition) even when mounted "read only". The mount
147 options "ro,noload" can be used to pre 147 options "ro,noload" can be used to prevent writes to the filesystem.
148 148
149 journal_checksum 149 journal_checksum
150 Enable checksumming of the journal tra 150 Enable checksumming of the journal transactions. This will allow the
151 recovery code in e2fsck and the kernel 151 recovery code in e2fsck and the kernel to detect corruption in the
152 kernel. It is a compatible change and 152 kernel. It is a compatible change and will be ignored by older
153 kernels. 153 kernels.
154 154
155 journal_async_commit 155 journal_async_commit
156 Commit block can be written to disk wi 156 Commit block can be written to disk without waiting for descriptor
157 blocks. If enabled older kernels canno 157 blocks. If enabled older kernels cannot mount the device. This will
158 enable 'journal_checksum' internally. 158 enable 'journal_checksum' internally.
159 159
160 journal_path=path, journal_dev=devnum 160 journal_path=path, journal_dev=devnum
161 When the external journal device's maj 161 When the external journal device's major/minor numbers have changed,
162 these options allow the user to specif 162 these options allow the user to specify the new journal location. The
163 journal device is identified through e 163 journal device is identified through either its new major/minor numbers
164 encoded in devnum, or via a path to th 164 encoded in devnum, or via a path to the device.
165 165
166 norecovery, noload 166 norecovery, noload
167 Don't load the journal on mounting. N 167 Don't load the journal on mounting. Note that if the filesystem was
168 not unmounted cleanly, skipping the jo 168 not unmounted cleanly, skipping the journal replay will lead to the
169 filesystem containing inconsistencies 169 filesystem containing inconsistencies that can lead to any number of
170 problems. 170 problems.
171 171
172 data=journal 172 data=journal
173 All data are committed into the journa 173 All data are committed into the journal prior to being written into the
174 main file system. Enabling this mode 174 main file system. Enabling this mode will disable delayed allocation
175 and O_DIRECT support. 175 and O_DIRECT support.
176 176
177 data=ordered (*) 177 data=ordered (*)
178 All data are forced directly out to th 178 All data are forced directly out to the main file system prior to its
179 metadata being committed to the journa 179 metadata being committed to the journal.
180 180
181 data=writeback 181 data=writeback
182 Data ordering is not preserved, data m 182 Data ordering is not preserved, data may be written into the main file
183 system after its metadata has been com 183 system after its metadata has been committed to the journal.
184 184
185 commit=nrsec (*) 185 commit=nrsec (*)
186 This setting limits the maximum age of 186 This setting limits the maximum age of the running transaction to
187 'nrsec' seconds. The default value is 187 'nrsec' seconds. The default value is 5 seconds. This means that if
188 you lose your power, you will lose as 188 you lose your power, you will lose as much as the latest 5 seconds of
189 metadata changes (your filesystem will 189 metadata changes (your filesystem will not be damaged though, thanks
190 to the journaling). This default value 190 to the journaling). This default value (or any low value) will hurt
191 performance, but it's good for data-sa 191 performance, but it's good for data-safety. Setting it to 0 will have
192 the same effect as leaving it at the d 192 the same effect as leaving it at the default (5 seconds). Setting it
193 to very large values will improve perf 193 to very large values will improve performance. Note that due to
194 delayed allocation even older data can 194 delayed allocation even older data can be lost on power failure since
195 writeback of those data begins only af 195 writeback of those data begins only after time set in
196 /proc/sys/vm/dirty_expire_centisecs. 196 /proc/sys/vm/dirty_expire_centisecs.
197 197
198 barrier=<0|1(*)>, barrier(*), nobarrier 198 barrier=<0|1(*)>, barrier(*), nobarrier
199 This enables/disables the use of write 199 This enables/disables the use of write barriers in the jbd code.
200 barrier=0 disables, barrier=1 enables. 200 barrier=0 disables, barrier=1 enables. This also requires an IO stack
201 which can support barriers, and if jbd 201 which can support barriers, and if jbd gets an error on a barrier
202 write, it will disable again with a wa 202 write, it will disable again with a warning. Write barriers enforce
203 proper on-disk ordering of journal com 203 proper on-disk ordering of journal commits, making volatile disk write
204 caches safe to use, at some performanc 204 caches safe to use, at some performance penalty. If your disks are
205 battery-backed in one way or another, 205 battery-backed in one way or another, disabling barriers may safely
206 improve performance. The mount option 206 improve performance. The mount options "barrier" and "nobarrier" can
207 also be used to enable or disable barr 207 also be used to enable or disable barriers, for consistency with other
208 ext4 mount options. 208 ext4 mount options.
209 209
210 inode_readahead_blks=n 210 inode_readahead_blks=n
211 This tuning parameter controls the max 211 This tuning parameter controls the maximum number of inode table blocks
212 that ext4's inode table readahead algo 212 that ext4's inode table readahead algorithm will pre-read into the
213 buffer cache. The default value is 32 213 buffer cache. The default value is 32 blocks.
214 214
>> 215 nouser_xattr
>> 216 Disables Extended User Attributes. See the attr(5) manual page for
>> 217 more information about extended attributes.
>> 218
>> 219 noacl
>> 220 This option disables POSIX Access Control List support. If ACL support
>> 221 is enabled in the kernel configuration (CONFIG_EXT4_FS_POSIX_ACL), ACL
>> 222 is enabled by default on mount. See the acl(5) manual page for more
>> 223 information about acl.
>> 224
215 bsddf (*) 225 bsddf (*)
216 Make 'df' act like BSD. 226 Make 'df' act like BSD.
217 227
218 minixdf 228 minixdf
219 Make 'df' act like Minix. 229 Make 'df' act like Minix.
220 230
221 debug 231 debug
222 Extra debugging information is sent to 232 Extra debugging information is sent to syslog.
223 233
224 abort 234 abort
225 Simulate the effects of calling ext4_a 235 Simulate the effects of calling ext4_abort() for debugging purposes.
226 This is normally used while remounting 236 This is normally used while remounting a filesystem which is already
227 mounted. 237 mounted.
228 238
229 errors=remount-ro 239 errors=remount-ro
230 Remount the filesystem read-only on an 240 Remount the filesystem read-only on an error.
231 241
232 errors=continue 242 errors=continue
233 Keep going on a filesystem error. 243 Keep going on a filesystem error.
234 244
235 errors=panic 245 errors=panic
236 Panic and halt the machine if an error 246 Panic and halt the machine if an error occurs. (These mount options
237 override the errors behavior specified 247 override the errors behavior specified in the superblock, which can be
238 configured using tune2fs) 248 configured using tune2fs)
239 249
240 data_err=ignore(*) 250 data_err=ignore(*)
241 Just print an error message if an erro 251 Just print an error message if an error occurs in a file data buffer in
242 ordered mode. 252 ordered mode.
243 data_err=abort 253 data_err=abort
244 Abort the journal if an error occurs i 254 Abort the journal if an error occurs in a file data buffer in ordered
245 mode. 255 mode.
246 256
247 grpid | bsdgroups 257 grpid | bsdgroups
248 New objects have the group ID of their 258 New objects have the group ID of their parent.
249 259
250 nogrpid (*) | sysvgroups 260 nogrpid (*) | sysvgroups
251 New objects have the group ID of their 261 New objects have the group ID of their creator.
252 262
253 resgid=n 263 resgid=n
254 The group ID which may use the reserve 264 The group ID which may use the reserved blocks.
255 265
256 resuid=n 266 resuid=n
257 The user ID which may use the reserved 267 The user ID which may use the reserved blocks.
258 268
259 sb= 269 sb=
260 Use alternate superblock at this locat 270 Use alternate superblock at this location.
261 271
262 quota, noquota, grpquota, usrquota 272 quota, noquota, grpquota, usrquota
263 These options are ignored by the files 273 These options are ignored by the filesystem. They are used only by
264 quota tools to recognize volumes where 274 quota tools to recognize volumes where quota should be turned on. See
265 documentation in the quota-tools packa 275 documentation in the quota-tools package for more details
266 (http://sourceforge.net/projects/linux 276 (http://sourceforge.net/projects/linuxquota).
267 277
268 jqfmt=<quota type>, usrjquota=<file>, grpjqu 278 jqfmt=<quota type>, usrjquota=<file>, grpjquota=<file>
269 These options tell filesystem details 279 These options tell filesystem details about quota so that quota
270 information can be properly updated du 280 information can be properly updated during journal replay. They replace
271 the above quota options. See documenta 281 the above quota options. See documentation in the quota-tools package
272 for more details (http://sourceforge.n 282 for more details (http://sourceforge.net/projects/linuxquota).
273 283
274 stripe=n 284 stripe=n
275 Number of filesystem blocks that mball 285 Number of filesystem blocks that mballoc will try to use for allocation
276 size and alignment. For RAID5/6 system 286 size and alignment. For RAID5/6 systems this should be the number of
277 data disks * RAID chunk size in file 287 data disks * RAID chunk size in file system blocks.
278 288
279 delalloc (*) 289 delalloc (*)
280 Defer block allocation until just befo 290 Defer block allocation until just before ext4 writes out the block(s)
281 in question. This allows ext4 to bett 291 in question. This allows ext4 to better allocation decisions more
282 efficiently. 292 efficiently.
283 293
284 nodelalloc 294 nodelalloc
285 Disable delayed allocation. Blocks ar 295 Disable delayed allocation. Blocks are allocated when the data is
286 copied from userspace to the page cach 296 copied from userspace to the page cache, either via the write(2) system
287 call or when an mmap'ed page which was 297 call or when an mmap'ed page which was previously unallocated is
288 written for the first time. 298 written for the first time.
289 299
290 max_batch_time=usec 300 max_batch_time=usec
291 Maximum amount of time ext4 should wai 301 Maximum amount of time ext4 should wait for additional filesystem
292 operations to be batch together with a 302 operations to be batch together with a synchronous write operation.
293 Since a synchronous write operation is 303 Since a synchronous write operation is going to force a commit and then
294 a wait for the I/O complete, it doesn' 304 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 305 throughput win, we wait for a small amount of time to see if any other
296 transactions can piggyback on the sync 306 transactions can piggyback on the synchronous write. The algorithm
297 used is designed to automatically tune 307 used is designed to automatically tune for the speed of the disk, by
298 measuring the amount of time (on avera 308 measuring the amount of time (on average) that it takes to finish
299 committing a transaction. Call this t 309 committing a transaction. Call this time the "commit time". If the
300 time that the transaction has been run 310 time that the transaction has been running is less than the commit
301 time, ext4 will try sleeping for the c 311 time, ext4 will try sleeping for the commit time to see if other
302 operations will join the transaction. 312 operations will join the transaction. The commit time is capped by
303 the max_batch_time, which defaults to 313 the max_batch_time, which defaults to 15000us (15ms). This
304 optimization can be turned off entirel 314 optimization can be turned off entirely by setting max_batch_time to 0.
305 315
306 min_batch_time=usec 316 min_batch_time=usec
307 This parameter sets the commit time (a 317 This parameter sets the commit time (as described above) to be at least
308 min_batch_time. It defaults to zero m 318 min_batch_time. It defaults to zero microseconds. Increasing this
309 parameter may improve the throughput o 319 parameter may improve the throughput of multi-threaded, synchronous
310 workloads on very fast disks, at the c 320 workloads on very fast disks, at the cost of increasing latency.
311 321
312 journal_ioprio=prio 322 journal_ioprio=prio
313 The I/O priority (from 0 to 7, where 0 323 The I/O priority (from 0 to 7, where 0 is the highest priority) which
314 should be used for I/O operations subm 324 should be used for I/O operations submitted by kjournald2 during a
315 commit operation. This defaults to 3, 325 commit operation. This defaults to 3, which is a slightly higher
316 priority than the default I/O priority 326 priority than the default I/O priority.
317 327
318 auto_da_alloc(*), noauto_da_alloc 328 auto_da_alloc(*), noauto_da_alloc
319 Many broken applications don't use fsy 329 Many broken applications don't use fsync() when replacing existing
320 files via patterns such as fd = open(" 330 files via patterns such as fd = open("foo.new")/write(fd,..)/close(fd)/
321 rename("foo.new", "foo"), or worse yet 331 rename("foo.new", "foo"), or worse yet, fd = open("foo",
322 O_TRUNC)/write(fd,..)/close(fd). If a 332 O_TRUNC)/write(fd,..)/close(fd). If auto_da_alloc is enabled, ext4
323 will detect the replace-via-rename and 333 will detect the replace-via-rename and replace-via-truncate patterns
324 and force that any delayed allocation 334 and force that any delayed allocation blocks are allocated such that at
325 the next journal commit, in the defaul 335 the next journal commit, in the default data=ordered mode, the data
326 blocks of the new file are forced to d 336 blocks of the new file are forced to disk before the rename() operation
327 is committed. This provides roughly t 337 is committed. This provides roughly the same level of guarantees as
328 ext3, and avoids the "zero-length" pro 338 ext3, and avoids the "zero-length" problem that can happen when a
329 system crashes before the delayed allo 339 system crashes before the delayed allocation blocks are forced to disk.
330 340
331 noinit_itable 341 noinit_itable
332 Do not initialize any uninitialized in 342 Do not initialize any uninitialized inode table blocks in the
333 background. This feature may be used 343 background. This feature may be used by installation CD's so that the
334 install process can complete as quickl 344 install process can complete as quickly as possible; the inode table
335 initialization process would then be d 345 initialization process would then be deferred until the next time the
336 file system is unmounted. 346 file system is unmounted.
337 347
338 init_itable=n 348 init_itable=n
339 The lazy itable init code will wait n 349 The lazy itable init code will wait n times the number of milliseconds
340 it took to zero out the previous block 350 it took to zero out the previous block group's inode table. This
341 minimizes the impact on the system per 351 minimizes the impact on the system performance while file system's
342 inode table is being initialized. 352 inode table is being initialized.
343 353
344 discard, nodiscard(*) 354 discard, nodiscard(*)
345 Controls whether ext4 should issue dis 355 Controls whether ext4 should issue discard/TRIM commands to the
346 underlying block device when blocks ar 356 underlying block device when blocks are freed. This is useful for SSD
347 devices and sparse/thinly-provisioned 357 devices and sparse/thinly-provisioned LUNs, but it is off by default
348 until sufficient testing has been done 358 until sufficient testing has been done.
349 359
350 nouid32 360 nouid32
351 Disables 32-bit UIDs and GIDs. This i 361 Disables 32-bit UIDs and GIDs. This is for interoperability with
352 older kernels which only store and exp 362 older kernels which only store and expect 16-bit values.
353 363
354 block_validity(*), noblock_validity 364 block_validity(*), noblock_validity
355 These options enable or disable the in 365 These options enable or disable the in-kernel facility for tracking
356 filesystem metadata blocks within inte 366 filesystem metadata blocks within internal data structures. This
357 allows multi- block allocator and othe 367 allows multi- block allocator and other routines to notice bugs or
358 corrupted allocation bitmaps which cau 368 corrupted allocation bitmaps which cause blocks to be allocated which
359 overlap with filesystem metadata block 369 overlap with filesystem metadata blocks.
360 370
361 dioread_lock, dioread_nolock 371 dioread_lock, dioread_nolock
362 Controls whether or not ext4 should us 372 Controls whether or not ext4 should use the DIO read locking. If the
363 dioread_nolock option is specified ext 373 dioread_nolock option is specified ext4 will allocate uninitialized
364 extent before buffer write and convert 374 extent before buffer write and convert the extent to initialized after
365 IO completes. This approach allows ext 375 IO completes. This approach allows ext4 code to avoid using inode
366 mutex, which improves scalability on h 376 mutex, which improves scalability on high speed storages. However this
367 does not work with data journaling and 377 does not work with data journaling and dioread_nolock option will be
368 ignored with kernel warning. Note that 378 ignored with kernel warning. Note that dioread_nolock code path is only
369 used for extent-based files. Because 379 used for extent-based files. Because of the restrictions this options
370 comprises it is off by default (e.g. d 380 comprises it is off by default (e.g. dioread_lock).
371 381
372 max_dir_size_kb=n 382 max_dir_size_kb=n
373 This limits the size of directories so 383 This limits the size of directories so that any attempt to expand them
374 beyond the specified limit in kilobyte 384 beyond the specified limit in kilobytes will cause an ENOSPC error.
375 This is useful in memory constrained e 385 This is useful in memory constrained environments, where a very large
376 directory can cause severe performance 386 directory can cause severe performance problems or even provoke the Out
377 Of Memory killer. (For example, if th 387 Of Memory killer. (For example, if there is only 512mb memory
378 available, a 176mb directory may serio 388 available, a 176mb directory may seriously cramp the system's style.)
379 389
380 i_version 390 i_version
381 Enable 64-bit inode version support. T 391 Enable 64-bit inode version support. This option is off by default.
382 392
383 dax 393 dax
384 Use direct access (no page cache). Se 394 Use direct access (no page cache). See
385 Documentation/filesystems/dax.rst. No !! 395 Documentation/filesystems/dax.txt. Note that this option is
386 incompatible with data=journal. 396 incompatible with data=journal.
387 397
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 398 Data Mode
396 ========= 399 =========
397 There are 3 different data modes: 400 There are 3 different data modes:
398 401
399 * writeback mode 402 * writeback mode
400 403
401 In data=writeback mode, ext4 does not journa 404 In data=writeback mode, ext4 does not journal data at all. This mode provides
402 a similar level of journaling as that of XFS 405 a similar level of journaling as that of XFS, JFS, and ReiserFS in its default
403 mode - metadata journaling. A crash+recover 406 mode - metadata journaling. A crash+recovery can cause incorrect data to
404 appear in files which were written shortly b 407 appear in files which were written shortly before the crash. This mode will
405 typically provide the best ext4 performance. 408 typically provide the best ext4 performance.
406 409
407 * ordered mode 410 * ordered mode
408 411
409 In data=ordered mode, ext4 only officially j 412 In data=ordered mode, ext4 only officially journals metadata, but it logically
410 groups metadata information related to data 413 groups metadata information related to data changes with the data blocks into
411 a single unit called a transaction. When it 414 a single unit called a transaction. When it's time to write the new metadata
412 out to disk, the associated data blocks are 415 out to disk, the associated data blocks are written first. In general, this
413 mode performs slightly slower than writeback 416 mode performs slightly slower than writeback but significantly faster than
414 journal mode. 417 journal mode.
415 418
416 * journal mode 419 * journal mode
417 420
418 data=journal mode provides full data and met 421 data=journal mode provides full data and metadata journaling. All new data is
419 written to the journal first, and then to it 422 written to the journal first, and then to its final location. In the event of
420 a crash, the journal can be replayed, bringi 423 a crash, the journal can be replayed, bringing both data and metadata into a
421 consistent state. This mode is the slowest 424 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 425 from and written to disk at the same time where it outperforms all others
423 modes. Enabling this mode will disable dela 426 modes. Enabling this mode will disable delayed allocation and O_DIRECT
424 support. 427 support.
425 428
426 /proc entries 429 /proc entries
427 ============= 430 =============
428 431
429 Information about mounted ext4 file systems ca 432 Information about mounted ext4 file systems can be found in
430 /proc/fs/ext4. Each mounted filesystem will h 433 /proc/fs/ext4. Each mounted filesystem will have a directory in
431 /proc/fs/ext4 based on its device name (i.e., 434 /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 435 /proc/fs/ext4/dm-0). The files in each per-device directory are shown
433 in table below. 436 in table below.
434 437
435 Files in /proc/fs/ext4/<devname> 438 Files in /proc/fs/ext4/<devname>
436 439
437 mb_groups 440 mb_groups
438 details of multiblock allocator buddy 441 details of multiblock allocator buddy cache of free blocks
439 442
440 /sys entries 443 /sys entries
441 ============ 444 ============
442 445
443 Information about mounted ext4 file systems ca 446 Information about mounted ext4 file systems can be found in
444 /sys/fs/ext4. Each mounted filesystem will ha 447 /sys/fs/ext4. Each mounted filesystem will have a directory in
445 /sys/fs/ext4 based on its device name (i.e., / 448 /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 449 /sys/fs/ext4/dm-0). The files in each per-device directory are shown
447 in table below. 450 in table below.
448 451
449 Files in /sys/fs/ext4/<devname>: 452 Files in /sys/fs/ext4/<devname>:
450 453
451 (see also Documentation/ABI/testing/sysfs-fs-e 454 (see also Documentation/ABI/testing/sysfs-fs-ext4)
452 455
453 delayed_allocation_blocks 456 delayed_allocation_blocks
454 This file is read-only and shows the n 457 This file is read-only and shows the number of blocks that are dirty in
455 the page cache, but which do not have 458 the page cache, but which do not have their location in the filesystem
456 allocated yet. 459 allocated yet.
457 460
458 inode_goal 461 inode_goal
459 Tuning parameter which (if non-zero) c 462 Tuning parameter which (if non-zero) controls the goal inode used by
460 the inode allocator in preference to a 463 the inode allocator in preference to all other allocation heuristics.
461 This is intended for debugging use onl 464 This is intended for debugging use only, and should be 0 on production
462 systems. 465 systems.
463 466
464 inode_readahead_blks 467 inode_readahead_blks
465 Tuning parameter which controls the ma 468 Tuning parameter which controls the maximum number of inode table
466 blocks that ext4's inode table readahe 469 blocks that ext4's inode table readahead algorithm will pre-read into
467 the buffer cache. 470 the buffer cache.
468 471
469 lifetime_write_kbytes 472 lifetime_write_kbytes
470 This file is read-only and shows the n 473 This file is read-only and shows the number of kilobytes of data that
471 have been written to this filesystem s 474 have been written to this filesystem since it was created.
472 475
473 max_writeback_mb_bump 476 max_writeback_mb_bump
474 The maximum number of megabytes the wr 477 The maximum number of megabytes the writeback code will try to write
475 out before move on to another inode. 478 out before move on to another inode.
476 479
477 mb_group_prealloc 480 mb_group_prealloc
478 The multiblock allocator will round up 481 The multiblock allocator will round up allocation requests to a
479 multiple of this tuning parameter if t 482 multiple of this tuning parameter if the stripe size is not set in the
480 ext4 superblock 483 ext4 superblock
481 484
>> 485 mb_max_inode_prealloc
>> 486 The maximum length of per-inode ext4_prealloc_space list.
>> 487
482 mb_max_to_scan 488 mb_max_to_scan
483 The maximum number of extents the mult 489 The maximum number of extents the multiblock allocator will search to
484 find the best extent. 490 find the best extent.
485 491
486 mb_min_to_scan 492 mb_min_to_scan
487 The minimum number of extents the mult 493 The minimum number of extents the multiblock allocator will search to
488 find the best extent. 494 find the best extent.
489 495
490 mb_order2_req 496 mb_order2_req
491 Tuning parameter which controls the mi 497 Tuning parameter which controls the minimum size for requests (as a
492 power of 2) where the buddy cache is u 498 power of 2) where the buddy cache is used.
493 499
494 mb_stats 500 mb_stats
495 Controls whether the multiblock alloca 501 Controls whether the multiblock allocator should collect statistics,
496 which are shown during the unmount. 1 502 which are shown during the unmount. 1 means to collect statistics, 0
497 means not to collect statistics. 503 means not to collect statistics.
498 504
499 mb_stream_req 505 mb_stream_req
500 Files which have fewer blocks than thi 506 Files which have fewer blocks than this tunable parameter will have
501 their blocks allocated out of a block 507 their blocks allocated out of a block group specific preallocation
502 pool, so that small files are packed c 508 pool, so that small files are packed closely together. Each large file
503 will have its blocks allocated out of 509 will have its blocks allocated out of its own unique preallocation
504 pool. 510 pool.
505 511
506 session_write_kbytes 512 session_write_kbytes
507 This file is read-only and shows the n 513 This file is read-only and shows the number of kilobytes of data that
508 have been written to this filesystem s 514 have been written to this filesystem since it was mounted.
509 515
510 reserved_clusters 516 reserved_clusters
511 This is RW file and contains number of 517 This is RW file and contains number of reserved clusters in the file
512 system which will be used in the speci 518 system which will be used in the specific situations to avoid costly
513 zeroout, unexpected ENOSPC, or possibl 519 zeroout, unexpected ENOSPC, or possible data loss. The default is 2% or
514 4096 clusters, whichever is smaller an 520 4096 clusters, whichever is smaller and this can be changed however it
515 can never exceed number of clusters in 521 can never exceed number of clusters in the file system. If there is not
516 enough space for the reserved space wh 522 enough space for the reserved space when mounting the file mount will
517 _not_ fail. 523 _not_ fail.
518 524
519 Ioctls 525 Ioctls
520 ====== 526 ======
521 527
522 Ext4 implements various ioctls which can be us !! 528 There is some Ext4 specific functionality which can be accessed by applications
523 ext4-specific functionality. An incomplete lis !! 529 through the system call interfaces. The list of all Ext4 specific ioctls are
524 table below. This list includes truly ext4-spe !! 530 shown in the table below.
525 well as ioctls that may have been ext4-specifi <<
526 by some other filesystem(s) too (``FS_IOC_*``) <<
527 531
528 Table of Ext4 ioctls !! 532 Table of Ext4 specific ioctls
529 533
530 FS_IOC_GETFLAGS !! 534 EXT4_IOC_GETFLAGS
531 Get additional attributes associated w 535 Get additional attributes associated with inode. The ioctl argument is
532 an integer bitfield, with bit values d !! 536 an integer bitfield, with bit values described in ext4.h. This ioctl is
>> 537 an alias for FS_IOC_GETFLAGS.
533 538
534 FS_IOC_SETFLAGS !! 539 EXT4_IOC_SETFLAGS
535 Set additional attributes associated w 540 Set additional attributes associated with inode. The ioctl argument is
536 an integer bitfield, with bit values d !! 541 an integer bitfield, with bit values described in ext4.h. This ioctl is
>> 542 an alias for FS_IOC_SETFLAGS.
537 543
538 EXT4_IOC_GETVERSION, EXT4_IOC_GETVERSION_OLD 544 EXT4_IOC_GETVERSION, EXT4_IOC_GETVERSION_OLD
539 Get the inode i_generation number stor 545 Get the inode i_generation number stored for each inode. The
540 i_generation number is normally change 546 i_generation number is normally changed only when new inode is created
541 and it is particularly useful for netw 547 and it is particularly useful for network filesystems. The '_OLD'
542 version of this ioctl is an alias for 548 version of this ioctl is an alias for FS_IOC_GETVERSION.
543 549
544 EXT4_IOC_SETVERSION, EXT4_IOC_SETVERSION_OLD 550 EXT4_IOC_SETVERSION, EXT4_IOC_SETVERSION_OLD
545 Set the inode i_generation number stor 551 Set the inode i_generation number stored for each inode. The '_OLD'
546 version of this ioctl is an alias for 552 version of this ioctl is an alias for FS_IOC_SETVERSION.
547 553
548 EXT4_IOC_GROUP_EXTEND 554 EXT4_IOC_GROUP_EXTEND
549 This ioctl has the same purpose as the 555 This ioctl has the same purpose as the resize mount option. It allows
550 to resize filesystem to the end of the 556 to resize filesystem to the end of the last existing block group,
551 further resize has to be done with res 557 further resize has to be done with resize2fs, either online, or
552 offline. The argument points to the un 558 offline. The argument points to the unsigned logn number representing
553 the filesystem new block count. 559 the filesystem new block count.
554 560
555 EXT4_IOC_MOVE_EXT 561 EXT4_IOC_MOVE_EXT
556 Move the block extents from orig_fd (t 562 Move the block extents from orig_fd (the one this ioctl is pointing to)
557 to the donor_fd (the one specified in 563 to the donor_fd (the one specified in move_extent structure passed as
558 an argument to this ioctl). Then, exch 564 an argument to this ioctl). Then, exchange inode metadata between
559 orig_fd and donor_fd. This is especia 565 orig_fd and donor_fd. This is especially useful for online
560 defragmentation, because the allocator 566 defragmentation, because the allocator has the opportunity to allocate
561 moved blocks better, ideally into one 567 moved blocks better, ideally into one contiguous extent.
562 568
563 EXT4_IOC_GROUP_ADD 569 EXT4_IOC_GROUP_ADD
564 Add a new group descriptor to an exist 570 Add a new group descriptor to an existing or new group descriptor
565 block. The new group descriptor is des 571 block. The new group descriptor is described by ext4_new_group_input
566 structure, which is passed as an argum 572 structure, which is passed as an argument to this ioctl. This is
567 especially useful in conjunction with 573 especially useful in conjunction with EXT4_IOC_GROUP_EXTEND, which
568 allows online resize of the filesystem 574 allows online resize of the filesystem to the end of the last existing
569 block group. Those two ioctls combine 575 block group. Those two ioctls combined is used in userspace online
570 resize tool (e.g. resize2fs). 576 resize tool (e.g. resize2fs).
571 577
572 EXT4_IOC_MIGRATE 578 EXT4_IOC_MIGRATE
573 This ioctl operates on the filesystem 579 This ioctl operates on the filesystem itself. It converts (migrates)
574 ext3 indirect block mapped inode to ex 580 ext3 indirect block mapped inode to ext4 extent mapped inode by walking
575 through indirect block mapping of the 581 through indirect block mapping of the original inode and converting
576 contiguous block ranges into ext4 exte 582 contiguous block ranges into ext4 extents of the temporary inode. Then,
577 inodes are swapped. This ioctl might h 583 inodes are swapped. This ioctl might help, when migrating from ext3 to
578 ext4 filesystem, however suggestion is 584 ext4 filesystem, however suggestion is to create fresh ext4 filesystem
579 and copy data from the backup. Note, t 585 and copy data from the backup. Note, that filesystem has to support
580 extents for this ioctl to work. 586 extents for this ioctl to work.
581 587
582 EXT4_IOC_ALLOC_DA_BLKS 588 EXT4_IOC_ALLOC_DA_BLKS
583 Force all of the delay allocated block 589 Force all of the delay allocated blocks to be allocated to preserve
584 application-expected ext3 behaviour. N 590 application-expected ext3 behaviour. Note that this will also start
585 triggering a write of the data blocks, 591 triggering a write of the data blocks, but this behaviour may change in
586 the future as it is not necessary and 592 the future as it is not necessary and has been done this way only for
587 sake of simplicity. 593 sake of simplicity.
588 594
589 EXT4_IOC_RESIZE_FS 595 EXT4_IOC_RESIZE_FS
590 Resize the filesystem to a new size. 596 Resize the filesystem to a new size. The number of blocks of resized
591 filesystem is passed in via 64 bit int 597 filesystem is passed in via 64 bit integer argument. The kernel
592 allocates bitmaps and inode table, the 598 allocates bitmaps and inode table, the userspace tool thus just passes
593 the new number of blocks. 599 the new number of blocks.
594 600
595 EXT4_IOC_SWAP_BOOT 601 EXT4_IOC_SWAP_BOOT
596 Swap i_blocks and associated attribute 602 Swap i_blocks and associated attributes (like i_blocks, i_size,
597 i_flags, ...) from the specified inode 603 i_flags, ...) from the specified inode with inode EXT4_BOOT_LOADER_INO
598 (#5). This is typically used to store 604 (#5). This is typically used to store a boot loader in a secure part of
599 the filesystem, where it can't be chan 605 the filesystem, where it can't be changed by a normal user by accident.
600 The data blocks of the previous boot l 606 The data blocks of the previous boot loader will be associated with the
601 given inode. 607 given inode.
602 608
603 References 609 References
604 ========== 610 ==========
605 611
606 kernel source: <file:fs/ext4/> 612 kernel source: <file:fs/ext4/>
607 <file:fs/jbd2/> 613 <file:fs/jbd2/>
608 614
609 programs: http://e2fsprogs.sourceforge.n 615 programs: http://e2fsprogs.sourceforge.net/
610 616
611 useful links: https://fedoraproject.org/wiki !! 617 useful links: http://fedoraproject.org/wiki/ext3-devel
612 http://www.bullopensource.org/ 618 http://www.bullopensource.org/ext4/
613 http://ext4.wiki.kernel.org/in 619 http://ext4.wiki.kernel.org/index.php/Main_Page
614 https://fedoraproject.org/wiki !! 620 http://fedoraproject.org/wiki/Features/Ext4
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