ext4.rst (28243B)
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SPDX-License-Identifier: GPL-2.0 2 3======================== 4ext4 General Information 5======================== 6 7Ext4 is an advanced level of the ext3 filesystem which incorporates 8scalability and reliability enhancements for supporting large filesystems 9(64 bit) in keeping with increasing disk capacities and state-of-the-art 10feature requirements. 11 12Mailing list: linux-ext4@vger.kernel.org 13Web site: http://ext4.wiki.kernel.org 14 15 16Quick usage instructions 17======================== 18 19Note: More extensive information for getting started with ext4 can be 20found at the ext4 wiki site at the URL: 21http://ext4.wiki.kernel.org/index.php/Ext4_Howto 22 23 - The latest version of e2fsprogs can be found at: 24 25 https://www.kernel.org/pub/linux/kernel/people/tytso/e2fsprogs/ 26 27 or 28 29 http://sourceforge.net/project/showfiles.php?group_id=2406 30 31 or grab the latest git repository from: 32 33 https://git.kernel.org/pub/scm/fs/ext2/e2fsprogs.git 34 35 - Create a new filesystem using the ext4 filesystem type: 36 37 # mke2fs -t ext4 /dev/hda1 38 39 Or to configure an existing ext3 filesystem to support extents: 40 41 # tune2fs -O extents /dev/hda1 42 43 If the filesystem was created with 128 byte inodes, it can be 44 converted to use 256 byte for greater efficiency via: 45 46 # tune2fs -I 256 /dev/hda1 47 48 - Mounting: 49 50 # mount -t ext4 /dev/hda1 /wherever 51 52 - When comparing performance with other filesystems, it's always 53 important to try multiple workloads; very often a subtle change in a 54 workload parameter can completely change the ranking of which 55 filesystems do well compared to others. When comparing versus ext3, 56 note that ext4 enables write barriers by default, while ext3 does 57 not enable write barriers by default. So it is useful to use 58 explicitly specify whether barriers are enabled or not when via the 59 '-o barriers=[0|1]' mount option for both ext3 and ext4 filesystems 60 for a fair comparison. When tuning ext3 for best benchmark numbers, 61 it is often worthwhile to try changing the data journaling mode; '-o 62 data=writeback' can be faster for some workloads. (Note however that 63 running mounted with data=writeback can potentially leave stale data 64 exposed in recently written files in case of an unclean shutdown, 65 which could be a security exposure in some situations.) Configuring 66 the filesystem with a large journal can also be helpful for 67 metadata-intensive workloads. 68 69Features 70======== 71 72Currently Available 73------------------- 74 75* ability to use filesystems > 16TB (e2fsprogs support not available yet) 76* extent format reduces metadata overhead (RAM, IO for access, transactions) 77* extent format more robust in face of on-disk corruption due to magics, 78* internal redundancy in tree 79* improved file allocation (multi-block alloc) 80* lift 32000 subdirectory limit imposed by i_links_count[1] 81* nsec timestamps for mtime, atime, ctime, create time 82* inode version field on disk (NFSv4, Lustre) 83* reduced e2fsck time via uninit_bg feature 84* journal checksumming for robustness, performance 85* persistent file preallocation (e.g for streaming media, databases) 86* ability to pack bitmaps and inode tables into larger virtual groups via the 87 flex_bg feature 88* large file support 89* inode allocation using large virtual block groups via flex_bg 90* delayed allocation 91* large block (up to pagesize) support 92* efficient new ordered mode in JBD2 and ext4 (avoid using buffer head to force 93 the ordering) 94* Case-insensitive file name lookups 95* file-based encryption support (fscrypt) 96* file-based verity support (fsverity) 97 98[1] Filesystems with a block size of 1k may see a limit imposed by the 99directory hash tree having a maximum depth of two. 100 101case-insensitive file name lookups 102====================================================== 103 104The case-insensitive file name lookup feature is supported on a 105per-directory basis, allowing the user to mix case-insensitive and 106case-sensitive directories in the same filesystem. It is enabled by 107flipping the +F inode attribute of an empty directory. The 108case-insensitive string match operation is only defined when we know how 109text in encoded in a byte sequence. For that reason, in order to enable 110case-insensitive directories, the filesystem must have the 111casefold feature, which stores the filesystem-wide encoding 112model used. By default, the charset adopted is the latest version of 113Unicode (12.1.0, by the time of this writing), encoded in the UTF-8 114form. The comparison algorithm is implemented by normalizing the 115strings to the Canonical decomposition form, as defined by Unicode, 116followed by a byte per byte comparison. 117 118The case-awareness is name-preserving on the disk, meaning that the file 119name provided by userspace is a byte-per-byte match to what is actually 120written in the disk. The Unicode normalization format used by the 121kernel is thus an internal representation, and not exposed to the 122userspace nor to the disk, with the important exception of disk hashes, 123used on large case-insensitive directories with DX feature. On DX 124directories, the hash must be calculated using the casefolded version of 125the filename, meaning that the normalization format used actually has an 126impact on where the directory entry is stored. 127 128When we change from viewing filenames as opaque byte sequences to seeing 129them as encoded strings we need to address what happens when a program 130tries to create a file with an invalid name. The Unicode subsystem 131within the kernel leaves the decision of what to do in this case to the 132filesystem, which select its preferred behavior by enabling/disabling 133the strict mode. When Ext4 encounters one of those strings and the 134filesystem did not require strict mode, it falls back to considering the 135entire string as an opaque byte sequence, which still allows the user to 136operate on that file, but the case-insensitive lookups won't work. 137 138Options 139======= 140 141When mounting an ext4 filesystem, the following option are accepted: 142(*) == default 143 144 ro 145 Mount filesystem read only. Note that ext4 will replay the journal (and 146 thus write to the partition) even when mounted "read only". The mount 147 options "ro,noload" can be used to prevent writes to the filesystem. 148 149 journal_checksum 150 Enable checksumming of the journal transactions. This will allow the 151 recovery code in e2fsck and the kernel to detect corruption in the 152 kernel. It is a compatible change and will be ignored by older 153 kernels. 154 155 journal_async_commit 156 Commit block can be written to disk without waiting for descriptor 157 blocks. If enabled older kernels cannot mount the device. This will 158 enable 'journal_checksum' internally. 159 160 journal_path=path, journal_dev=devnum 161 When the external journal device's major/minor numbers have changed, 162 these options allow the user to specify the new journal location. The 163 journal device is identified through either its new major/minor numbers 164 encoded in devnum, or via a path to the device. 165 166 norecovery, noload 167 Don't load the journal on mounting. Note that if the filesystem was 168 not unmounted cleanly, skipping the journal replay will lead to the 169 filesystem containing inconsistencies that can lead to any number of 170 problems. 171 172 data=journal 173 All data are committed into the journal prior to being written into the 174 main file system. Enabling this mode will disable delayed allocation 175 and O_DIRECT support. 176 177 data=ordered (*) 178 All data are forced directly out to the main file system prior to its 179 metadata being committed to the journal. 180 181 data=writeback 182 Data ordering is not preserved, data may be written into the main file 183 system after its metadata has been committed to the journal. 184 185 commit=nrsec (*) 186 This setting limits the maximum age of the running transaction to 187 'nrsec' seconds. The default value is 5 seconds. This means that if 188 you lose your power, you will lose as much as the latest 5 seconds of 189 metadata changes (your filesystem will not be damaged though, thanks 190 to the journaling). This default value (or any low value) will hurt 191 performance, but it's good for data-safety. Setting it to 0 will have 192 the same effect as leaving it at the default (5 seconds). Setting it 193 to very large values will improve performance. Note that due to 194 delayed allocation even older data can be lost on power failure since 195 writeback of those data begins only after time set in 196 /proc/sys/vm/dirty_expire_centisecs. 197 198 barrier=<0|1(*)>, barrier(*), nobarrier 199 This enables/disables the use of write barriers in the jbd code. 200 barrier=0 disables, barrier=1 enables. This also requires an IO stack 201 which can support barriers, and if jbd gets an error on a barrier 202 write, it will disable again with a warning. Write barriers enforce 203 proper on-disk ordering of journal commits, making volatile disk write 204 caches safe to use, at some performance penalty. If your disks are 205 battery-backed in one way or another, disabling barriers may safely 206 improve performance. The mount options "barrier" and "nobarrier" can 207 also be used to enable or disable barriers, for consistency with other 208 ext4 mount options. 209 210 inode_readahead_blks=n 211 This tuning parameter controls the maximum number of inode table blocks 212 that ext4's inode table readahead algorithm will pre-read into the 213 buffer cache. The default value is 32 blocks. 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. 224 225 bsddf (*) 226 Make 'df' act like BSD. 227 228 minixdf 229 Make 'df' act like Minix. 230 231 debug 232 Extra debugging information is sent to syslog. 233 234 abort 235 Simulate the effects of calling ext4_abort() for debugging purposes. 236 This is normally used while remounting a filesystem which is already 237 mounted. 238 239 errors=remount-ro 240 Remount the filesystem read-only on an error. 241 242 errors=continue 243 Keep going on a filesystem error. 244 245 errors=panic 246 Panic and halt the machine if an error occurs. (These mount options 247 override the errors behavior specified in the superblock, which can be 248 configured using tune2fs) 249 250 data_err=ignore(*) 251 Just print an error message if an error occurs in a file data buffer in 252 ordered mode. 253 data_err=abort 254 Abort the journal if an error occurs in a file data buffer in ordered 255 mode. 256 257 grpid | bsdgroups 258 New objects have the group ID of their parent. 259 260 nogrpid (*) | sysvgroups 261 New objects have the group ID of their creator. 262 263 resgid=n 264 The group ID which may use the reserved blocks. 265 266 resuid=n 267 The user ID which may use the reserved blocks. 268 269 sb= 270 Use alternate superblock at this location. 271 272 quota, noquota, grpquota, usrquota 273 These options are ignored by the filesystem. They are used only by 274 quota tools to recognize volumes where quota should be turned on. See 275 documentation in the quota-tools package for more details 276 (http://sourceforge.net/projects/linuxquota). 277 278 jqfmt=<quota type>, usrjquota=<file>, grpjquota=<file> 279 These options tell filesystem details about quota so that quota 280 information can be properly updated during journal replay. They replace 281 the above quota options. See documentation in the quota-tools package 282 for more details (http://sourceforge.net/projects/linuxquota). 283 284 stripe=n 285 Number of filesystem blocks that mballoc will try to use for allocation 286 size and alignment. For RAID5/6 systems this should be the number of 287 data disks * RAID chunk size in file system blocks. 288 289 delalloc (*) 290 Defer block allocation until just before ext4 writes out the block(s) 291 in question. This allows ext4 to better allocation decisions more 292 efficiently. 293 294 nodelalloc 295 Disable delayed allocation. Blocks are allocated when the data is 296 copied from userspace to the page cache, either via the write(2) system 297 call or when an mmap'ed page which was previously unallocated is 298 written for the first time. 299 300 max_batch_time=usec 301 Maximum amount of time ext4 should wait for additional filesystem 302 operations to be batch together with a synchronous write operation. 303 Since a synchronous write operation is going to force a commit and then 304 a wait for the I/O complete, it doesn't cost much, and can be a huge 305 throughput win, we wait for a small amount of time to see if any other 306 transactions can piggyback on the synchronous write. The algorithm 307 used is designed to automatically tune for the speed of the disk, by 308 measuring the amount of time (on average) that it takes to finish 309 committing a transaction. Call this time the "commit time". If the 310 time that the transaction has been running is less than the commit 311 time, ext4 will try sleeping for the commit time to see if other 312 operations will join the transaction. The commit time is capped by 313 the max_batch_time, which defaults to 15000us (15ms). This 314 optimization can be turned off entirely by setting max_batch_time to 0. 315 316 min_batch_time=usec 317 This parameter sets the commit time (as described above) to be at least 318 min_batch_time. It defaults to zero microseconds. Increasing this 319 parameter may improve the throughput of multi-threaded, synchronous 320 workloads on very fast disks, at the cost of increasing latency. 321 322 journal_ioprio=prio 323 The I/O priority (from 0 to 7, where 0 is the highest priority) which 324 should be used for I/O operations submitted by kjournald2 during a 325 commit operation. This defaults to 3, which is a slightly higher 326 priority than the default I/O priority. 327 328 auto_da_alloc(*), noauto_da_alloc 329 Many broken applications don't use fsync() when replacing existing 330 files via patterns such as fd = open("foo.new")/write(fd,..)/close(fd)/ 331 rename("foo.new", "foo"), or worse yet, fd = open("foo", 332 O_TRUNC)/write(fd,..)/close(fd). If auto_da_alloc is enabled, ext4 333 will detect the replace-via-rename and replace-via-truncate patterns 334 and force that any delayed allocation blocks are allocated such that at 335 the next journal commit, in the default data=ordered mode, the data 336 blocks of the new file are forced to disk before the rename() operation 337 is committed. This provides roughly the same level of guarantees as 338 ext3, and avoids the "zero-length" problem that can happen when a 339 system crashes before the delayed allocation blocks are forced to disk. 340 341 noinit_itable 342 Do not initialize any uninitialized inode table blocks in the 343 background. This feature may be used by installation CD's so that the 344 install process can complete as quickly as possible; the inode table 345 initialization process would then be deferred until the next time the 346 file system is unmounted. 347 348 init_itable=n 349 The lazy itable init code will wait n times the number of milliseconds 350 it took to zero out the previous block group's inode table. This 351 minimizes the impact on the system performance while file system's 352 inode table is being initialized. 353 354 discard, nodiscard(*) 355 Controls whether ext4 should issue discard/TRIM commands to the 356 underlying block device when blocks are freed. This is useful for SSD 357 devices and sparse/thinly-provisioned LUNs, but it is off by default 358 until sufficient testing has been done. 359 360 nouid32 361 Disables 32-bit UIDs and GIDs. This is for interoperability with 362 older kernels which only store and expect 16-bit values. 363 364 block_validity(*), noblock_validity 365 These options enable or disable the in-kernel facility for tracking 366 filesystem metadata blocks within internal data structures. This 367 allows multi- block allocator and other routines to notice bugs or 368 corrupted allocation bitmaps which cause blocks to be allocated which 369 overlap with filesystem metadata blocks. 370 371 dioread_lock, dioread_nolock 372 Controls whether or not ext4 should use the DIO read locking. If the 373 dioread_nolock option is specified ext4 will allocate uninitialized 374 extent before buffer write and convert the extent to initialized after 375 IO completes. This approach allows ext4 code to avoid using inode 376 mutex, which improves scalability on high speed storages. However this 377 does not work with data journaling and dioread_nolock option will be 378 ignored with kernel warning. Note that dioread_nolock code path is only 379 used for extent-based files. Because of the restrictions this options 380 comprises it is off by default (e.g. dioread_lock). 381 382 max_dir_size_kb=n 383 This limits the size of directories so that any attempt to expand them 384 beyond the specified limit in kilobytes will cause an ENOSPC error. 385 This is useful in memory constrained environments, where a very large 386 directory can cause severe performance problems or even provoke the Out 387 Of Memory killer. (For example, if there is only 512mb memory 388 available, a 176mb directory may seriously cramp the system's style.) 389 390 i_version 391 Enable 64-bit inode version support. This option is off by default. 392 393 dax 394 Use direct access (no page cache). See 395 Documentation/filesystems/dax.rst. Note that this option is 396 incompatible with data=journal. 397 398 inlinecrypt 399 When possible, encrypt/decrypt the contents of encrypted files using the 400 blk-crypto framework rather than filesystem-layer encryption. This 401 allows the use of inline encryption hardware. The on-disk format is 402 unaffected. For more details, see 403 Documentation/block/inline-encryption.rst. 404 405Data Mode 406========= 407There are 3 different data modes: 408 409* writeback mode 410 411 In data=writeback mode, ext4 does not journal data at all. This mode provides 412 a similar level of journaling as that of XFS, JFS, and ReiserFS in its default 413 mode - metadata journaling. A crash+recovery can cause incorrect data to 414 appear in files which were written shortly before the crash. This mode will 415 typically provide the best ext4 performance. 416 417* ordered mode 418 419 In data=ordered mode, ext4 only officially journals metadata, but it logically 420 groups metadata information related to data changes with the data blocks into 421 a single unit called a transaction. When it's time to write the new metadata 422 out to disk, the associated data blocks are written first. In general, this 423 mode performs slightly slower than writeback but significantly faster than 424 journal mode. 425 426* journal mode 427 428 data=journal mode provides full data and metadata journaling. All new data is 429 written to the journal first, and then to its final location. In the event of 430 a crash, the journal can be replayed, bringing both data and metadata into a 431 consistent state. This mode is the slowest except when data needs to be read 432 from and written to disk at the same time where it outperforms all others 433 modes. Enabling this mode will disable delayed allocation and O_DIRECT 434 support. 435 436/proc entries 437============= 438 439Information about mounted ext4 file systems can be found in 440/proc/fs/ext4. Each mounted filesystem will have a directory in 441/proc/fs/ext4 based on its device name (i.e., /proc/fs/ext4/hdc or 442/proc/fs/ext4/dm-0). The files in each per-device directory are shown 443in table below. 444 445Files in /proc/fs/ext4/<devname> 446 447 mb_groups 448 details of multiblock allocator buddy cache of free blocks 449 450/sys entries 451============ 452 453Information about mounted ext4 file systems can be found in 454/sys/fs/ext4. Each mounted filesystem will have a directory in 455/sys/fs/ext4 based on its device name (i.e., /sys/fs/ext4/hdc or 456/sys/fs/ext4/dm-0). The files in each per-device directory are shown 457in table below. 458 459Files in /sys/fs/ext4/<devname>: 460 461(see also Documentation/ABI/testing/sysfs-fs-ext4) 462 463 delayed_allocation_blocks 464 This file is read-only and shows the number of blocks that are dirty in 465 the page cache, but which do not have their location in the filesystem 466 allocated yet. 467 468 inode_goal 469 Tuning parameter which (if non-zero) controls the goal inode used by 470 the inode allocator in preference to all other allocation heuristics. 471 This is intended for debugging use only, and should be 0 on production 472 systems. 473 474 inode_readahead_blks 475 Tuning parameter which controls the maximum number of inode table 476 blocks that ext4's inode table readahead algorithm will pre-read into 477 the buffer cache. 478 479 lifetime_write_kbytes 480 This file is read-only and shows the number of kilobytes of data that 481 have been written to this filesystem since it was created. 482 483 max_writeback_mb_bump 484 The maximum number of megabytes the writeback code will try to write 485 out before move on to another inode. 486 487 mb_group_prealloc 488 The multiblock allocator will round up allocation requests to a 489 multiple of this tuning parameter if the stripe size is not set in the 490 ext4 superblock 491 492 mb_max_inode_prealloc 493 The maximum length of per-inode ext4_prealloc_space list. 494 495 mb_max_to_scan 496 The maximum number of extents the multiblock allocator will search to 497 find the best extent. 498 499 mb_min_to_scan 500 The minimum number of extents the multiblock allocator will search to 501 find the best extent. 502 503 mb_order2_req 504 Tuning parameter which controls the minimum size for requests (as a 505 power of 2) where the buddy cache is used. 506 507 mb_stats 508 Controls whether the multiblock allocator should collect statistics, 509 which are shown during the unmount. 1 means to collect statistics, 0 510 means not to collect statistics. 511 512 mb_stream_req 513 Files which have fewer blocks than this tunable parameter will have 514 their blocks allocated out of a block group specific preallocation 515 pool, so that small files are packed closely together. Each large file 516 will have its blocks allocated out of its own unique preallocation 517 pool. 518 519 session_write_kbytes 520 This file is read-only and shows the number of kilobytes of data that 521 have been written to this filesystem since it was mounted. 522 523 reserved_clusters 524 This is RW file and contains number of reserved clusters in the file 525 system which will be used in the specific situations to avoid costly 526 zeroout, unexpected ENOSPC, or possible data loss. The default is 2% or 527 4096 clusters, whichever is smaller and this can be changed however it 528 can never exceed number of clusters in the file system. If there is not 529 enough space for the reserved space when mounting the file mount will 530 _not_ fail. 531 532Ioctls 533====== 534 535Ext4 implements various ioctls which can be used by applications to access 536ext4-specific functionality. An incomplete list of these ioctls is shown in the 537table below. This list includes truly ext4-specific ioctls (``EXT4_IOC_*``) as 538well as ioctls that may have been ext4-specific originally but are now supported 539by some other filesystem(s) too (``FS_IOC_*``). 540 541Table of Ext4 ioctls 542 543 FS_IOC_GETFLAGS 544 Get additional attributes associated with inode. The ioctl argument is 545 an integer bitfield, with bit values described in ext4.h. 546 547 FS_IOC_SETFLAGS 548 Set additional attributes associated with inode. The ioctl argument is 549 an integer bitfield, with bit values described in ext4.h. 550 551 EXT4_IOC_GETVERSION, EXT4_IOC_GETVERSION_OLD 552 Get the inode i_generation number stored for each inode. The 553 i_generation number is normally changed only when new inode is created 554 and it is particularly useful for network filesystems. The '_OLD' 555 version of this ioctl is an alias for FS_IOC_GETVERSION. 556 557 EXT4_IOC_SETVERSION, EXT4_IOC_SETVERSION_OLD 558 Set the inode i_generation number stored for each inode. The '_OLD' 559 version of this ioctl is an alias for FS_IOC_SETVERSION. 560 561 EXT4_IOC_GROUP_EXTEND 562 This ioctl has the same purpose as the resize mount option. It allows 563 to resize filesystem to the end of the last existing block group, 564 further resize has to be done with resize2fs, either online, or 565 offline. The argument points to the unsigned logn number representing 566 the filesystem new block count. 567 568 EXT4_IOC_MOVE_EXT 569 Move the block extents from orig_fd (the one this ioctl is pointing to) 570 to the donor_fd (the one specified in move_extent structure passed as 571 an argument to this ioctl). Then, exchange inode metadata between 572 orig_fd and donor_fd. This is especially useful for online 573 defragmentation, because the allocator has the opportunity to allocate 574 moved blocks better, ideally into one contiguous extent. 575 576 EXT4_IOC_GROUP_ADD 577 Add a new group descriptor to an existing or new group descriptor 578 block. The new group descriptor is described by ext4_new_group_input 579 structure, which is passed as an argument to this ioctl. This is 580 especially useful in conjunction with EXT4_IOC_GROUP_EXTEND, which 581 allows online resize of the filesystem to the end of the last existing 582 block group. Those two ioctls combined is used in userspace online 583 resize tool (e.g. resize2fs). 584 585 EXT4_IOC_MIGRATE 586 This ioctl operates on the filesystem itself. It converts (migrates) 587 ext3 indirect block mapped inode to ext4 extent mapped inode by walking 588 through indirect block mapping of the original inode and converting 589 contiguous block ranges into ext4 extents of the temporary inode. Then, 590 inodes are swapped. This ioctl might help, when migrating from ext3 to 591 ext4 filesystem, however suggestion is to create fresh ext4 filesystem 592 and copy data from the backup. Note, that filesystem has to support 593 extents for this ioctl to work. 594 595 EXT4_IOC_ALLOC_DA_BLKS 596 Force all of the delay allocated blocks to be allocated to preserve 597 application-expected ext3 behaviour. Note that this will also start 598 triggering a write of the data blocks, but this behaviour may change in 599 the future as it is not necessary and has been done this way only for 600 sake of simplicity. 601 602 EXT4_IOC_RESIZE_FS 603 Resize the filesystem to a new size. The number of blocks of resized 604 filesystem is passed in via 64 bit integer argument. The kernel 605 allocates bitmaps and inode table, the userspace tool thus just passes 606 the new number of blocks. 607 608 EXT4_IOC_SWAP_BOOT 609 Swap i_blocks and associated attributes (like i_blocks, i_size, 610 i_flags, ...) from the specified inode with inode EXT4_BOOT_LOADER_INO 611 (#5). This is typically used to store a boot loader in a secure part of 612 the filesystem, where it can't be changed by a normal user by accident. 613 The data blocks of the previous boot loader will be associated with the 614 given inode. 615 616References 617========== 618 619kernel source: <file:fs/ext4/> 620 <file:fs/jbd2/> 621 622programs: http://e2fsprogs.sourceforge.net/ 623 624useful links: https://fedoraproject.org/wiki/ext3-devel 625 http://www.bullopensource.org/ext4/ 626 http://ext4.wiki.kernel.org/index.php/Main_Page 627 https://fedoraproject.org/wiki/Features/Ext4