1.. 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. This feature is deprecated and support 176 will be removed in January 2028. 177 178 data=ordered (*) 179 All data are forced directly out to the main file system prior to its 180 metadata being committed to the journal. 181 182 data=writeback 183 Data ordering is not preserved, data may be written into the main file 184 system after its metadata has been committed to the journal. 185 186 commit=nrsec (*) 187 This setting limits the maximum age of the running transaction to 188 'nrsec' seconds. The default value is 5 seconds. This means that if 189 you lose your power, you will lose as much as the latest 5 seconds of 190 metadata changes (your filesystem will not be damaged though, thanks 191 to the journaling). This default value (or any low value) will hurt 192 performance, but it's good for data-safety. Setting it to 0 will have 193 the same effect as leaving it at the default (5 seconds). Setting it 194 to very large values will improve performance. Note that due to 195 delayed allocation even older data can be lost on power failure since 196 writeback of those data begins only after time set in 197 /proc/sys/vm/dirty_expire_centisecs. 198 199 barrier=<0|1(*)>, barrier(*), nobarrier 200 This enables/disables the use of write barriers in the jbd code. 201 barrier=0 disables, barrier=1 enables. This also requires an IO stack 202 which can support barriers, and if jbd gets an error on a barrier 203 write, it will disable again with a warning. Write barriers enforce 204 proper on-disk ordering of journal commits, making volatile disk write 205 caches safe to use, at some performance penalty. If your disks are 206 battery-backed in one way or another, disabling barriers may safely 207 improve performance. The mount options "barrier" and "nobarrier" can 208 also be used to enable or disable barriers, for consistency with other 209 ext4 mount options. 210 211 inode_readahead_blks=n 212 This tuning parameter controls the maximum number of inode table blocks 213 that ext4's inode table readahead algorithm will pre-read into the 214 buffer cache. The default value is 32 blocks. 215 216 bsddf (*) 217 Make 'df' act like BSD. 218 219 minixdf 220 Make 'df' act like Minix. 221 222 debug 223 Extra debugging information is sent to syslog. 224 225 abort 226 Simulate the effects of calling ext4_abort() for debugging purposes. 227 This is normally used while remounting a filesystem which is already 228 mounted. 229 230 errors=remount-ro 231 Remount the filesystem read-only on an error. 232 233 errors=continue 234 Keep going on a filesystem error. 235 236 errors=panic 237 Panic and halt the machine if an error occurs. (These mount options 238 override the errors behavior specified in the superblock, which can be 239 configured using tune2fs) 240 241 data_err=ignore(*) 242 Just print an error message if an error occurs in a file data buffer. 243 244 data_err=abort 245 Abort the journal if an error occurs in a file data buffer. 246 247 grpid | bsdgroups 248 New objects have the group ID of their parent. 249 250 nogrpid (*) | sysvgroups 251 New objects have the group ID of their creator. 252 253 resgid=n 254 The group ID which may use the reserved blocks. 255 256 resuid=n 257 The user ID which may use the reserved blocks. 258 259 sb= 260 Use alternate superblock at this location. 261 262 quota, noquota, grpquota, usrquota 263 These options are ignored by the filesystem. They are used only by 264 quota tools to recognize volumes where quota should be turned on. See 265 documentation in the quota-tools package for more details 266 (http://sourceforge.net/projects/linuxquota). 267 268 jqfmt=<quota type>, usrjquota=<file>, grpjquota=<file> 269 These options tell filesystem details about quota so that quota 270 information can be properly updated during journal replay. They replace 271 the above quota options. See documentation in the quota-tools package 272 for more details (http://sourceforge.net/projects/linuxquota). 273 274 stripe=n 275 Number of filesystem blocks that mballoc will try to use for allocation 276 size and alignment. For RAID5/6 systems this should be the number of 277 data disks * RAID chunk size in file system blocks. 278 279 delalloc (*) 280 Defer block allocation until just before ext4 writes out the block(s) 281 in question. This allows ext4 to better allocation decisions more 282 efficiently. 283 284 nodelalloc 285 Disable delayed allocation. Blocks are allocated when the data is 286 copied from userspace to the page cache, either via the write(2) system 287 call or when an mmap'ed page which was previously unallocated is 288 written for the first time. 289 290 max_batch_time=usec 291 Maximum amount of time ext4 should wait for additional filesystem 292 operations to be batch together with a synchronous write operation. 293 Since a synchronous write operation is going to force a commit and then 294 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 amount of time to see if any other 296 transactions can piggyback on the synchronous write. The algorithm 297 used is designed to automatically tune for the speed of the disk, by 298 measuring the amount of time (on average) that it takes to finish 299 committing a transaction. Call this time the "commit time". If the 300 time that the transaction has been running is less than the commit 301 time, ext4 will try sleeping for the commit time to see if other 302 operations will join the transaction. The commit time is capped by 303 the max_batch_time, which defaults to 15000us (15ms). This 304 optimization can be turned off entirely by setting max_batch_time to 0. 305 306 min_batch_time=usec 307 This parameter sets the commit time (as described above) to be at least 308 min_batch_time. It defaults to zero microseconds. Increasing this 309 parameter may improve the throughput of multi-threaded, synchronous 310 workloads on very fast disks, at the cost of increasing latency. 311 312 journal_ioprio=prio 313 The I/O priority (from 0 to 7, where 0 is the highest priority) which 314 should be used for I/O operations submitted by kjournald2 during a 315 commit operation. This defaults to 3, which is a slightly higher 316 priority than the default I/O priority. 317 318 auto_da_alloc(*), noauto_da_alloc 319 Many broken applications don't use fsync() when replacing existing 320 files via patterns such as fd = open("foo.new")/write(fd,..)/close(fd)/ 321 rename("foo.new", "foo"), or worse yet, fd = open("foo", 322 O_TRUNC)/write(fd,..)/close(fd). If auto_da_alloc is enabled, ext4 323 will detect the replace-via-rename and replace-via-truncate patterns 324 and force that any delayed allocation blocks are allocated such that at 325 the next journal commit, in the default data=ordered mode, the data 326 blocks of the new file are forced to disk before the rename() operation 327 is committed. This provides roughly the same level of guarantees as 328 ext3, and avoids the "zero-length" problem that can happen when a 329 system crashes before the delayed allocation blocks are forced to disk. 330 331 noinit_itable 332 Do not initialize any uninitialized inode table blocks in the 333 background. This feature may be used by installation CD's so that the 334 install process can complete as quickly as possible; the inode table 335 initialization process would then be deferred until the next time the 336 file system is unmounted. 337 338 init_itable=n 339 The lazy itable init code will wait n times the number of milliseconds 340 it took to zero out the previous block group's inode table. This 341 minimizes the impact on the system performance while file system's 342 inode table is being initialized. 343 344 discard, nodiscard(*) 345 Controls whether ext4 should issue discard/TRIM commands to the 346 underlying block device when blocks are freed. This is useful for SSD 347 devices and sparse/thinly-provisioned LUNs, but it is off by default 348 until sufficient testing has been done. 349 350 nouid32 351 Disables 32-bit UIDs and GIDs. This is for interoperability with 352 older kernels which only store and expect 16-bit values. 353 354 block_validity(*), noblock_validity 355 These options enable or disable the in-kernel facility for tracking 356 filesystem metadata blocks within internal data structures. This 357 allows multi- block allocator and other routines to notice bugs or 358 corrupted allocation bitmaps which cause blocks to be allocated which 359 overlap with filesystem metadata blocks. 360 361 dioread_lock, dioread_nolock 362 Controls whether or not ext4 should use the DIO read locking. If the 363 dioread_nolock option is specified ext4 will allocate uninitialized 364 extent before buffer write and convert the extent to initialized after 365 IO completes. This approach allows ext4 code to avoid using inode 366 mutex, which improves scalability on high speed storages. However this 367 does not work with data journaling and dioread_nolock option will be 368 ignored with kernel warning. Note that dioread_nolock code path is only 369 used for extent-based files. Because of the restrictions this options 370 comprises it is off by default (e.g. dioread_lock). 371 372 max_dir_size_kb=n 373 This limits the size of directories so that any attempt to expand them 374 beyond the specified limit in kilobytes will cause an ENOSPC error. 375 This is useful in memory constrained environments, where a very large 376 directory can cause severe performance problems or even provoke the Out 377 Of Memory killer. (For example, if there is only 512mb memory 378 available, a 176mb directory may seriously cramp the system's style.) 379 380 i_version 381 Enable 64-bit inode version support. This option is off by default. 382 383 dax 384 Use direct access (no page cache). See 385 Documentation/filesystems/dax.rst. Note that this option is 386 incompatible with data=journal. 387 388 inlinecrypt 389 When possible, encrypt/decrypt the contents of encrypted files using 390 inline encryption hardware rather than the CPU. For more details, see 391 Documentation/filesystems/fscrypt.rst. 392 393Data Mode 394========= 395There are 3 different data modes: 396 397* writeback mode 398 399 In data=writeback mode, ext4 does not journal data at all. This mode provides 400 a similar level of journaling as that of XFS and JFS in its default 401 mode - metadata journaling. A crash+recovery can cause incorrect data to 402 appear in files which were written shortly before the crash. This mode will 403 typically provide the best ext4 performance. 404 405* ordered mode 406 407 In data=ordered mode, ext4 only officially journals metadata, but it logically 408 groups metadata information related to data changes with the data blocks into 409 a single unit called a transaction. When it's time to write the new metadata 410 out to disk, the associated data blocks are written first. In general, this 411 mode performs slightly slower than writeback but significantly faster than 412 journal mode. 413 414* journal mode 415 416 data=journal mode provides full data and metadata journaling. All new data is 417 written to the journal first, and then to its final location. In the event of 418 a crash, the journal can be replayed, bringing both data and metadata into a 419 consistent state. This mode is the slowest except when data needs to be read 420 from and written to disk at the same time where it outperforms all others 421 modes. Enabling this mode will disable delayed allocation and O_DIRECT 422 support. The data=journal feature is deprecated and support will be 423 removed in January 2028. 424 425/proc entries 426============= 427 428Information about mounted ext4 file systems can be found in 429/proc/fs/ext4. Each mounted filesystem will have a directory in 430/proc/fs/ext4 based on its device name (i.e., /proc/fs/ext4/hdc or 431/proc/fs/ext4/dm-0). The files in each per-device directory are shown 432in table below. 433 434Files in /proc/fs/ext4/<devname> 435 436 mb_groups 437 details of multiblock allocator buddy cache of free blocks 438 439/sys entries 440============ 441 442Information about mounted ext4 file systems can be found in 443/sys/fs/ext4. Each mounted filesystem will have a directory in 444/sys/fs/ext4 based on its device name (i.e., /sys/fs/ext4/hdc or 445/sys/fs/ext4/dm-0). The files in each per-device directory are shown 446in table below. 447 448Files in /sys/fs/ext4/<devname>: 449 450(see also Documentation/ABI/testing/sysfs-fs-ext4) 451 452 delayed_allocation_blocks 453 This file is read-only and shows the number of blocks that are dirty in 454 the page cache, but which do not have their location in the filesystem 455 allocated yet. 456 457 inode_goal 458 Tuning parameter which (if non-zero) controls the goal inode used by 459 the inode allocator in preference to all other allocation heuristics. 460 This is intended for debugging use only, and should be 0 on production 461 systems. 462 463 inode_readahead_blks 464 Tuning parameter which controls the maximum number of inode table 465 blocks that ext4's inode table readahead algorithm will pre-read into 466 the buffer cache. 467 468 lifetime_write_kbytes 469 This file is read-only and shows the number of kilobytes of data that 470 have been written to this filesystem since it was created. 471 472 max_writeback_mb_bump 473 The maximum number of megabytes the writeback code will try to write 474 out before move on to another inode. 475 476 mb_group_prealloc 477 The multiblock allocator will round up allocation requests to a 478 multiple of this tuning parameter if the stripe size is not set in the 479 ext4 superblock 480 481 mb_max_to_scan 482 The maximum number of extents the multiblock allocator will search to 483 find the best extent. 484 485 mb_min_to_scan 486 The minimum number of extents the multiblock allocator will search to 487 find the best extent. 488 489 mb_order2_req 490 Tuning parameter which controls the minimum size for requests (as a 491 power of 2) where the buddy cache is used. 492 493 mb_stats 494 Controls whether the multiblock allocator should collect statistics, 495 which are shown during the unmount. 1 means to collect statistics, 0 496 means not to collect statistics. 497 498 mb_stream_req 499 Files which have fewer blocks than this tunable parameter will have 500 their blocks allocated out of a block group specific preallocation 501 pool, so that small files are packed closely together. Each large file 502 will have its blocks allocated out of its own unique preallocation 503 pool. 504 505 session_write_kbytes 506 This file is read-only and shows the number of kilobytes of data that 507 have been written to this filesystem since it was mounted. 508 509 reserved_clusters 510 This is RW file and contains number of reserved clusters in the file 511 system which will be used in the specific situations to avoid costly 512 zeroout, unexpected ENOSPC, or possible data loss. The default is 2% or 513 4096 clusters, whichever is smaller and this can be changed however it 514 can never exceed number of clusters in the file system. If there is not 515 enough space for the reserved space when mounting the file mount will 516 _not_ fail. 517 518Ioctls 519====== 520 521Ext4 implements various ioctls which can be used by applications to access 522ext4-specific functionality. An incomplete list of these ioctls is shown in the 523table below. This list includes truly ext4-specific ioctls (``EXT4_IOC_*``) as 524well as ioctls that may have been ext4-specific originally but are now supported 525by some other filesystem(s) too (``FS_IOC_*``). 526 527Table of Ext4 ioctls 528 529 FS_IOC_GETFLAGS 530 Get additional attributes associated with inode. The ioctl argument is 531 an integer bitfield, with bit values described in ext4.h. 532 533 FS_IOC_SETFLAGS 534 Set additional attributes associated with inode. The ioctl argument is 535 an integer bitfield, with bit values described in ext4.h. 536 537 EXT4_IOC_GETVERSION, EXT4_IOC_GETVERSION_OLD 538 Get the inode i_generation number stored for each inode. The 539 i_generation number is normally changed only when new inode is created 540 and it is particularly useful for network filesystems. The '_OLD' 541 version of this ioctl is an alias for FS_IOC_GETVERSION. 542 543 EXT4_IOC_SETVERSION, EXT4_IOC_SETVERSION_OLD 544 Set the inode i_generation number stored for each inode. The '_OLD' 545 version of this ioctl is an alias for FS_IOC_SETVERSION. 546 547 EXT4_IOC_GROUP_EXTEND 548 This ioctl has the same purpose as the resize mount option. It allows 549 to resize filesystem to the end of the last existing block group, 550 further resize has to be done with resize2fs, either online, or 551 offline. The argument points to the unsigned logn number representing 552 the filesystem new block count. 553 554 EXT4_IOC_MOVE_EXT 555 Move the block extents from orig_fd (the one this ioctl is pointing to) 556 to the donor_fd (the one specified in move_extent structure passed as 557 an argument to this ioctl). Then, exchange inode metadata between 558 orig_fd and donor_fd. This is especially useful for online 559 defragmentation, because the allocator has the opportunity to allocate 560 moved blocks better, ideally into one contiguous extent. 561 562 EXT4_IOC_GROUP_ADD 563 Add a new group descriptor to an existing or new group descriptor 564 block. The new group descriptor is described by ext4_new_group_input 565 structure, which is passed as an argument to this ioctl. This is 566 especially useful in conjunction with EXT4_IOC_GROUP_EXTEND, which 567 allows online resize of the filesystem to the end of the last existing 568 block group. Those two ioctls combined is used in userspace online 569 resize tool (e.g. resize2fs). 570 571 EXT4_IOC_MIGRATE 572 This ioctl operates on the filesystem itself. It converts (migrates) 573 ext3 indirect block mapped inode to ext4 extent mapped inode by walking 574 through indirect block mapping of the original inode and converting 575 contiguous block ranges into ext4 extents of the temporary inode. Then, 576 inodes are swapped. This ioctl might help, when migrating from ext3 to 577 ext4 filesystem, however suggestion is to create fresh ext4 filesystem 578 and copy data from the backup. Note, that filesystem has to support 579 extents for this ioctl to work. 580 581 EXT4_IOC_ALLOC_DA_BLKS 582 Force all of the delay allocated blocks to be allocated to preserve 583 application-expected ext3 behaviour. Note that this will also start 584 triggering a write of the data blocks, but this behaviour may change in 585 the future as it is not necessary and has been done this way only for 586 sake of simplicity. 587 588 EXT4_IOC_RESIZE_FS 589 Resize the filesystem to a new size. The number of blocks of resized 590 filesystem is passed in via 64 bit integer argument. The kernel 591 allocates bitmaps and inode table, the userspace tool thus just passes 592 the new number of blocks. 593 594 EXT4_IOC_SWAP_BOOT 595 Swap i_blocks and associated attributes (like i_blocks, i_size, 596 i_flags, ...) from the specified inode with inode EXT4_BOOT_LOADER_INO 597 (#5). This is typically used to store a boot loader in a secure part of 598 the filesystem, where it can't be changed by a normal user by accident. 599 The data blocks of the previous boot loader will be associated with the 600 given inode. 601 602References 603========== 604 605kernel source: <file:fs/ext4/> 606 <file:fs/jbd2/> 607 608programs: http://e2fsprogs.sourceforge.net/ 609 610useful links: https://fedoraproject.org/wiki/ext3-devel 611 http://www.bullopensource.org/ext4/ 612 http://ext4.wiki.kernel.org/index.php/Main_Page 613 https://fedoraproject.org/wiki/Features/Ext4 614