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