Based on kernel version 4.16.1. Page generated on 2018-04-09 11:53 EST.
1 2 Ext4 Filesystem 3 =============== 4 5 Ext4 is an advanced level of the ext3 filesystem which incorporates 6 scalability and reliability enhancements for supporting large filesystems 7 (64 bit) in keeping with increasing disk capacities and state-of-the-art 8 feature requirements. 9 10 Mailing list: email@example.com 11 Web site: http://ext4.wiki.kernel.org 12 13 14 1. Quick usage instructions: 15 =========================== 16 17 Note: More extensive information for getting started with ext4 can be 18 found at the ext4 wiki site at the URL: 19 http://ext4.wiki.kernel.org/index.php/Ext4_Howto 20 21 - Compile and install the latest version of e2fsprogs (as of this 22 writing version 1.41.3) from: 23 24 http://sourceforge.net/project/showfiles.php?group_id=2406 25 26 or 27 28 https://www.kernel.org/pub/linux/kernel/people/tytso/e2fsprogs/ 29 30 or grab the latest git repository from: 31 32 git://git.kernel.org/pub/scm/fs/ext2/e2fsprogs.git 33 34 - Note that it is highly important to install the mke2fs.conf file 35 that comes with the e2fsprogs 1.41.x sources in /etc/mke2fs.conf. If 36 you have edited the /etc/mke2fs.conf file installed on your system, 37 you will need to merge your changes with the version from e2fsprogs 38 1.41.x. 39 40 - Create a new filesystem using the ext4 filesystem type: 41 42 # mke2fs -t ext4 /dev/hda1 43 44 Or to configure an existing ext3 filesystem to support extents: 45 46 # tune2fs -O extents /dev/hda1 47 48 If the filesystem was created with 128 byte inodes, it can be 49 converted to use 256 byte for greater efficiency via: 50 51 # tune2fs -I 256 /dev/hda1 52 53 (Note: we currently do not have tools to convert an ext4 54 filesystem back to ext3; so please do not do try this on production 55 filesystems.) 56 57 - Mounting: 58 59 # mount -t ext4 /dev/hda1 /wherever 60 61 - When comparing performance with other filesystems, it's always 62 important to try multiple workloads; very often a subtle change in a 63 workload parameter can completely change the ranking of which 64 filesystems do well compared to others. When comparing versus ext3, 65 note that ext4 enables write barriers by default, while ext3 does 66 not enable write barriers by default. So it is useful to use 67 explicitly specify whether barriers are enabled or not when via the 68 '-o barriers=[0|1]' mount option for both ext3 and ext4 filesystems 69 for a fair comparison. When tuning ext3 for best benchmark numbers, 70 it is often worthwhile to try changing the data journaling mode; '-o 71 data=writeback' can be faster for some workloads. (Note however that 72 running mounted with data=writeback can potentially leave stale data 73 exposed in recently written files in case of an unclean shutdown, 74 which could be a security exposure in some situations.) Configuring 75 the filesystem with a large journal can also be helpful for 76 metadata-intensive workloads. 77 78 2. Features 79 =========== 80 81 2.1 Currently available 82 83 * ability to use filesystems > 16TB (e2fsprogs support not available yet) 84 * extent format reduces metadata overhead (RAM, IO for access, transactions) 85 * extent format more robust in face of on-disk corruption due to magics, 86 * internal redundancy in tree 87 * improved file allocation (multi-block alloc) 88 * lift 32000 subdirectory limit imposed by i_links_count 89 * nsec timestamps for mtime, atime, ctime, create time 90 * inode version field on disk (NFSv4, Lustre) 91 * reduced e2fsck time via uninit_bg feature 92 * journal checksumming for robustness, performance 93 * persistent file preallocation (e.g for streaming media, databases) 94 * ability to pack bitmaps and inode tables into larger virtual groups via the 95 flex_bg feature 96 * large file support 97 * inode allocation using large virtual block groups via flex_bg 98 * delayed allocation 99 * large block (up to pagesize) support 100 * efficient new ordered mode in JBD2 and ext4 (avoid using buffer head to force 101 the ordering) 102 103  Filesystems with a block size of 1k may see a limit imposed by the 104 directory hash tree having a maximum depth of two. 105 106 2.2 Candidate features for future inclusion 107 108 * online defrag (patches available but not well tested) 109 * reduced mke2fs time via lazy itable initialization in conjunction with 110 the uninit_bg feature (capability to do this is available in e2fsprogs 111 but a kernel thread to do lazy zeroing of unused inode table blocks 112 after filesystem is first mounted is required for safety) 113 114 There are several others under discussion, whether they all make it in is 115 partly a function of how much time everyone has to work on them. Features like 116 metadata checksumming have been discussed and planned for a bit but no patches 117 exist yet so I'm not sure they're in the near-term roadmap. 118 119 The big performance win will come with mballoc, delalloc and flex_bg 120 grouping of bitmaps and inode tables. Some test results available here: 121 122 - http://www.bullopensource.org/ext4/20080818-ffsb/ffsb-write-2.6.27-rc1.html 123 - http://www.bullopensource.org/ext4/20080818-ffsb/ffsb-readwrite-2.6.27-rc1.html 124 125 3. Options 126 ========== 127 128 When mounting an ext4 filesystem, the following option are accepted: 129 (*) == default 130 131 ro Mount filesystem read only. Note that ext4 will 132 replay the journal (and thus write to the 133 partition) even when mounted "read only". The 134 mount options "ro,noload" can be used to prevent 135 writes to the filesystem. 136 137 journal_checksum Enable checksumming of the journal transactions. 138 This will allow the recovery code in e2fsck and the 139 kernel to detect corruption in the kernel. It is a 140 compatible change and will be ignored by older kernels. 141 142 journal_async_commit Commit block can be written to disk without waiting 143 for descriptor blocks. If enabled older kernels cannot 144 mount the device. This will enable 'journal_checksum' 145 internally. 146 147 journal_path=path 148 journal_dev=devnum When the external journal device's major/minor numbers 149 have changed, these options allow the user to specify 150 the new journal location. The journal device is 151 identified through either its new major/minor numbers 152 encoded in devnum, or via a path to the device. 153 154 norecovery Don't load the journal on mounting. Note that 155 noload if the filesystem was not unmounted cleanly, 156 skipping the journal replay will lead to the 157 filesystem containing inconsistencies that can 158 lead to any number of problems. 159 160 data=journal All data are committed into the journal prior to being 161 written into the main file system. Enabling 162 this mode will disable delayed allocation and 163 O_DIRECT support. 164 165 data=ordered (*) All data are forced directly out to the main file 166 system prior to its metadata being committed to the 167 journal. 168 169 data=writeback Data ordering is not preserved, data may be written 170 into the main file system after its metadata has been 171 committed to the journal. 172 173 commit=nrsec (*) Ext4 can be told to sync all its data and metadata 174 every 'nrsec' seconds. The default value is 5 seconds. 175 This means that if you lose your power, you will lose 176 as much as the latest 5 seconds of work (your 177 filesystem will not be damaged though, thanks to the 178 journaling). This default value (or any low value) 179 will hurt performance, but it's good for data-safety. 180 Setting it to 0 will have the same effect as leaving 181 it at the default (5 seconds). 182 Setting it to very large values will improve 183 performance. 184 185 barrier=<0|1(*)> This enables/disables the use of write barriers in 186 barrier(*) the jbd code. barrier=0 disables, barrier=1 enables. 187 nobarrier This also requires an IO stack which can support 188 barriers, and if jbd gets an error on a barrier 189 write, it will disable again with a warning. 190 Write barriers enforce proper on-disk ordering 191 of journal commits, making volatile disk write caches 192 safe to use, at some performance penalty. If 193 your disks are battery-backed in one way or another, 194 disabling barriers may safely improve performance. 195 The mount options "barrier" and "nobarrier" can 196 also be used to enable or disable barriers, for 197 consistency with other ext4 mount options. 198 199 inode_readahead_blks=n This tuning parameter controls the maximum 200 number of inode table blocks that ext4's inode 201 table readahead algorithm will pre-read into 202 the buffer cache. The default value is 32 blocks. 203 204 nouser_xattr Disables Extended User Attributes. See the 205 attr(5) manual page for more information about 206 extended attributes. 207 208 noacl This option disables POSIX Access Control List 209 support. If ACL support is enabled in the kernel 210 configuration (CONFIG_EXT4_FS_POSIX_ACL), ACL is 211 enabled by default on mount. See the acl(5) manual 212 page for more information about acl. 213 214 bsddf (*) Make 'df' act like BSD. 215 minixdf Make 'df' act like Minix. 216 217 debug Extra debugging information is sent to syslog. 218 219 abort Simulate the effects of calling ext4_abort() for 220 debugging purposes. This is normally used while 221 remounting a filesystem which is already mounted. 222 223 errors=remount-ro Remount the filesystem read-only on an error. 224 errors=continue Keep going on a filesystem error. 225 errors=panic Panic and halt the machine if an error occurs. 226 (These mount options override the errors behavior 227 specified in the superblock, which can be configured 228 using tune2fs) 229 230 data_err=ignore(*) Just print an error message if an error occurs 231 in a file data buffer in ordered mode. 232 data_err=abort Abort the journal if an error occurs in a file 233 data buffer in ordered mode. 234 235 grpid New objects have the group ID of their parent. 236 bsdgroups 237 238 nogrpid (*) New objects have the group ID of their creator. 239 sysvgroups 240 241 resgid=n The group ID which may use the reserved blocks. 242 243 resuid=n The user ID which may use the reserved blocks. 244 245 sb=n Use alternate superblock at this location. 246 247 quota These options are ignored by the filesystem. They 248 noquota are used only by quota tools to recognize volumes 249 grpquota where quota should be turned on. See documentation 250 usrquota in the quota-tools package for more details 251 (http://sourceforge.net/projects/linuxquota). 252 253 jqfmt=<quota type> These options tell filesystem details about quota 254 usrjquota=<file> so that quota information can be properly updated 255 grpjquota=<file> during journal replay. They replace the above 256 quota options. See documentation in the quota-tools 257 package for more details 258 (http://sourceforge.net/projects/linuxquota). 259 260 stripe=n Number of filesystem blocks that mballoc will try 261 to use for allocation size and alignment. For RAID5/6 262 systems this should be the number of data 263 disks * RAID chunk size in file system blocks. 264 265 delalloc (*) Defer block allocation until just before ext4 266 writes out the block(s) in question. This 267 allows ext4 to better allocation decisions 268 more efficiently. 269 nodelalloc Disable delayed allocation. Blocks are allocated 270 when the data is copied from userspace to the 271 page cache, either via the write(2) system call 272 or when an mmap'ed page which was previously 273 unallocated is written for the first time. 274 275 max_batch_time=usec Maximum amount of time ext4 should wait for 276 additional filesystem operations to be batch 277 together with a synchronous write operation. 278 Since a synchronous write operation is going to 279 force a commit and then a wait for the I/O 280 complete, it doesn't cost much, and can be a 281 huge throughput win, we wait for a small amount 282 of time to see if any other transactions can 283 piggyback on the synchronous write. The 284 algorithm used is designed to automatically tune 285 for the speed of the disk, by measuring the 286 amount of time (on average) that it takes to 287 finish committing a transaction. Call this time 288 the "commit time". If the time that the 289 transaction has been running is less than the 290 commit time, ext4 will try sleeping for the 291 commit time to see if other operations will join 292 the transaction. The commit time is capped by 293 the max_batch_time, which defaults to 15000us 294 (15ms). This optimization can be turned off 295 entirely by setting max_batch_time to 0. 296 297 min_batch_time=usec This parameter sets the commit time (as 298 described above) to be at least min_batch_time. 299 It defaults to zero microseconds. Increasing 300 this parameter may improve the throughput of 301 multi-threaded, synchronous workloads on very 302 fast disks, at the cost of increasing latency. 303 304 journal_ioprio=prio The I/O priority (from 0 to 7, where 0 is the 305 highest priority) which should be used for I/O 306 operations submitted by kjournald2 during a 307 commit operation. This defaults to 3, which is 308 a slightly higher priority than the default I/O 309 priority. 310 311 auto_da_alloc(*) Many broken applications don't use fsync() when 312 noauto_da_alloc replacing existing files via patterns such as 313 fd = open("foo.new")/write(fd,..)/close(fd)/ 314 rename("foo.new", "foo"), or worse yet, 315 fd = open("foo", O_TRUNC)/write(fd,..)/close(fd). 316 If auto_da_alloc is enabled, ext4 will detect 317 the replace-via-rename and replace-via-truncate 318 patterns and force that any delayed allocation 319 blocks are allocated such that at the next 320 journal commit, in the default data=ordered 321 mode, the data blocks of the new file are forced 322 to disk before the rename() operation is 323 committed. This provides roughly the same level 324 of guarantees as ext3, and avoids the 325 "zero-length" problem that can happen when a 326 system crashes before the delayed allocation 327 blocks are forced to disk. 328 329 noinit_itable Do not initialize any uninitialized inode table 330 blocks in the background. This feature may be 331 used by installation CD's so that the install 332 process can complete as quickly as possible; the 333 inode table initialization process would then be 334 deferred until the next time the file system 335 is unmounted. 336 337 init_itable=n The lazy itable init code will wait n times the 338 number of milliseconds it took to zero out the 339 previous block group's inode table. This 340 minimizes the impact on the system performance 341 while file system's inode table is being initialized. 342 343 discard Controls whether ext4 should issue discard/TRIM 344 nodiscard(*) commands to the underlying block device when 345 blocks are freed. This is useful for SSD devices 346 and sparse/thinly-provisioned LUNs, but it is off 347 by default until sufficient testing has been done. 348 349 nouid32 Disables 32-bit UIDs and GIDs. This is for 350 interoperability with older kernels which only 351 store and expect 16-bit values. 352 353 block_validity(*) These options enable or disable the in-kernel 354 noblock_validity facility for tracking filesystem metadata blocks 355 within internal data structures. This allows multi- 356 block allocator and other routines to notice 357 bugs or corrupted allocation bitmaps which cause 358 blocks to be allocated which overlap with 359 filesystem metadata blocks. 360 361 dioread_lock Controls whether or not ext4 should use the DIO read 362 dioread_nolock locking. If the dioread_nolock option is specified 363 ext4 will allocate uninitialized extent before buffer 364 write and convert the extent to initialized after IO 365 completes. This approach allows ext4 code to avoid 366 using inode mutex, which improves scalability on high 367 speed storages. However this does not work with 368 data journaling and dioread_nolock option will be 369 ignored with kernel warning. Note that dioread_nolock 370 code path is only used for extent-based files. 371 Because of the restrictions this options comprises 372 it is off by default (e.g. dioread_lock). 373 374 max_dir_size_kb=n This limits the size of directories so that any 375 attempt to expand them beyond the specified 376 limit in kilobytes will cause an ENOSPC error. 377 This is useful in memory constrained 378 environments, where a very large directory can 379 cause severe performance problems or even 380 provoke the Out Of Memory killer. (For example, 381 if there is only 512mb memory available, a 176mb 382 directory may seriously cramp the system's style.) 383 384 i_version Enable 64-bit inode version support. This option is 385 off by default. 386 387 dax Use direct access (no page cache). See 388 Documentation/filesystems/dax.txt. Note that 389 this option is incompatible with data=journal. 390 391 Data Mode 392 ========= 393 There are 3 different data modes: 394 395 * writeback mode 396 In data=writeback mode, ext4 does not journal data at all. This mode provides 397 a similar level of journaling as that of XFS, JFS, and ReiserFS in its default 398 mode - metadata journaling. A crash+recovery can cause incorrect data to 399 appear in files which were written shortly before the crash. This mode will 400 typically provide the best ext4 performance. 401 402 * ordered mode 403 In data=ordered mode, ext4 only officially journals metadata, but it logically 404 groups metadata information related to data changes with the data blocks into a 405 single unit called a transaction. When it's time to write the new metadata 406 out to disk, the associated data blocks are written first. In general, 407 this mode performs slightly slower than writeback but significantly faster than journal mode. 408 409 * journal mode 410 data=journal mode provides full data and metadata journaling. All new data is 411 written to the journal first, and then to its final location. 412 In the event of a crash, the journal can be replayed, bringing both data and 413 metadata into a consistent state. This mode is the slowest except when data 414 needs to be read from and written to disk at the same time where it 415 outperforms all others modes. Enabling this mode will disable delayed 416 allocation and O_DIRECT support. 417 418 /proc entries 419 ============= 420 421 Information about mounted ext4 file systems can be found in 422 /proc/fs/ext4. Each mounted filesystem will have a directory in 423 /proc/fs/ext4 based on its device name (i.e., /proc/fs/ext4/hdc or 424 /proc/fs/ext4/dm-0). The files in each per-device directory are shown 425 in table below. 426 427 Files in /proc/fs/ext4/<devname> 428 .............................................................................. 429 File Content 430 mb_groups details of multiblock allocator buddy cache of free blocks 431 .............................................................................. 432 433 /sys entries 434 ============ 435 436 Information about mounted ext4 file systems can be found in 437 /sys/fs/ext4. Each mounted filesystem will have a directory in 438 /sys/fs/ext4 based on its device name (i.e., /sys/fs/ext4/hdc or 439 /sys/fs/ext4/dm-0). The files in each per-device directory are shown 440 in table below. 441 442 Files in /sys/fs/ext4/<devname> 443 (see also Documentation/ABI/testing/sysfs-fs-ext4) 444 .............................................................................. 445 File Content 446 447 delayed_allocation_blocks This file is read-only and shows the number of 448 blocks that are dirty in the page cache, but 449 which do not have their location in the 450 filesystem allocated yet. 451 452 inode_goal Tuning parameter which (if non-zero) controls 453 the goal inode used by the inode allocator in 454 preference to all other allocation heuristics. 455 This is intended for debugging use only, and 456 should be 0 on production systems. 457 458 inode_readahead_blks Tuning parameter which controls the maximum 459 number of inode table blocks that ext4's inode 460 table readahead algorithm will pre-read into 461 the buffer cache 462 463 lifetime_write_kbytes This file is read-only and shows the number of 464 kilobytes of data that have been written to this 465 filesystem since it was created. 466 467 max_writeback_mb_bump The maximum number of megabytes the writeback 468 code will try to write out before move on to 469 another inode. 470 471 mb_group_prealloc The multiblock allocator will round up allocation 472 requests to a multiple of this tuning parameter if 473 the stripe size is not set in the ext4 superblock 474 475 mb_max_to_scan The maximum number of extents the multiblock 476 allocator will search to find the best extent 477 478 mb_min_to_scan The minimum number of extents the multiblock 479 allocator will search to find the best extent 480 481 mb_order2_req Tuning parameter which controls the minimum size 482 for requests (as a power of 2) where the buddy 483 cache is used 484 485 mb_stats Controls whether the multiblock allocator should 486 collect statistics, which are shown during the 487 unmount. 1 means to collect statistics, 0 means 488 not to collect statistics 489 490 mb_stream_req Files which have fewer blocks than this tunable 491 parameter will have their blocks allocated out 492 of a block group specific preallocation pool, so 493 that small files are packed closely together. 494 Each large file will have its blocks allocated 495 out of its own unique preallocation pool. 496 497 session_write_kbytes This file is read-only and shows the number of 498 kilobytes of data that have been written to this 499 filesystem since it was mounted. 500 501 reserved_clusters This is RW file and contains number of reserved 502 clusters in the file system which will be used 503 in the specific situations to avoid costly 504 zeroout, unexpected ENOSPC, or possible data 505 loss. The default is 2% or 4096 clusters, 506 whichever is smaller and this can be changed 507 however it can never exceed number of clusters 508 in the file system. If there is not enough space 509 for the reserved space when mounting the file 510 mount will _not_ fail. 511 .............................................................................. 512 513 Ioctls 514 ====== 515 516 There is some Ext4 specific functionality which can be accessed by applications 517 through the system call interfaces. The list of all Ext4 specific ioctls are 518 shown in the table below. 519 520 Table of Ext4 specific ioctls 521 .............................................................................. 522 Ioctl Description 523 EXT4_IOC_GETFLAGS Get additional attributes associated with inode. 524 The ioctl argument is an integer bitfield, with 525 bit values described in ext4.h. This ioctl is an 526 alias for FS_IOC_GETFLAGS. 527 528 EXT4_IOC_SETFLAGS Set additional attributes associated with inode. 529 The ioctl argument is an integer bitfield, with 530 bit values described in ext4.h. This ioctl is an 531 alias for FS_IOC_SETFLAGS. 532 533 EXT4_IOC_GETVERSION 534 EXT4_IOC_GETVERSION_OLD 535 Get the inode i_generation number stored for 536 each inode. The i_generation number is normally 537 changed only when new inode is created and it is 538 particularly useful for network filesystems. The 539 '_OLD' version of this ioctl is an alias for 540 FS_IOC_GETVERSION. 541 542 EXT4_IOC_SETVERSION 543 EXT4_IOC_SETVERSION_OLD 544 Set the inode i_generation number stored for 545 each inode. The '_OLD' version of this ioctl 546 is an alias for FS_IOC_SETVERSION. 547 548 EXT4_IOC_GROUP_EXTEND This ioctl has the same purpose as the resize 549 mount option. It allows to resize filesystem 550 to the end of the last existing block group, 551 further resize has to be done with resize2fs, 552 either online, or offline. The argument points 553 to the unsigned logn number representing the 554 filesystem new block count. 555 556 EXT4_IOC_MOVE_EXT Move the block extents from orig_fd (the one 557 this ioctl is pointing to) to the donor_fd (the 558 one specified in move_extent structure passed 559 as an argument to this ioctl). Then, exchange 560 inode metadata between orig_fd and donor_fd. 561 This is especially useful for online 562 defragmentation, because the allocator has the 563 opportunity to allocate moved blocks better, 564 ideally into one contiguous extent. 565 566 EXT4_IOC_GROUP_ADD Add a new group descriptor to an existing or 567 new group descriptor block. The new group 568 descriptor is described by ext4_new_group_input 569 structure, which is passed as an argument to 570 this ioctl. This is especially useful in 571 conjunction with EXT4_IOC_GROUP_EXTEND, 572 which allows online resize of the filesystem 573 to the end of the last existing block group. 574 Those two ioctls combined is used in userspace 575 online resize tool (e.g. resize2fs). 576 577 EXT4_IOC_MIGRATE This ioctl operates on the filesystem itself. 578 It converts (migrates) ext3 indirect block mapped 579 inode to ext4 extent mapped inode by walking 580 through indirect block mapping of the original 581 inode and converting contiguous block ranges 582 into ext4 extents of the temporary inode. Then, 583 inodes are swapped. This ioctl might help, when 584 migrating from ext3 to ext4 filesystem, however 585 suggestion is to create fresh ext4 filesystem 586 and copy data from the backup. Note, that 587 filesystem has to support extents for this ioctl 588 to work. 589 590 EXT4_IOC_ALLOC_DA_BLKS Force all of the delay allocated blocks to be 591 allocated to preserve application-expected ext3 592 behaviour. Note that this will also start 593 triggering a write of the data blocks, but this 594 behaviour may change in the future as it is 595 not necessary and has been done this way only 596 for sake of simplicity. 597 598 EXT4_IOC_RESIZE_FS Resize the filesystem to a new size. The number 599 of blocks of resized filesystem is passed in via 600 64 bit integer argument. The kernel allocates 601 bitmaps and inode table, the userspace tool thus 602 just passes the new number of blocks. 603 604 EXT4_IOC_SWAP_BOOT Swap i_blocks and associated attributes 605 (like i_blocks, i_size, i_flags, ...) from 606 the specified inode with inode 607 EXT4_BOOT_LOADER_INO (#5). This is typically 608 used to store a boot loader in a secure part of 609 the filesystem, where it can't be changed by a 610 normal user by accident. 611 The data blocks of the previous boot loader 612 will be associated with the given inode. 613 614 .............................................................................. 615 616 References 617 ========== 618 619 kernel source: <file:fs/ext4/> 620 <file:fs/jbd2/> 621 622 programs: http://e2fsprogs.sourceforge.net/ 623 624 useful links: http://fedoraproject.org/wiki/ext3-devel 625 http://www.bullopensource.org/ext4/ 626 http://ext4.wiki.kernel.org/index.php/Main_Page 627 http://fedoraproject.org/wiki/Features/Ext4