Based on kernel version 4.2. Page generated on 2015-09-09 12:13 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 ftp://ftp.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 and http://acl.bestbits.at/ 206 for more information about 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 and http://acl.bestbits.at/ for more information 213 about acl. 214 215 bsddf (*) Make 'df' act like BSD. 216 minixdf Make 'df' act like Minix. 217 218 debug Extra debugging information is sent to syslog. 219 220 abort Simulate the effects of calling ext4_abort() for 221 debugging purposes. This is normally used while 222 remounting a filesystem which is already mounted. 223 224 errors=remount-ro Remount the filesystem read-only on an error. 225 errors=continue Keep going on a filesystem error. 226 errors=panic Panic and halt the machine if an error occurs. 227 (These mount options override the errors behavior 228 specified in the superblock, which can be configured 229 using tune2fs) 230 231 data_err=ignore(*) Just print an error message if an error occurs 232 in a file data buffer in ordered mode. 233 data_err=abort Abort the journal if an error occurs in a file 234 data buffer in ordered mode. 235 236 grpid Give objects the same group ID as their creator. 237 bsdgroups 238 239 nogrpid (*) New objects have the group ID of their creator. 240 sysvgroups 241 242 resgid=n The group ID which may use the reserved blocks. 243 244 resuid=n The user ID which may use the reserved blocks. 245 246 sb=n Use alternate superblock at this location. 247 248 quota These options are ignored by the filesystem. They 249 noquota are used only by quota tools to recognize volumes 250 grpquota where quota should be turned on. See documentation 251 usrquota in the quota-tools package for more details 252 (http://sourceforge.net/projects/linuxquota). 253 254 jqfmt=<quota type> These options tell filesystem details about quota 255 usrjquota=<file> so that quota information can be properly updated 256 grpjquota=<file> during journal replay. They replace the above 257 quota options. See documentation in the quota-tools 258 package for more details 259 (http://sourceforge.net/projects/linuxquota). 260 261 stripe=n Number of filesystem blocks that mballoc will try 262 to use for allocation size and alignment. For RAID5/6 263 systems this should be the number of data 264 disks * RAID chunk size in file system blocks. 265 266 delalloc (*) Defer block allocation until just before ext4 267 writes out the block(s) in question. This 268 allows ext4 to better allocation decisions 269 more efficiently. 270 nodelalloc Disable delayed allocation. Blocks are allocated 271 when the data is copied from userspace to the 272 page cache, either via the write(2) system call 273 or when an mmap'ed page which was previously 274 unallocated is written for the first time. 275 276 max_batch_time=usec Maximum amount of time ext4 should wait for 277 additional filesystem operations to be batch 278 together with a synchronous write operation. 279 Since a synchronous write operation is going to 280 force a commit and then a wait for the I/O 281 complete, it doesn't cost much, and can be a 282 huge throughput win, we wait for a small amount 283 of time to see if any other transactions can 284 piggyback on the synchronous write. The 285 algorithm used is designed to automatically tune 286 for the speed of the disk, by measuring the 287 amount of time (on average) that it takes to 288 finish committing a transaction. Call this time 289 the "commit time". If the time that the 290 transaction has been running is less than the 291 commit time, ext4 will try sleeping for the 292 commit time to see if other operations will join 293 the transaction. The commit time is capped by 294 the max_batch_time, which defaults to 15000us 295 (15ms). This optimization can be turned off 296 entirely by setting max_batch_time to 0. 297 298 min_batch_time=usec This parameter sets the commit time (as 299 described above) to be at least min_batch_time. 300 It defaults to zero microseconds. Increasing 301 this parameter may improve the throughput of 302 multi-threaded, synchronous workloads on very 303 fast disks, at the cost of increasing latency. 304 305 journal_ioprio=prio The I/O priority (from 0 to 7, where 0 is the 306 highest priority) which should be used for I/O 307 operations submitted by kjournald2 during a 308 commit operation. This defaults to 3, which is 309 a slightly higher priority than the default I/O 310 priority. 311 312 auto_da_alloc(*) Many broken applications don't use fsync() when 313 noauto_da_alloc replacing existing files via patterns such as 314 fd = open("foo.new")/write(fd,..)/close(fd)/ 315 rename("foo.new", "foo"), or worse yet, 316 fd = open("foo", O_TRUNC)/write(fd,..)/close(fd). 317 If auto_da_alloc is enabled, ext4 will detect 318 the replace-via-rename and replace-via-truncate 319 patterns and force that any delayed allocation 320 blocks are allocated such that at the next 321 journal commit, in the default data=ordered 322 mode, the data blocks of the new file are forced 323 to disk before the rename() operation is 324 committed. This provides roughly the same level 325 of guarantees as ext3, and avoids the 326 "zero-length" problem that can happen when a 327 system crashes before the delayed allocation 328 blocks are forced to disk. 329 330 noinit_itable Do not initialize any uninitialized inode table 331 blocks in the background. This feature may be 332 used by installation CD's so that the install 333 process can complete as quickly as possible; the 334 inode table initialization process would then be 335 deferred until the next time the file system 336 is unmounted. 337 338 init_itable=n The lazy itable init code will wait n times the 339 number of milliseconds it took to zero out the 340 previous block group's inode table. This 341 minimizes the impact on the system performance 342 while file system's inode table is being initialized. 343 344 discard Controls whether ext4 should issue discard/TRIM 345 nodiscard(*) commands to the underlying block device when 346 blocks are freed. This is useful for SSD devices 347 and sparse/thinly-provisioned LUNs, but it is off 348 by default until sufficient testing has been done. 349 350 nouid32 Disables 32-bit UIDs and GIDs. This is for 351 interoperability with older kernels which only 352 store and expect 16-bit values. 353 354 block_validity This options allows to enables/disables the in-kernel 355 noblock_validity facility for tracking filesystem metadata blocks 356 within internal data structures. This allows multi- 357 block allocator and other routines to quickly locate 358 extents which might overlap with filesystem metadata 359 blocks. This option is intended for debugging 360 purposes and since it negatively affects the 361 performance, it is off by default. 362 363 dioread_lock Controls whether or not ext4 should use the DIO read 364 dioread_nolock locking. If the dioread_nolock option is specified 365 ext4 will allocate uninitialized extent before buffer 366 write and convert the extent to initialized after IO 367 completes. This approach allows ext4 code to avoid 368 using inode mutex, which improves scalability on high 369 speed storages. However this does not work with 370 data journaling and dioread_nolock option will be 371 ignored with kernel warning. Note that dioread_nolock 372 code path is only used for extent-based files. 373 Because of the restrictions this options comprises 374 it is off by default (e.g. dioread_lock). 375 376 max_dir_size_kb=n This limits the size of directories so that any 377 attempt to expand them beyond the specified 378 limit in kilobytes will cause an ENOSPC error. 379 This is useful in memory constrained 380 environments, where a very large directory can 381 cause severe performance problems or even 382 provoke the Out Of Memory killer. (For example, 383 if there is only 512mb memory available, a 176mb 384 directory may seriously cramp the system's style.) 385 386 i_version Enable 64-bit inode version support. This option is 387 off by default. 388 389 dax Use direct access (no page cache). See 390 Documentation/filesystems/dax.txt. Note that 391 this option is incompatible with data=journal. 392 393 Data Mode 394 ========= 395 There are 3 different data modes: 396 397 * writeback mode 398 In data=writeback mode, ext4 does not journal data at all. This mode provides 399 a similar level of journaling as that of XFS, JFS, and ReiserFS in its default 400 mode - metadata journaling. A crash+recovery can cause incorrect data to 401 appear in files which were written shortly before the crash. This mode will 402 typically provide the best ext4 performance. 403 404 * ordered mode 405 In data=ordered mode, ext4 only officially journals metadata, but it logically 406 groups metadata information related to data changes with the data blocks into a 407 single unit called a transaction. When it's time to write the new metadata 408 out to disk, the associated data blocks are written first. In general, 409 this mode performs slightly slower than writeback but significantly faster than journal mode. 410 411 * journal mode 412 data=journal mode provides full data and metadata journaling. All new data is 413 written to the journal first, and then to its final location. 414 In the event of a crash, the journal can be replayed, bringing both data and 415 metadata into a consistent state. This mode is the slowest except when data 416 needs to be read from and written to disk at the same time where it 417 outperforms all others modes. Enabling this mode will disable delayed 418 allocation and O_DIRECT support. 419 420 /proc entries 421 ============= 422 423 Information about mounted ext4 file systems can be found in 424 /proc/fs/ext4. Each mounted filesystem will have a directory in 425 /proc/fs/ext4 based on its device name (i.e., /proc/fs/ext4/hdc or 426 /proc/fs/ext4/dm-0). The files in each per-device directory are shown 427 in table below. 428 429 Files in /proc/fs/ext4/<devname> 430 .............................................................................. 431 File Content 432 mb_groups details of multiblock allocator buddy cache of free blocks 433 .............................................................................. 434 435 /sys entries 436 ============ 437 438 Information about mounted ext4 file systems can be found in 439 /sys/fs/ext4. Each mounted filesystem will have a directory in 440 /sys/fs/ext4 based on its device name (i.e., /sys/fs/ext4/hdc or 441 /sys/fs/ext4/dm-0). The files in each per-device directory are shown 442 in table below. 443 444 Files in /sys/fs/ext4/<devname> 445 (see also Documentation/ABI/testing/sysfs-fs-ext4) 446 .............................................................................. 447 File Content 448 449 delayed_allocation_blocks This file is read-only and shows the number of 450 blocks that are dirty in the page cache, but 451 which do not have their location in the 452 filesystem allocated yet. 453 454 inode_goal Tuning parameter which (if non-zero) controls 455 the goal inode used by the inode allocator in 456 preference to all other allocation heuristics. 457 This is intended for debugging use only, and 458 should be 0 on production systems. 459 460 inode_readahead_blks Tuning parameter which controls the maximum 461 number of inode table blocks that ext4's inode 462 table readahead algorithm will pre-read into 463 the buffer cache 464 465 lifetime_write_kbytes This file is read-only and shows the number of 466 kilobytes of data that have been written to this 467 filesystem since it was created. 468 469 max_writeback_mb_bump The maximum number of megabytes the writeback 470 code will try to write out before move on to 471 another inode. 472 473 mb_group_prealloc The multiblock allocator will round up allocation 474 requests to a multiple of this tuning parameter if 475 the stripe size is not set in the ext4 superblock 476 477 mb_max_to_scan The maximum number of extents the multiblock 478 allocator will search to find the best extent 479 480 mb_min_to_scan The minimum number of extents the multiblock 481 allocator will search to find the best extent 482 483 mb_order2_req Tuning parameter which controls the minimum size 484 for requests (as a power of 2) where the buddy 485 cache is used 486 487 mb_stats Controls whether the multiblock allocator should 488 collect statistics, which are shown during the 489 unmount. 1 means to collect statistics, 0 means 490 not to collect statistics 491 492 mb_stream_req Files which have fewer blocks than this tunable 493 parameter will have their blocks allocated out 494 of a block group specific preallocation pool, so 495 that small files are packed closely together. 496 Each large file will have its blocks allocated 497 out of its own unique preallocation pool. 498 499 session_write_kbytes This file is read-only and shows the number of 500 kilobytes of data that have been written to this 501 filesystem since it was mounted. 502 503 reserved_clusters This is RW file and contains number of reserved 504 clusters in the file system which will be used 505 in the specific situations to avoid costly 506 zeroout, unexpected ENOSPC, or possible data 507 loss. The default is 2% or 4096 clusters, 508 whichever is smaller and this can be changed 509 however it can never exceed number of clusters 510 in the file system. If there is not enough space 511 for the reserved space when mounting the file 512 mount will _not_ fail. 513 .............................................................................. 514 515 Ioctls 516 ====== 517 518 There is some Ext4 specific functionality which can be accessed by applications 519 through the system call interfaces. The list of all Ext4 specific ioctls are 520 shown in the table below. 521 522 Table of Ext4 specific ioctls 523 .............................................................................. 524 Ioctl Description 525 EXT4_IOC_GETFLAGS Get additional attributes associated with inode. 526 The ioctl argument is an integer bitfield, with 527 bit values described in ext4.h. This ioctl is an 528 alias for FS_IOC_GETFLAGS. 529 530 EXT4_IOC_SETFLAGS Set additional attributes associated with inode. 531 The ioctl argument is an integer bitfield, with 532 bit values described in ext4.h. This ioctl is an 533 alias for FS_IOC_SETFLAGS. 534 535 EXT4_IOC_GETVERSION 536 EXT4_IOC_GETVERSION_OLD 537 Get the inode i_generation number stored for 538 each inode. The i_generation number is normally 539 changed only when new inode is created and it is 540 particularly useful for network filesystems. The 541 '_OLD' version of this ioctl is an alias for 542 FS_IOC_GETVERSION. 543 544 EXT4_IOC_SETVERSION 545 EXT4_IOC_SETVERSION_OLD 546 Set the inode i_generation number stored for 547 each inode. The '_OLD' version of this ioctl 548 is an alias for FS_IOC_SETVERSION. 549 550 EXT4_IOC_GROUP_EXTEND This ioctl has the same purpose as the resize 551 mount option. It allows to resize filesystem 552 to the end of the last existing block group, 553 further resize has to be done with resize2fs, 554 either online, or offline. The argument points 555 to the unsigned logn number representing the 556 filesystem new block count. 557 558 EXT4_IOC_MOVE_EXT Move the block extents from orig_fd (the one 559 this ioctl is pointing to) to the donor_fd (the 560 one specified in move_extent structure passed 561 as an argument to this ioctl). Then, exchange 562 inode metadata between orig_fd and donor_fd. 563 This is especially useful for online 564 defragmentation, because the allocator has the 565 opportunity to allocate moved blocks better, 566 ideally into one contiguous extent. 567 568 EXT4_IOC_GROUP_ADD Add a new group descriptor to an existing or 569 new group descriptor block. The new group 570 descriptor is described by ext4_new_group_input 571 structure, which is passed as an argument to 572 this ioctl. This is especially useful in 573 conjunction with EXT4_IOC_GROUP_EXTEND, 574 which allows online resize of the filesystem 575 to the end of the last existing block group. 576 Those two ioctls combined is used in userspace 577 online resize tool (e.g. resize2fs). 578 579 EXT4_IOC_MIGRATE This ioctl operates on the filesystem itself. 580 It converts (migrates) ext3 indirect block mapped 581 inode to ext4 extent mapped inode by walking 582 through indirect block mapping of the original 583 inode and converting contiguous block ranges 584 into ext4 extents of the temporary inode. Then, 585 inodes are swapped. This ioctl might help, when 586 migrating from ext3 to ext4 filesystem, however 587 suggestion is to create fresh ext4 filesystem 588 and copy data from the backup. Note, that 589 filesystem has to support extents for this ioctl 590 to work. 591 592 EXT4_IOC_ALLOC_DA_BLKS Force all of the delay allocated blocks to be 593 allocated to preserve application-expected ext3 594 behaviour. Note that this will also start 595 triggering a write of the data blocks, but this 596 behaviour may change in the future as it is 597 not necessary and has been done this way only 598 for sake of simplicity. 599 600 EXT4_IOC_RESIZE_FS Resize the filesystem to a new size. The number 601 of blocks of resized filesystem is passed in via 602 64 bit integer argument. The kernel allocates 603 bitmaps and inode table, the userspace tool thus 604 just passes the new number of blocks. 605 606 EXT4_IOC_SWAP_BOOT Swap i_blocks and associated attributes 607 (like i_blocks, i_size, i_flags, ...) from 608 the specified inode with inode 609 EXT4_BOOT_LOADER_INO (#5). This is typically 610 used to store a boot loader in a secure part of 611 the filesystem, where it can't be changed by a 612 normal user by accident. 613 The data blocks of the previous boot loader 614 will be associated with the given inode. 615 616 .............................................................................. 617 618 References 619 ========== 620 621 kernel source: <file:fs/ext4/> 622 <file:fs/jbd2/> 623 624 programs: http://e2fsprogs.sourceforge.net/ 625 626 useful links: http://fedoraproject.org/wiki/ext3-devel 627 http://www.bullopensource.org/ext4/ 628 http://ext4.wiki.kernel.org/index.php/Main_Page 629 http://fedoraproject.org/wiki/Features/Ext4