1 2Ext4 Filesystem 3=============== 4 5Ext4 is an an advanced level of the ext3 filesystem which incorporates 6scalability and reliability enhancements for supporting large filesystems 7(64 bit) in keeping with increasing disk capacities and state-of-the-art 8feature requirements. 9 10Mailing list: linux-ext4@vger.kernel.org 11Web site: http://ext4.wiki.kernel.org 12 13 141. Quick usage instructions: 15=========================== 16 17Note: 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. 77 782. Features 79=========== 80 812.1 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[1] 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[1] Filesystems with a block size of 1k may see a limit imposed by the 104directory hash tree having a maximum depth of two. 105 1062.2 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 114There are several others under discussion, whether they all make it in is 115partly a function of how much time everyone has to work on them. Features like 116metadata checksumming have been discussed and planned for a bit but no patches 117exist yet so I'm not sure they're in the near-term roadmap. 118 119The big performance win will come with mballoc, delalloc and flex_bg 120grouping 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 1253. Options 126========== 127 128When mounting an ext4 filesystem, the following option are accepted: 129(*) == default 130 131ro 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 137journal_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 142journal_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 147journal_dev=devnum When the external journal device's major/minor numbers 148 have changed, this option allows the user to specify 149 the new journal location. The journal device is 150 identified through its new major/minor numbers encoded 151 in devnum. 152 153norecovery Don't load the journal on mounting. Note that 154noload if the filesystem was not unmounted cleanly, 155 skipping the journal replay will lead to the 156 filesystem containing inconsistencies that can 157 lead to any number of problems. 158 159data=journal All data are committed into the journal prior to being 160 written into the main file system. Enabling 161 this mode will disable delayed allocation and 162 O_DIRECT support. 163 164data=ordered (*) All data are forced directly out to the main file 165 system prior to its metadata being committed to the 166 journal. 167 168data=writeback Data ordering is not preserved, data may be written 169 into the main file system after its metadata has been 170 committed to the journal. 171 172commit=nrsec (*) Ext4 can be told to sync all its data and metadata 173 every 'nrsec' seconds. The default value is 5 seconds. 174 This means that if you lose your power, you will lose 175 as much as the latest 5 seconds of work (your 176 filesystem will not be damaged though, thanks to the 177 journaling). This default value (or any low value) 178 will hurt performance, but it's good for data-safety. 179 Setting it to 0 will have the same effect as leaving 180 it at the default (5 seconds). 181 Setting it to very large values will improve 182 performance. 183 184barrier=<0|1(*)> This enables/disables the use of write barriers in 185barrier(*) the jbd code. barrier=0 disables, barrier=1 enables. 186nobarrier This also requires an IO stack which can support 187 barriers, and if jbd gets an error on a barrier 188 write, it will disable again with a warning. 189 Write barriers enforce proper on-disk ordering 190 of journal commits, making volatile disk write caches 191 safe to use, at some performance penalty. If 192 your disks are battery-backed in one way or another, 193 disabling barriers may safely improve performance. 194 The mount options "barrier" and "nobarrier" can 195 also be used to enable or disable barriers, for 196 consistency with other ext4 mount options. 197 198inode_readahead_blks=n This tuning parameter controls the maximum 199 number of inode table blocks that ext4's inode 200 table readahead algorithm will pre-read into 201 the buffer cache. The default value is 32 blocks. 202 203nouser_xattr Disables Extended User Attributes. If you have extended 204 attribute support enabled in the kernel configuration 205 (CONFIG_EXT4_FS_XATTR), extended attribute support 206 is enabled by default on mount. See the attr(5) manual 207 page and http://acl.bestbits.at/ for more information 208 about extended attributes. 209 210noacl This option disables POSIX Access Control List 211 support. If ACL support is enabled in the kernel 212 configuration (CONFIG_EXT4_FS_POSIX_ACL), ACL is 213 enabled by default on mount. See the acl(5) manual 214 page and http://acl.bestbits.at/ for more information 215 about acl. 216 217bsddf (*) Make 'df' act like BSD. 218minixdf Make 'df' act like Minix. 219 220debug Extra debugging information is sent to syslog. 221 222abort Simulate the effects of calling ext4_abort() for 223 debugging purposes. This is normally used while 224 remounting a filesystem which is already mounted. 225 226errors=remount-ro Remount the filesystem read-only on an error. 227errors=continue Keep going on a filesystem error. 228errors=panic Panic and halt the machine if an error occurs. 229 (These mount options override the errors behavior 230 specified in the superblock, which can be configured 231 using tune2fs) 232 233data_err=ignore(*) Just print an error message if an error occurs 234 in a file data buffer in ordered mode. 235data_err=abort Abort the journal if an error occurs in a file 236 data buffer in ordered mode. 237 238grpid Give objects the same group ID as their creator. 239bsdgroups 240 241nogrpid (*) New objects have the group ID of their creator. 242sysvgroups 243 244resgid=n The group ID which may use the reserved blocks. 245 246resuid=n The user ID which may use the reserved blocks. 247 248sb=n Use alternate superblock at this location. 249 250quota These options are ignored by the filesystem. They 251noquota are used only by quota tools to recognize volumes 252grpquota where quota should be turned on. See documentation 253usrquota in the quota-tools package for more details 254 (http://sourceforge.net/projects/linuxquota). 255 256jqfmt=<quota type> These options tell filesystem details about quota 257usrjquota=<file> so that quota information can be properly updated 258grpjquota=<file> during journal replay. They replace the above 259 quota options. See documentation in the quota-tools 260 package for more details 261 (http://sourceforge.net/projects/linuxquota). 262 263stripe=n Number of filesystem blocks that mballoc will try 264 to use for allocation size and alignment. For RAID5/6 265 systems this should be the number of data 266 disks * RAID chunk size in file system blocks. 267 268delalloc (*) Defer block allocation until just before ext4 269 writes out the block(s) in question. This 270 allows ext4 to better allocation decisions 271 more efficiently. 272nodelalloc Disable delayed allocation. Blocks are allocated 273 when the data is copied from userspace to the 274 page cache, either via the write(2) system call 275 or when an mmap'ed page which was previously 276 unallocated is written for the first time. 277 278max_batch_time=usec Maximum amount of time ext4 should wait for 279 additional filesystem operations to be batch 280 together with a synchronous write operation. 281 Since a synchronous write operation is going to 282 force a commit and then a wait for the I/O 283 complete, it doesn't cost much, and can be a 284 huge throughput win, we wait for a small amount 285 of time to see if any other transactions can 286 piggyback on the synchronous write. The 287 algorithm used is designed to automatically tune 288 for the speed of the disk, by measuring the 289 amount of time (on average) that it takes to 290 finish committing a transaction. Call this time 291 the "commit time". If the time that the 292 transaction has been running is less than the 293 commit time, ext4 will try sleeping for the 294 commit time to see if other operations will join 295 the transaction. The commit time is capped by 296 the max_batch_time, which defaults to 15000us 297 (15ms). This optimization can be turned off 298 entirely by setting max_batch_time to 0. 299 300min_batch_time=usec This parameter sets the commit time (as 301 described above) to be at least min_batch_time. 302 It defaults to zero microseconds. Increasing 303 this parameter may improve the throughput of 304 multi-threaded, synchronous workloads on very 305 fast disks, at the cost of increasing latency. 306 307journal_ioprio=prio The I/O priority (from 0 to 7, where 0 is the 308 highest priority) which should be used for I/O 309 operations submitted by kjournald2 during a 310 commit operation. This defaults to 3, which is 311 a slightly higher priority than the default I/O 312 priority. 313 314auto_da_alloc(*) Many broken applications don't use fsync() when 315noauto_da_alloc replacing existing files via patterns such as 316 fd = open("foo.new")/write(fd,..)/close(fd)/ 317 rename("foo.new", "foo"), or worse yet, 318 fd = open("foo", O_TRUNC)/write(fd,..)/close(fd). 319 If auto_da_alloc is enabled, ext4 will detect 320 the replace-via-rename and replace-via-truncate 321 patterns and force that any delayed allocation 322 blocks are allocated such that at the next 323 journal commit, in the default data=ordered 324 mode, the data blocks of the new file are forced 325 to disk before the rename() operation is 326 committed. This provides roughly the same level 327 of guarantees as ext3, and avoids the 328 "zero-length" problem that can happen when a 329 system crashes before the delayed allocation 330 blocks are forced to disk. 331 332noinit_itable Do not initialize any uninitialized inode table 333 blocks in the background. This feature may be 334 used by installation CD's so that the install 335 process can complete as quickly as possible; the 336 inode table initialization process would then be 337 deferred until the next time the file system 338 is unmounted. 339 340init_itable=n The lazy itable init code will wait n times the 341 number of milliseconds it took to zero out the 342 previous block group's inode table. This 343 minimizes the impact on the system performance 344 while file system's inode table is being initialized. 345 346discard Controls whether ext4 should issue discard/TRIM 347nodiscard(*) commands to the underlying block device when 348 blocks are freed. This is useful for SSD devices 349 and sparse/thinly-provisioned LUNs, but it is off 350 by default until sufficient testing has been done. 351 352nouid32 Disables 32-bit UIDs and GIDs. This is for 353 interoperability with older kernels which only 354 store and expect 16-bit values. 355 356block_validity This options allows to enables/disables the in-kernel 357noblock_validity facility for tracking filesystem metadata blocks 358 within internal data structures. This allows multi- 359 block allocator and other routines to quickly locate 360 extents which might overlap with filesystem metadata 361 blocks. This option is intended for debugging 362 purposes and since it negatively affects the 363 performance, it is off by default. 364 365dioread_lock Controls whether or not ext4 should use the DIO read 366dioread_nolock locking. If the dioread_nolock option is specified 367 ext4 will allocate uninitialized extent before buffer 368 write and convert the extent to initialized after IO 369 completes. This approach allows ext4 code to avoid 370 using inode mutex, which improves scalability on high 371 speed storages. However this does not work with 372 data journaling and dioread_nolock option will be 373 ignored with kernel warning. Note that dioread_nolock 374 code path is only used for extent-based files. 375 Because of the restrictions this options comprises 376 it is off by default (e.g. dioread_lock). 377 378max_dir_size_kb=n This limits the size of directories so that any 379 attempt to expand them beyond the specified 380 limit in kilobytes will cause an ENOSPC error. 381 This is useful in memory constrained 382 environments, where a very large directory can 383 cause severe performance problems or even 384 provoke the Out Of Memory killer. (For example, 385 if there is only 512mb memory available, a 176mb 386 directory may seriously cramp the system's style.) 387 388i_version Enable 64-bit inode version support. This option is 389 off by default. 390 391Data Mode 392========= 393There are 3 different data modes: 394 395* writeback mode 396In data=writeback mode, ext4 does not journal data at all. This mode provides 397a similar level of journaling as that of XFS, JFS, and ReiserFS in its default 398mode - metadata journaling. A crash+recovery can cause incorrect data to 399appear in files which were written shortly before the crash. This mode will 400typically provide the best ext4 performance. 401 402* ordered mode 403In data=ordered mode, ext4 only officially journals metadata, but it logically 404groups metadata information related to data changes with the data blocks into a 405single unit called a transaction. When it's time to write the new metadata 406out to disk, the associated data blocks are written first. In general, 407this mode performs slightly slower than writeback but significantly faster than journal mode. 408 409* journal mode 410data=journal mode provides full data and metadata journaling. All new data is 411written to the journal first, and then to its final location. 412In the event of a crash, the journal can be replayed, bringing both data and 413metadata into a consistent state. This mode is the slowest except when data 414needs to be read from and written to disk at the same time where it 415outperforms all others modes. Enabling this mode will disable delayed 416allocation and O_DIRECT support. 417 418/proc entries 419============= 420 421Information 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 425in table below. 426 427Files 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 436Information 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 440in table below. 441 442Files 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 502Ioctls 503====== 504 505There is some Ext4 specific functionality which can be accessed by applications 506through the system call interfaces. The list of all Ext4 specific ioctls are 507shown in the table below. 508 509Table of Ext4 specific ioctls 510.............................................................................. 511 Ioctl Description 512 EXT4_IOC_GETFLAGS Get additional attributes associated with inode. 513 The ioctl argument is an integer bitfield, with 514 bit values described in ext4.h. This ioctl is an 515 alias for FS_IOC_GETFLAGS. 516 517 EXT4_IOC_SETFLAGS Set additional attributes associated with inode. 518 The ioctl argument is an integer bitfield, with 519 bit values described in ext4.h. This ioctl is an 520 alias for FS_IOC_SETFLAGS. 521 522 EXT4_IOC_GETVERSION 523 EXT4_IOC_GETVERSION_OLD 524 Get the inode i_generation number stored for 525 each inode. The i_generation number is normally 526 changed only when new inode is created and it is 527 particularly useful for network filesystems. The 528 '_OLD' version of this ioctl is an alias for 529 FS_IOC_GETVERSION. 530 531 EXT4_IOC_SETVERSION 532 EXT4_IOC_SETVERSION_OLD 533 Set the inode i_generation number stored for 534 each inode. The '_OLD' version of this ioctl 535 is an alias for FS_IOC_SETVERSION. 536 537 EXT4_IOC_GROUP_EXTEND This ioctl has the same purpose as the resize 538 mount option. It allows to resize filesystem 539 to the end of the last existing block group, 540 further resize has to be done with resize2fs, 541 either online, or offline. The argument points 542 to the unsigned logn number representing the 543 filesystem new block count. 544 545 EXT4_IOC_MOVE_EXT Move the block extents from orig_fd (the one 546 this ioctl is pointing to) to the donor_fd (the 547 one specified in move_extent structure passed 548 as an argument to this ioctl). Then, exchange 549 inode metadata between orig_fd and donor_fd. 550 This is especially useful for online 551 defragmentation, because the allocator has the 552 opportunity to allocate moved blocks better, 553 ideally into one contiguous extent. 554 555 EXT4_IOC_GROUP_ADD Add a new group descriptor to an existing or 556 new group descriptor block. The new group 557 descriptor is described by ext4_new_group_input 558 structure, which is passed as an argument to 559 this ioctl. This is especially useful in 560 conjunction with EXT4_IOC_GROUP_EXTEND, 561 which allows online resize of the filesystem 562 to the end of the last existing block group. 563 Those two ioctls combined is used in userspace 564 online resize tool (e.g. resize2fs). 565 566 EXT4_IOC_MIGRATE This ioctl operates on the filesystem itself. 567 It converts (migrates) ext3 indirect block mapped 568 inode to ext4 extent mapped inode by walking 569 through indirect block mapping of the original 570 inode and converting contiguous block ranges 571 into ext4 extents of the temporary inode. Then, 572 inodes are swapped. This ioctl might help, when 573 migrating from ext3 to ext4 filesystem, however 574 suggestion is to create fresh ext4 filesystem 575 and copy data from the backup. Note, that 576 filesystem has to support extents for this ioctl 577 to work. 578 579 EXT4_IOC_ALLOC_DA_BLKS Force all of the delay allocated blocks to be 580 allocated to preserve application-expected ext3 581 behaviour. Note that this will also start 582 triggering a write of the data blocks, but this 583 behaviour may change in the future as it is 584 not necessary and has been done this way only 585 for sake of simplicity. 586 587 EXT4_IOC_RESIZE_FS Resize the filesystem to a new size. The number 588 of blocks of resized filesystem is passed in via 589 64 bit integer argument. The kernel allocates 590 bitmaps and inode table, the userspace tool thus 591 just passes the new number of blocks. 592 593.............................................................................. 594 595References 596========== 597 598kernel source: <file:fs/ext4/> 599 <file:fs/jbd2/> 600 601programs: http://e2fsprogs.sourceforge.net/ 602 603useful links: http://fedoraproject.org/wiki/ext3-devel 604 http://www.bullopensource.org/ext4/ 605 http://ext4.wiki.kernel.org/index.php/Main_Page 606 http://fedoraproject.org/wiki/Features/Ext4 607