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=update Update the ext4 file system's journal to the current 148 format. 149 150journal_dev=devnum When the external journal device's major/minor numbers 151 have changed, this option allows the user to specify 152 the new journal location. The journal device is 153 identified through its new major/minor numbers encoded 154 in devnum. 155 156norecovery Don't load the journal on mounting. Note that 157noload if the filesystem was not unmounted cleanly, 158 skipping the journal replay will lead to the 159 filesystem containing inconsistencies that can 160 lead to any number of problems. 161 162data=journal All data are committed into the journal prior to being 163 written into the main file system. Enabling 164 this mode will disable delayed allocation and 165 O_DIRECT support. 166 167data=ordered (*) All data are forced directly out to the main file 168 system prior to its metadata being committed to the 169 journal. 170 171data=writeback Data ordering is not preserved, data may be written 172 into the main file system after its metadata has been 173 committed to the journal. 174 175commit=nrsec (*) Ext4 can be told to sync all its data and metadata 176 every 'nrsec' seconds. The default value is 5 seconds. 177 This means that if you lose your power, you will lose 178 as much as the latest 5 seconds of work (your 179 filesystem will not be damaged though, thanks to the 180 journaling). This default value (or any low value) 181 will hurt performance, but it's good for data-safety. 182 Setting it to 0 will have the same effect as leaving 183 it at the default (5 seconds). 184 Setting it to very large values will improve 185 performance. 186 187barrier=<0|1(*)> This enables/disables the use of write barriers in 188barrier(*) the jbd code. barrier=0 disables, barrier=1 enables. 189nobarrier This also requires an IO stack which can support 190 barriers, and if jbd gets an error on a barrier 191 write, it will disable again with a warning. 192 Write barriers enforce proper on-disk ordering 193 of journal commits, making volatile disk write caches 194 safe to use, at some performance penalty. If 195 your disks are battery-backed in one way or another, 196 disabling barriers may safely improve performance. 197 The mount options "barrier" and "nobarrier" can 198 also be used to enable or disable barriers, for 199 consistency with other ext4 mount options. 200 201inode_readahead_blks=n This tuning parameter controls the maximum 202 number of inode table blocks that ext4's inode 203 table readahead algorithm will pre-read into 204 the buffer cache. The default value is 32 blocks. 205 206nouser_xattr Disables Extended User Attributes. If you have extended 207 attribute support enabled in the kernel configuration 208 (CONFIG_EXT4_FS_XATTR), extended attribute support 209 is enabled by default on mount. See the attr(5) manual 210 page and http://acl.bestbits.at/ for more information 211 about extended attributes. 212 213noacl This option disables POSIX Access Control List 214 support. If ACL support is enabled in the kernel 215 configuration (CONFIG_EXT4_FS_POSIX_ACL), ACL is 216 enabled by default on mount. See the acl(5) manual 217 page and http://acl.bestbits.at/ for more information 218 about acl. 219 220bsddf (*) Make 'df' act like BSD. 221minixdf Make 'df' act like Minix. 222 223debug Extra debugging information is sent to syslog. 224 225abort Simulate the effects of calling ext4_abort() for 226 debugging purposes. This is normally used while 227 remounting a filesystem which is already mounted. 228 229errors=remount-ro Remount the filesystem read-only on an error. 230errors=continue Keep going on a filesystem error. 231errors=panic Panic and halt the machine if an error occurs. 232 (These mount options override the errors behavior 233 specified in the superblock, which can be configured 234 using tune2fs) 235 236data_err=ignore(*) Just print an error message if an error occurs 237 in a file data buffer in ordered mode. 238data_err=abort Abort the journal if an error occurs in a file 239 data buffer in ordered mode. 240 241grpid Give objects the same group ID as their creator. 242bsdgroups 243 244nogrpid (*) New objects have the group ID of their creator. 245sysvgroups 246 247resgid=n The group ID which may use the reserved blocks. 248 249resuid=n The user ID which may use the reserved blocks. 250 251sb=n Use alternate superblock at this location. 252 253quota These options are ignored by the filesystem. They 254noquota are used only by quota tools to recognize volumes 255grpquota where quota should be turned on. See documentation 256usrquota in the quota-tools package for more details 257 (http://sourceforge.net/projects/linuxquota). 258 259jqfmt=<quota type> These options tell filesystem details about quota 260usrjquota=<file> so that quota information can be properly updated 261grpjquota=<file> during journal replay. They replace the above 262 quota options. See documentation in the quota-tools 263 package for more details 264 (http://sourceforge.net/projects/linuxquota). 265 266stripe=n Number of filesystem blocks that mballoc will try 267 to use for allocation size and alignment. For RAID5/6 268 systems this should be the number of data 269 disks * RAID chunk size in file system blocks. 270 271delalloc (*) Defer block allocation until just before ext4 272 writes out the block(s) in question. This 273 allows ext4 to better allocation decisions 274 more efficiently. 275nodelalloc Disable delayed allocation. Blocks are allocated 276 when the data is copied from userspace to the 277 page cache, either via the write(2) system call 278 or when an mmap'ed page which was previously 279 unallocated is written for the first time. 280 281max_batch_time=usec Maximum amount of time ext4 should wait for 282 additional filesystem operations to be batch 283 together with a synchronous write operation. 284 Since a synchronous write operation is going to 285 force a commit and then a wait for the I/O 286 complete, it doesn't cost much, and can be a 287 huge throughput win, we wait for a small amount 288 of time to see if any other transactions can 289 piggyback on the synchronous write. The 290 algorithm used is designed to automatically tune 291 for the speed of the disk, by measuring the 292 amount of time (on average) that it takes to 293 finish committing a transaction. Call this time 294 the "commit time". If the time that the 295 transaction has been running is less than the 296 commit time, ext4 will try sleeping for the 297 commit time to see if other operations will join 298 the transaction. The commit time is capped by 299 the max_batch_time, which defaults to 15000us 300 (15ms). This optimization can be turned off 301 entirely by setting max_batch_time to 0. 302 303min_batch_time=usec This parameter sets the commit time (as 304 described above) to be at least min_batch_time. 305 It defaults to zero microseconds. Increasing 306 this parameter may improve the throughput of 307 multi-threaded, synchronous workloads on very 308 fast disks, at the cost of increasing latency. 309 310journal_ioprio=prio The I/O priority (from 0 to 7, where 0 is the 311 highest priorty) which should be used for I/O 312 operations submitted by kjournald2 during a 313 commit operation. This defaults to 3, which is 314 a slightly higher priority than the default I/O 315 priority. 316 317auto_da_alloc(*) Many broken applications don't use fsync() when 318noauto_da_alloc replacing existing files via patterns such as 319 fd = open("foo.new")/write(fd,..)/close(fd)/ 320 rename("foo.new", "foo"), or worse yet, 321 fd = open("foo", O_TRUNC)/write(fd,..)/close(fd). 322 If auto_da_alloc is enabled, ext4 will detect 323 the replace-via-rename and replace-via-truncate 324 patterns and force that any delayed allocation 325 blocks are allocated such that at the next 326 journal commit, in the default data=ordered 327 mode, the data blocks of the new file are forced 328 to disk before the rename() operation is 329 committed. This provides roughly the same level 330 of guarantees as ext3, and avoids the 331 "zero-length" problem that can happen when a 332 system crashes before the delayed allocation 333 blocks are forced to disk. 334 335noinit_itable Do not initialize any uninitialized inode table 336 blocks in the background. This feature may be 337 used by installation CD's so that the install 338 process can complete as quickly as possible; the 339 inode table initialization process would then be 340 deferred until the next time the file system 341 is unmounted. 342 343init_itable=n The lazy itable init code will wait n times the 344 number of milliseconds it took to zero out the 345 previous block group's inode table. This 346 minimizes the impact on the systme performance 347 while file system's inode table is being initialized. 348 349discard Controls whether ext4 should issue discard/TRIM 350nodiscard(*) commands to the underlying block device when 351 blocks are freed. This is useful for SSD devices 352 and sparse/thinly-provisioned LUNs, but it is off 353 by default until sufficient testing has been done. 354 355nouid32 Disables 32-bit UIDs and GIDs. This is for 356 interoperability with older kernels which only 357 store and expect 16-bit values. 358 359resize Allows to resize filesystem to the end of the last 360 existing block group, further resize has to be done 361 with resize2fs either online, or offline. It can be 362 used only with conjunction with remount. 363 364block_validity This options allows to enables/disables the in-kernel 365noblock_validity facility for tracking filesystem metadata blocks 366 within internal data structures. This allows multi- 367 block allocator and other routines to quickly locate 368 extents which might overlap with filesystem metadata 369 blocks. This option is intended for debugging 370 purposes and since it negatively affects the 371 performance, it is off by default. 372 373dioread_lock Controls whether or not ext4 should use the DIO read 374dioread_nolock locking. If the dioread_nolock option is specified 375 ext4 will allocate uninitialized extent before buffer 376 write and convert the extent to initialized after IO 377 completes. This approach allows ext4 code to avoid 378 using inode mutex, which improves scalability on high 379 speed storages. However this does not work with 380 data journaling and dioread_nolock option will be 381 ignored with kernel warning. Note that dioread_nolock 382 code path is only used for extent-based files. 383 Because of the restrictions this options comprises 384 it is off by default (e.g. dioread_lock). 385 386i_version Enable 64-bit inode version support. This option is 387 off by default. 388 389Data Mode 390========= 391There are 3 different data modes: 392 393* writeback mode 394In data=writeback mode, ext4 does not journal data at all. This mode provides 395a similar level of journaling as that of XFS, JFS, and ReiserFS in its default 396mode - metadata journaling. A crash+recovery can cause incorrect data to 397appear in files which were written shortly before the crash. This mode will 398typically provide the best ext4 performance. 399 400* ordered mode 401In data=ordered mode, ext4 only officially journals metadata, but it logically 402groups metadata information related to data changes with the data blocks into a 403single unit called a transaction. When it's time to write the new metadata 404out to disk, the associated data blocks are written first. In general, 405this mode performs slightly slower than writeback but significantly faster than journal mode. 406 407* journal mode 408data=journal mode provides full data and metadata journaling. All new data is 409written to the journal first, and then to its final location. 410In the event of a crash, the journal can be replayed, bringing both data and 411metadata into a consistent state. This mode is the slowest except when data 412needs to be read from and written to disk at the same time where it 413outperforms all others modes. Enabling this mode will disable delayed 414allocation and O_DIRECT support. 415 416/proc entries 417============= 418 419Information about mounted ext4 file systems can be found in 420/proc/fs/ext4. Each mounted filesystem will have a directory in 421/proc/fs/ext4 based on its device name (i.e., /proc/fs/ext4/hdc or 422/proc/fs/ext4/dm-0). The files in each per-device directory are shown 423in table below. 424 425Files in /proc/fs/ext4/<devname> 426.............................................................................. 427 File Content 428 mb_groups details of multiblock allocator buddy cache of free blocks 429.............................................................................. 430 431/sys entries 432============ 433 434Information about mounted ext4 file systems can be found in 435/sys/fs/ext4. Each mounted filesystem will have a directory in 436/sys/fs/ext4 based on its device name (i.e., /sys/fs/ext4/hdc or 437/sys/fs/ext4/dm-0). The files in each per-device directory are shown 438in table below. 439 440Files in /sys/fs/ext4/<devname> 441(see also Documentation/ABI/testing/sysfs-fs-ext4) 442.............................................................................. 443 File Content 444 445 delayed_allocation_blocks This file is read-only and shows the number of 446 blocks that are dirty in the page cache, but 447 which do not have their location in the 448 filesystem allocated yet. 449 450 inode_goal Tuning parameter which (if non-zero) controls 451 the goal inode used by the inode allocator in 452 preference to all other allocation heuristics. 453 This is intended for debugging use only, and 454 should be 0 on production systems. 455 456 inode_readahead_blks Tuning parameter which controls the maximum 457 number of inode table blocks that ext4's inode 458 table readahead algorithm will pre-read into 459 the buffer cache 460 461 lifetime_write_kbytes This file is read-only and shows the number of 462 kilobytes of data that have been written to this 463 filesystem since it was created. 464 465 max_writeback_mb_bump The maximum number of megabytes the writeback 466 code will try to write out before move on to 467 another inode. 468 469 mb_group_prealloc The multiblock allocator will round up allocation 470 requests to a multiple of this tuning parameter if 471 the stripe size is not set in the ext4 superblock 472 473 mb_max_to_scan The maximum number of extents the multiblock 474 allocator will search to find the best extent 475 476 mb_min_to_scan The minimum number of extents the multiblock 477 allocator will search to find the best extent 478 479 mb_order2_req Tuning parameter which controls the minimum size 480 for requests (as a power of 2) where the buddy 481 cache is used 482 483 mb_stats Controls whether the multiblock allocator should 484 collect statistics, which are shown during the 485 unmount. 1 means to collect statistics, 0 means 486 not to collect statistics 487 488 mb_stream_req Files which have fewer blocks than this tunable 489 parameter will have their blocks allocated out 490 of a block group specific preallocation pool, so 491 that small files are packed closely together. 492 Each large file will have its blocks allocated 493 out of its own unique preallocation pool. 494 495 session_write_kbytes This file is read-only and shows the number of 496 kilobytes of data that have been written to this 497 filesystem since it was mounted. 498.............................................................................. 499 500Ioctls 501====== 502 503There is some Ext4 specific functionality which can be accessed by applications 504through the system call interfaces. The list of all Ext4 specific ioctls are 505shown in the table below. 506 507Table of Ext4 specific ioctls 508.............................................................................. 509 Ioctl Description 510 EXT4_IOC_GETFLAGS Get additional attributes associated with inode. 511 The ioctl argument is an integer bitfield, with 512 bit values described in ext4.h. This ioctl is an 513 alias for FS_IOC_GETFLAGS. 514 515 EXT4_IOC_SETFLAGS Set additional attributes associated with inode. 516 The ioctl argument is an integer bitfield, with 517 bit values described in ext4.h. This ioctl is an 518 alias for FS_IOC_SETFLAGS. 519 520 EXT4_IOC_GETVERSION 521 EXT4_IOC_GETVERSION_OLD 522 Get the inode i_generation number stored for 523 each inode. The i_generation number is normally 524 changed only when new inode is created and it is 525 particularly useful for network filesystems. The 526 '_OLD' version of this ioctl is an alias for 527 FS_IOC_GETVERSION. 528 529 EXT4_IOC_SETVERSION 530 EXT4_IOC_SETVERSION_OLD 531 Set the inode i_generation number stored for 532 each inode. The '_OLD' version of this ioctl 533 is an alias for FS_IOC_SETVERSION. 534 535 EXT4_IOC_GROUP_EXTEND This ioctl has the same purpose as the resize 536 mount option. It allows to resize filesystem 537 to the end of the last existing block group, 538 further resize has to be done with resize2fs, 539 either online, or offline. The argument points 540 to the unsigned logn number representing the 541 filesystem new block count. 542 543 EXT4_IOC_MOVE_EXT Move the block extents from orig_fd (the one 544 this ioctl is pointing to) to the donor_fd (the 545 one specified in move_extent structure passed 546 as an argument to this ioctl). Then, exchange 547 inode metadata between orig_fd and donor_fd. 548 This is especially useful for online 549 defragmentation, because the allocator has the 550 opportunity to allocate moved blocks better, 551 ideally into one contiguous extent. 552 553 EXT4_IOC_GROUP_ADD Add a new group descriptor to an existing or 554 new group descriptor block. The new group 555 descriptor is described by ext4_new_group_input 556 structure, which is passed as an argument to 557 this ioctl. This is especially useful in 558 conjunction with EXT4_IOC_GROUP_EXTEND, 559 which allows online resize of the filesystem 560 to the end of the last existing block group. 561 Those two ioctls combined is used in userspace 562 online resize tool (e.g. resize2fs). 563 564 EXT4_IOC_MIGRATE This ioctl operates on the filesystem itself. 565 It converts (migrates) ext3 indirect block mapped 566 inode to ext4 extent mapped inode by walking 567 through indirect block mapping of the original 568 inode and converting contiguous block ranges 569 into ext4 extents of the temporary inode. Then, 570 inodes are swapped. This ioctl might help, when 571 migrating from ext3 to ext4 filesystem, however 572 suggestion is to create fresh ext4 filesystem 573 and copy data from the backup. Note, that 574 filesystem has to support extents for this ioctl 575 to work. 576 577 EXT4_IOC_ALLOC_DA_BLKS Force all of the delay allocated blocks to be 578 allocated to preserve application-expected ext3 579 behaviour. Note that this will also start 580 triggering a write of the data blocks, but this 581 behaviour may change in the future as it is 582 not necessary and has been done this way only 583 for sake of simplicity. 584 585 EXT4_IOC_RESIZE_FS Resize the filesystem to a new size. The number 586 of blocks of resized filesystem is passed in via 587 64 bit integer argument. The kernel allocates 588 bitmaps and inode table, the userspace tool thus 589 just passes the new number of blocks. 590 591.............................................................................. 592 593References 594========== 595 596kernel source: <file:fs/ext4/> 597 <file:fs/jbd2/> 598 599programs: http://e2fsprogs.sourceforge.net/ 600 601useful links: http://fedoraproject.org/wiki/ext3-devel 602 http://www.bullopensource.org/ext4/ 603 http://ext4.wiki.kernel.org/index.php/Main_Page 604 http://fedoraproject.org/wiki/Features/Ext4 605