Documentation/admin-guide/ext4.rst

   1 .. SPDX-License-Identifier: GPL-2.0
   2
   3 ========================
   4 ext4 General Information
   5 ========================
   6
   7 Ext4 is an advanced level of the ext3 filesystem which incorporates
   8 scalability and reliability enhancements for supporting large filesystems
   9 (64 bit) in keeping with increasing disk capacities and state-of-the-art
  10 feature requirements.
  11
  12 Mailing list:   linux-ext4@vger.kernel.org
  13 Web site:       http://ext4.wiki.kernel.org
  14
  15
  16 Quick usage instructions
  17 ========================
  18
  19 Note: More extensive information for getting started with ext4 can be
  20 found at the ext4 wiki site at the URL:
  21 http://ext4.wiki.kernel.org/index.php/Ext4_Howto
  22
  23   - The latest version of e2fsprogs can be found at:
  24
  25     https://www.kernel.org/pub/linux/kernel/people/tytso/e2fsprogs/
  26
  27         or
  28
  29     http://sourceforge.net/project/showfiles.php?group_id=2406
  30
  31         or grab the latest git repository from:
  32
  33    https://git.kernel.org/pub/scm/fs/ext2/e2fsprogs.git
  34
  35   - Create a new filesystem using the ext4 filesystem type:
  36
  37         # mke2fs -t ext4 /dev/hda1
  38
  39     Or to configure an existing ext3 filesystem to support extents:
  40
  41         # tune2fs -O extents /dev/hda1
  42
  43     If the filesystem was created with 128 byte inodes, it can be
  44     converted to use 256 byte for greater efficiency via:
  45
  46         # tune2fs -I 256 /dev/hda1
  47
  48   - Mounting:
  49
  50         # mount -t ext4 /dev/hda1 /wherever
  51
  52   - When comparing performance with other filesystems, it's always
  53     important to try multiple workloads; very often a subtle change in a
  54     workload parameter can completely change the ranking of which
  55     filesystems do well compared to others.  When comparing versus ext3,
  56     note that ext4 enables write barriers by default, while ext3 does
  57     not enable write barriers by default.  So it is useful to use
  58     explicitly specify whether barriers are enabled or not when via the
  59     '-o barriers=[0|1]' mount option for both ext3 and ext4 filesystems
  60     for a fair comparison.  When tuning ext3 for best benchmark numbers,
  61     it is often worthwhile to try changing the data journaling mode; '-o
  62     data=writeback' can be faster for some workloads.  (Note however that
  63     running mounted with data=writeback can potentially leave stale data
  64     exposed in recently written files in case of an unclean shutdown,
  65     which could be a security exposure in some situations.)  Configuring
  66     the filesystem with a large journal can also be helpful for
  67     metadata-intensive workloads.
  68
  69 Features
  70 ========
  71
  72 Currently Available
  73 -------------------
  74
  75 * ability to use filesystems > 16TB (e2fsprogs support not available yet)
  76 * extent format reduces metadata overhead (RAM, IO for access, transactions)
  77 * extent format more robust in face of on-disk corruption due to magics,
  78 * internal redundancy in tree
  79 * improved file allocation (multi-block alloc)
  80 * lift 32000 subdirectory limit imposed by i_links_count[1]
  81 * nsec timestamps for mtime, atime, ctime, create time
  82 * inode version field on disk (NFSv4, Lustre)
  83 * reduced e2fsck time via uninit_bg feature
  84 * journal checksumming for robustness, performance
  85 * persistent file preallocation (e.g for streaming media, databases)
  86 * ability to pack bitmaps and inode tables into larger virtual groups via the
  87   flex_bg feature
  88 * large file support
  89 * inode allocation using large virtual block groups via flex_bg
  90 * delayed allocation
  91 * large block (up to pagesize) support
  92 * efficient new ordered mode in JBD2 and ext4 (avoid using buffer head to force
  93   the ordering)
  94 * Case-insensitive file name lookups
  95 * file-based encryption support (fscrypt)
  96 * file-based verity support (fsverity)
  97
  98 [1] Filesystems with a block size of 1k may see a limit imposed by the
  99 directory hash tree having a maximum depth of two.
 100
 101 case-insensitive file name lookups
 102 ======================================================
 103
 104 The case-insensitive file name lookup feature is supported on a
 105 per-directory basis, allowing the user to mix case-insensitive and
 106 case-sensitive directories in the same filesystem.  It is enabled by
 107 flipping the +F inode attribute of an empty directory.  The
 108 case-insensitive string match operation is only defined when we know how
 109 text in encoded in a byte sequence.  For that reason, in order to enable
 110 case-insensitive directories, the filesystem must have the
 111 casefold feature, which stores the filesystem-wide encoding
 112 model used.  By default, the charset adopted is the latest version of
 113 Unicode (12.1.0, by the time of this writing), encoded in the UTF-8
 114 form.  The comparison algorithm is implemented by normalizing the
 115 strings to the Canonical decomposition form, as defined by Unicode,
 116 followed by a byte per byte comparison.
 117
 118 The case-awareness is name-preserving on the disk, meaning that the file
 119 name provided by userspace is a byte-per-byte match to what is actually
 120 written in the disk.  The Unicode normalization format used by the
 121 kernel is thus an internal representation, and not exposed to the
 122 userspace nor to the disk, with the important exception of disk hashes,
 123 used on large case-insensitive directories with DX feature.  On DX
 124 directories, the hash must be calculated using the casefolded version of
 125 the filename, meaning that the normalization format used actually has an
 126 impact on where the directory entry is stored.
 127
 128 When we change from viewing filenames as opaque byte sequences to seeing
 129 them as encoded strings we need to address what happens when a program
 130 tries to create a file with an invalid name.  The Unicode subsystem
 131 within the kernel leaves the decision of what to do in this case to the
 132 filesystem, which select its preferred behavior by enabling/disabling
 133 the strict mode.  When Ext4 encounters one of those strings and the
 134 filesystem did not require strict mode, it falls back to considering the
 135 entire string as an opaque byte sequence, which still allows the user to
 136 operate on that file, but the case-insensitive lookups won't work.
 137
 138 Options
 139 =======
 140
 141 When mounting an ext4 filesystem, the following option are accepted:
 142 (*) == default
 143
 144   ro
 145         Mount filesystem read only. Note that ext4 will replay the journal (and
 146         thus write to the partition) even when mounted "read only". The mount
 147         options "ro,noload" can be used to prevent writes to the filesystem.
 148
 149   journal_checksum
 150         Enable checksumming of the journal transactions.  This will allow the
 151         recovery code in e2fsck and the kernel to detect corruption in the
 152         kernel.  It is a compatible change and will be ignored by older
 153         kernels.
 154
 155   journal_async_commit
 156         Commit block can be written to disk without waiting for descriptor
 157         blocks. If enabled older kernels cannot mount the device. This will
 158         enable 'journal_checksum' internally.
 159
 160   journal_path=path, journal_dev=devnum
 161         When the external journal device's major/minor numbers have changed,
 162         these options allow the user to specify the new journal location.  The
 163         journal device is identified through either its new major/minor numbers
 164         encoded in devnum, or via a path to the device.
 165
 166   norecovery, noload
 167         Don't load the journal on mounting.  Note that if the filesystem was
 168         not unmounted cleanly, skipping the journal replay will lead to the
 169         filesystem containing inconsistencies that can lead to any number of
 170         problems.
 171
 172   data=journal
 173         All data are committed into the journal prior to being written into the
 174         main file system.  Enabling this mode will disable delayed allocation
 175         and O_DIRECT support.
 176
 177   data=ordered  (*)
 178         All data are forced directly out to the main file system prior to its
 179         metadata being committed to the journal.
 180
 181   data=writeback
 182         Data ordering is not preserved, data may be written into the main file
 183         system after its metadata has been committed to the journal.
 184
 185   commit=nrsec  (*)
 186         This setting limits the maximum age of the running transaction to
 187         'nrsec' seconds.  The default value is 5 seconds.  This means that if
 188         you lose your power, you will lose as much as the latest 5 seconds of
 189         metadata changes (your filesystem will not be damaged though, thanks
 190         to the journaling). This default value (or any low value) will hurt
 191         performance, but it's good for data-safety.  Setting it to 0 will have
 192         the same effect as leaving it at the default (5 seconds).  Setting it
 193         to very large values will improve performance.  Note that due to
 194         delayed allocation even older data can be lost on power failure since
 195         writeback of those data begins only after time set in
 196         /proc/sys/vm/dirty_expire_centisecs.
 197
 198   barrier=<0|1(*)>, barrier(*), nobarrier
 199         This enables/disables the use of write barriers in the jbd code.
 200         barrier=0 disables, barrier=1 enables.  This also requires an IO stack
 201         which can support barriers, and if jbd gets an error on a barrier
 202         write, it will disable again with a warning.  Write barriers enforce
 203         proper on-disk ordering of journal commits, making volatile disk write
 204         caches safe to use, at some performance penalty.  If your disks are
 205         battery-backed in one way or another, disabling barriers may safely
 206         improve performance.  The mount options "barrier" and "nobarrier" can
 207         also be used to enable or disable barriers, for consistency with other
 208         ext4 mount options.
 209
 210   inode_readahead_blks=n
 211         This tuning parameter controls the maximum number of inode table blocks
 212         that ext4's inode table readahead algorithm will pre-read into the
 213         buffer cache.  The default value is 32 blocks.
 214
 215   nouser_xattr
 216         Disables Extended User Attributes.  See the attr(5) manual page for
 217         more information about extended attributes.
 218
 219   noacl
 220         This option disables POSIX Access Control List support. If ACL support
 221         is enabled in the kernel configuration (CONFIG_EXT4_FS_POSIX_ACL), ACL
 222         is enabled by default on mount. See the acl(5) manual page for more
 223         information about acl.
 224
 225   bsddf (*)
 226         Make 'df' act like BSD.
 227
 228   minixdf
 229         Make 'df' act like Minix.
 230
 231   debug
 232         Extra debugging information is sent to syslog.
 233
 234   abort
 235         Simulate the effects of calling ext4_abort() for debugging purposes.
 236         This is normally used while remounting a filesystem which is already
 237         mounted.
 238
 239   errors=remount-ro
 240         Remount the filesystem read-only on an error.
 241
 242   errors=continue
 243         Keep going on a filesystem error.
 244
 245   errors=panic
 246         Panic and halt the machine if an error occurs.  (These mount options
 247         override the errors behavior specified in the superblock, which can be
 248         configured using tune2fs)
 249
 250   data_err=ignore(*)
 251         Just print an error message if an error occurs in a file data buffer in
 252         ordered mode.
 253   data_err=abort
 254         Abort the journal if an error occurs in a file data buffer in ordered
 255         mode.
 256
 257   grpid | bsdgroups
 258         New objects have the group ID of their parent.
 259
 260   nogrpid (*) | sysvgroups
 261         New objects have the group ID of their creator.
 262
 263   resgid=n
 264         The group ID which may use the reserved blocks.
 265
 266   resuid=n
 267         The user ID which may use the reserved blocks.
 268
 269   sb=
 270         Use alternate superblock at this location.
 271
 272   quota, noquota, grpquota, usrquota
 273         These options are ignored by the filesystem. They are used only by
 274         quota tools to recognize volumes where quota should be turned on. See
 275         documentation in the quota-tools package for more details
 276         (http://sourceforge.net/projects/linuxquota).
 277
 278   jqfmt=<quota type>, usrjquota=<file>, grpjquota=<file>
 279         These options tell filesystem details about quota so that quota
 280         information can be properly updated during journal replay. They replace
 281         the above quota options. See documentation in the quota-tools package
 282         for more details (http://sourceforge.net/projects/linuxquota).
 283
 284   stripe=n
 285         Number of filesystem blocks that mballoc will try to use for allocation
 286         size and alignment. For RAID5/6 systems this should be the number of
 287         data disks *  RAID chunk size in file system blocks.
 288
 289   delalloc      (*)
 290         Defer block allocation until just before ext4 writes out the block(s)
 291         in question.  This allows ext4 to better allocation decisions more
 292         efficiently.
 293
 294   nodelalloc
 295         Disable delayed allocation.  Blocks are allocated when the data is
 296         copied from userspace to the page cache, either via the write(2) system
 297         call or when an mmap'ed page which was previously unallocated is
 298         written for the first time.
 299
 300   max_batch_time=usec
 301         Maximum amount of time ext4 should wait for additional filesystem
 302         operations to be batch together with a synchronous write operation.
 303         Since a synchronous write operation is going to force a commit and then
 304         a wait for the I/O complete, it doesn't cost much, and can be a huge
 305         throughput win, we wait for a small amount of time to see if any other
 306         transactions can piggyback on the synchronous write.   The algorithm
 307         used is designed to automatically tune for the speed of the disk, by
 308         measuring the amount of time (on average) that it takes to finish
 309         committing a transaction.  Call this time the "commit time".  If the
 310         time that the transaction has been running is less than the commit
 311         time, ext4 will try sleeping for the commit time to see if other
 312         operations will join the transaction.   The commit time is capped by
 313         the max_batch_time, which defaults to 15000us (15ms).   This
 314         optimization can be turned off entirely by setting max_batch_time to 0.
 315
 316   min_batch_time=usec
 317         This parameter sets the commit time (as described above) to be at least
 318         min_batch_time.  It defaults to zero microseconds.  Increasing this
 319         parameter may improve the throughput of multi-threaded, synchronous
 320         workloads on very fast disks, at the cost of increasing latency.
 321
 322   journal_ioprio=prio
 323         The I/O priority (from 0 to 7, where 0 is the highest priority) which
 324         should be used for I/O operations submitted by kjournald2 during a
 325         commit operation.  This defaults to 3, which is a slightly higher
 326         priority than the default I/O priority.
 327
 328   auto_da_alloc(*), noauto_da_alloc
 329         Many broken applications don't use fsync() when replacing existing
 330         files via patterns such as fd = open("foo.new")/write(fd,..)/close(fd)/
 331         rename("foo.new", "foo"), or worse yet, fd = open("foo",
 332         O_TRUNC)/write(fd,..)/close(fd).  If auto_da_alloc is enabled, ext4
 333         will detect the replace-via-rename and replace-via-truncate patterns
 334         and force that any delayed allocation blocks are allocated such that at
 335         the next journal commit, in the default data=ordered mode, the data
 336         blocks of the new file are forced to disk before the rename() operation
 337         is committed.  This provides roughly the same level of guarantees as
 338         ext3, and avoids the "zero-length" problem that can happen when a
 339         system crashes before the delayed allocation blocks are forced to disk.
 340
 341   noinit_itable
 342         Do not initialize any uninitialized inode table blocks in the
 343         background.  This feature may be used by installation CD's so that the
 344         install process can complete as quickly as possible; the inode table
 345         initialization process would then be deferred until the next time the
 346         file system is unmounted.
 347
 348   init_itable=n
 349         The lazy itable init code will wait n times the number of milliseconds
 350         it took to zero out the previous block group's inode table.  This
 351         minimizes the impact on the system performance while file system's
 352         inode table is being initialized.
 353
 354   discard, nodiscard(*)
 355         Controls whether ext4 should issue discard/TRIM commands to the
 356         underlying block device when blocks are freed.  This is useful for SSD
 357         devices and sparse/thinly-provisioned LUNs, but it is off by default
 358         until sufficient testing has been done.
 359
 360   nouid32
 361         Disables 32-bit UIDs and GIDs.  This is for interoperability  with
 362         older kernels which only store and expect 16-bit values.
 363
 364   block_validity(*), noblock_validity
 365         These options enable or disable the in-kernel facility for tracking
 366         filesystem metadata blocks within internal data structures.  This
 367         allows multi- block allocator and other routines to notice bugs or
 368         corrupted allocation bitmaps which cause blocks to be allocated which
 369         overlap with filesystem metadata blocks.
 370
 371   dioread_lock, dioread_nolock
 372         Controls whether or not ext4 should use the DIO read locking. If the
 373         dioread_nolock option is specified ext4 will allocate uninitialized
 374         extent before buffer write and convert the extent to initialized after
 375         IO completes. This approach allows ext4 code to avoid using inode
 376         mutex, which improves scalability on high speed storages. However this
 377         does not work with data journaling and dioread_nolock option will be
 378         ignored with kernel warning. Note that dioread_nolock code path is only
 379         used for extent-based files.  Because of the restrictions this options
 380         comprises it is off by default (e.g. dioread_lock).
 381
 382   max_dir_size_kb=n
 383         This limits the size of directories so that any attempt to expand them
 384         beyond the specified limit in kilobytes will cause an ENOSPC error.
 385         This is useful in memory constrained environments, where a very large
 386         directory can cause severe performance problems or even provoke the Out
 387         Of Memory killer.  (For example, if there is only 512mb memory
 388         available, a 176mb directory may seriously cramp the system's style.)
 389
 390   i_version
 391         Enable 64-bit inode version support. This option is off by default.
 392
 393   dax
 394         Use direct access (no page cache).  See
 395         Documentation/filesystems/dax.txt.  Note that this option is
 396         incompatible with data=journal.
 397
 398 Data Mode
 399 =========
 400 There are 3 different data modes:
 401
 402 * writeback mode
 403
 404   In data=writeback mode, ext4 does not journal data at all.  This mode provides
 405   a similar level of journaling as that of XFS, JFS, and ReiserFS in its default
 406   mode - metadata journaling.  A crash+recovery can cause incorrect data to
 407   appear in files which were written shortly before the crash.  This mode will
 408   typically provide the best ext4 performance.
 409
 410 * ordered mode
 411
 412   In data=ordered mode, ext4 only officially journals metadata, but it logically
 413   groups metadata information related to data changes with the data blocks into
 414   a single unit called a transaction.  When it's time to write the new metadata
 415   out to disk, the associated data blocks are written first.  In general, this
 416   mode performs slightly slower than writeback but significantly faster than
 417   journal mode.
 418
 419 * journal mode
 420
 421   data=journal mode provides full data and metadata journaling.  All new data is
 422   written to the journal first, and then to its final location.  In the event of
 423   a crash, the journal can be replayed, bringing both data and metadata into a
 424   consistent state.  This mode is the slowest except when data needs to be read
 425   from and written to disk at the same time where it outperforms all others
 426   modes.  Enabling this mode will disable delayed allocation and O_DIRECT
 427   support.
 428
 429 /proc entries
 430 =============
 431
 432 Information about mounted ext4 file systems can be found in
 433 /proc/fs/ext4.  Each mounted filesystem will have a directory in
 434 /proc/fs/ext4 based on its device name (i.e., /proc/fs/ext4/hdc or
 435 /proc/fs/ext4/dm-0).   The files in each per-device directory are shown
 436 in table below.
 437
 438 Files in /proc/fs/ext4/<devname>
 439
 440   mb_groups
 441         details of multiblock allocator buddy cache of free blocks
 442
 443 /sys entries
 444 ============
 445
 446 Information about mounted ext4 file systems can be found in
 447 /sys/fs/ext4.  Each mounted filesystem will have a directory in
 448 /sys/fs/ext4 based on its device name (i.e., /sys/fs/ext4/hdc or
 449 /sys/fs/ext4/dm-0).   The files in each per-device directory are shown
 450 in table below.
 451
 452 Files in /sys/fs/ext4/<devname>:
 453
 454 (see also Documentation/ABI/testing/sysfs-fs-ext4)
 455
 456   delayed_allocation_blocks
 457         This file is read-only and shows the number of blocks that are dirty in
 458         the page cache, but which do not have their location in the filesystem
 459         allocated yet.
 460
 461   inode_goal
 462         Tuning parameter which (if non-zero) controls the goal inode used by
 463         the inode allocator in preference to all other allocation heuristics.
 464         This is intended for debugging use only, and should be 0 on production
 465         systems.
 466
 467   inode_readahead_blks
 468         Tuning parameter which controls the maximum number of inode table
 469         blocks that ext4's inode table readahead algorithm will pre-read into
 470         the buffer cache.
 471
 472   lifetime_write_kbytes
 473         This file is read-only and shows the number of kilobytes of data that
 474         have been written to this filesystem since it was created.
 475
 476   max_writeback_mb_bump
 477         The maximum number of megabytes the writeback code will try to write
 478         out before move on to another inode.
 479
 480   mb_group_prealloc
 481         The multiblock allocator will round up allocation requests to a
 482         multiple of this tuning parameter if the stripe size is not set in the
 483         ext4 superblock
 484
 485   mb_max_to_scan
 486         The maximum number of extents the multiblock allocator will search to
 487         find the best extent.
 488
 489   mb_min_to_scan
 490         The minimum number of extents the multiblock allocator will search to
 491         find the best extent.
 492
 493   mb_order2_req
 494         Tuning parameter which controls the minimum size for requests (as a
 495         power of 2) where the buddy cache is used.
 496
 497   mb_stats
 498         Controls whether the multiblock allocator should collect statistics,
 499         which are shown during the unmount. 1 means to collect statistics, 0
 500         means not to collect statistics.
 501
 502   mb_stream_req
 503         Files which have fewer blocks than this tunable parameter will have
 504         their blocks allocated out of a block group specific preallocation
 505         pool, so that small files are packed closely together.  Each large file
 506         will have its blocks allocated out of its own unique preallocation
 507         pool.
 508
 509   session_write_kbytes
 510         This file is read-only and shows the number of kilobytes of data that
 511         have been written to this filesystem since it was mounted.
 512
 513   reserved_clusters
 514         This is RW file and contains number of reserved clusters in the file
 515         system which will be used in the specific situations to avoid costly
 516         zeroout, unexpected ENOSPC, or possible data loss. The default is 2% or
 517         4096 clusters, whichever is smaller and this can be changed however it
 518         can never exceed number of clusters in the file system. If there is not
 519         enough space for the reserved space when mounting the file mount will
 520         _not_ fail.
 521
 522 Ioctls
 523 ======
 524
 525 There is some Ext4 specific functionality which can be accessed by applications
 526 through the system call interfaces. The list of all Ext4 specific ioctls are
 527 shown in the table below.
 528
 529 Table of Ext4 specific ioctls
 530
 531   EXT4_IOC_GETFLAGS
 532         Get additional attributes associated with inode.  The ioctl argument is
 533         an integer bitfield, with bit values described in ext4.h. This ioctl is
 534         an alias for FS_IOC_GETFLAGS.
 535
 536   EXT4_IOC_SETFLAGS
 537         Set additional attributes associated with inode.  The ioctl argument is
 538         an integer bitfield, with bit values described in ext4.h. This ioctl is
 539         an alias for FS_IOC_SETFLAGS.
 540
 541   EXT4_IOC_GETVERSION, EXT4_IOC_GETVERSION_OLD
 542         Get the inode i_generation number stored for each inode. The
 543         i_generation number is normally changed only when new inode is created
 544         and it is particularly useful for network filesystems. The '_OLD'
 545         version of this ioctl is an alias for FS_IOC_GETVERSION.
 546
 547   EXT4_IOC_SETVERSION, EXT4_IOC_SETVERSION_OLD
 548         Set the inode i_generation number stored for each inode. The '_OLD'
 549         version of this ioctl is an alias for FS_IOC_SETVERSION.
 550
 551   EXT4_IOC_GROUP_EXTEND
 552         This ioctl has the same purpose as the resize mount option. It allows
 553         to resize filesystem to the end of the last existing block group,
 554         further resize has to be done with resize2fs, either online, or
 555         offline. The argument points to the unsigned logn number representing
 556         the filesystem new block count.
 557
 558   EXT4_IOC_MOVE_EXT
 559         Move the block extents from orig_fd (the one this ioctl is pointing to)
 560         to the donor_fd (the one specified in move_extent structure passed as
 561         an argument to this ioctl). Then, exchange inode metadata between
 562         orig_fd and donor_fd.  This is especially useful for online
 563         defragmentation, because the allocator has the opportunity to allocate
 564         moved blocks better, ideally into one contiguous extent.
 565
 566   EXT4_IOC_GROUP_ADD
 567         Add a new group descriptor to an existing or new group descriptor
 568         block. The new group descriptor is described by ext4_new_group_input
 569         structure, which is passed as an argument to this ioctl. This is
 570         especially useful in conjunction with EXT4_IOC_GROUP_EXTEND, which
 571         allows online resize of the filesystem to the end of the last existing
 572         block group.  Those two ioctls combined is used in userspace online
 573         resize tool (e.g. resize2fs).
 574
 575   EXT4_IOC_MIGRATE
 576         This ioctl operates on the filesystem itself.  It converts (migrates)
 577         ext3 indirect block mapped inode to ext4 extent mapped inode by walking
 578         through indirect block mapping of the original inode and converting
 579         contiguous block ranges into ext4 extents of the temporary inode. Then,
 580         inodes are swapped. This ioctl might help, when migrating from ext3 to
 581         ext4 filesystem, however suggestion is to create fresh ext4 filesystem
 582         and copy data from the backup. Note, that filesystem has to support
 583         extents for this ioctl to work.
 584
 585   EXT4_IOC_ALLOC_DA_BLKS
 586         Force all of the delay allocated blocks to be allocated to preserve
 587         application-expected ext3 behaviour. Note that this will also start
 588         triggering a write of the data blocks, but this behaviour may change in
 589         the future as it is not necessary and has been done this way only for
 590         sake of simplicity.
 591
 592   EXT4_IOC_RESIZE_FS
 593         Resize the filesystem to a new size.  The number of blocks of resized
 594         filesystem is passed in via 64 bit integer argument.  The kernel
 595         allocates bitmaps and inode table, the userspace tool thus just passes
 596         the new number of blocks.
 597
 598   EXT4_IOC_SWAP_BOOT
 599         Swap i_blocks and associated attributes (like i_blocks, i_size,
 600         i_flags, ...) from the specified inode with inode EXT4_BOOT_LOADER_INO
 601         (#5). This is typically used to store a boot loader in a secure part of
 602         the filesystem, where it can't be changed by a normal user by accident.
 603         The data blocks of the previous boot loader will be associated with the
 604         given inode.
 605
 606 References
 607 ==========
 608
 609 kernel source:  <file:fs/ext4/>
 610                 <file:fs/jbd2/>
 611
 612 programs:       http://e2fsprogs.sourceforge.net/
 613
 614 useful links:   http://fedoraproject.org/wiki/ext3-devel
 615                 http://www.bullopensource.org/ext4/
 616                 http://ext4.wiki.kernel.org/index.php/Main_Page
 617                 http://fedoraproject.org/wiki/Features/Ext4