2Ext4 Filesystem
   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.
  10Mailing list:
  11Web site:
  141. Quick usage instructions:
  17Note: More extensive information for getting started with ext4 can be
  18      found at the ext4 wiki site at the URL:
  21  - Compile and install the latest version of e2fsprogs (as of this
  22    writing version 1.41.3) from:
  26        or
  30        or grab the latest git repository from:
  32    git://
  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.
  40  - Create a new filesystem using the ext4 filesystem type:
  42        # mke2fs -t ext4 /dev/hda1
  44    Or to configure an existing ext3 filesystem to support extents: 
  46        # tune2fs -O extents /dev/hda1
  48    If the filesystem was created with 128 byte inodes, it can be
  49    converted to use 256 byte for greater efficiency via:
  51        # tune2fs -I 256 /dev/hda1
  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.)
  57  - Mounting:
  59        # mount -t ext4 /dev/hda1 /wherever
  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.
  782. Features
  812.1 Currently available
  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)
 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.
 1062.2 Candidate features for future inclusion
 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)
 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.
 119The big performance win will come with mballoc, delalloc and flex_bg
 120grouping of bitmaps and inode tables.  Some test results available here:
 122 -
 123 -
 1253. Options
 128When mounting an ext4 filesystem, the following option are accepted:
 129(*) == default
 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.
 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.
 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.
 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.
 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.
 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.
 164data=ordered    (*)     All data are forced directly out to the main file
 165                        system prior to its metadata being committed to the
 166                        journal.
 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.
 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.
 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.
 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.
 203nouser_xattr            Disables Extended User Attributes.  See the
 204                        attr(5) manual page and
 205                        for more information about extended attributes.
 207noacl                   This option disables POSIX Access Control List
 208                        support. If ACL support is enabled in the kernel
 209                        configuration (CONFIG_EXT4_FS_POSIX_ACL), ACL is
 210                        enabled by default on mount. See the acl(5) manual
 211                        page and for more information
 212                        about acl.
 214bsddf           (*)     Make 'df' act like BSD.
 215minixdf                 Make 'df' act like Minix.
 217debug                   Extra debugging information is sent to syslog.
 219abort                   Simulate the effects of calling ext4_abort() for
 220                        debugging purposes.  This is normally used while
 221                        remounting a filesystem which is already mounted.
 223errors=remount-ro       Remount the filesystem read-only on an error.
 224errors=continue         Keep going on a filesystem error.
 225errors=panic            Panic and halt the machine if an error occurs.
 226                        (These mount options override the errors behavior
 227                        specified in the superblock, which can be configured
 228                        using tune2fs)
 230data_err=ignore(*)      Just print an error message if an error occurs
 231                        in a file data buffer in ordered mode.
 232data_err=abort          Abort the journal if an error occurs in a file
 233                        data buffer in ordered mode.
 235grpid                   Give objects the same group ID as their creator.
 238nogrpid         (*)     New objects have the group ID of their creator.
 241resgid=n                The group ID which may use the reserved blocks.
 243resuid=n                The user ID which may use the reserved blocks.
 245sb=n                    Use alternate superblock at this location.
 247quota                   These options are ignored by the filesystem. They
 248noquota                 are used only by quota tools to recognize volumes
 249grpquota                where quota should be turned on. See documentation
 250usrquota                in the quota-tools package for more details
 251                        (
 253jqfmt=<quota type>      These options tell filesystem details about quota
 254usrjquota=<file>        so that quota information can be properly updated
 255grpjquota=<file>        during journal replay. They replace the above
 256                        quota options. See documentation in the quota-tools
 257                        package for more details
 258                        (
 260stripe=n                Number of filesystem blocks that mballoc will try
 261                        to use for allocation size and alignment. For RAID5/6
 262                        systems this should be the number of data
 263                        disks *  RAID chunk size in file system blocks.
 265delalloc        (*)     Defer block allocation until just before ext4
 266                        writes out the block(s) in question.  This
 267                        allows ext4 to better allocation decisions
 268                        more efficiently.
 269nodelalloc              Disable delayed allocation.  Blocks are allocated
 270                        when the data is copied from userspace to the
 271                        page cache, either via the write(2) system call
 272                        or when an mmap'ed page which was previously
 273                        unallocated is written for the first time.
 275max_batch_time=usec     Maximum amount of time ext4 should wait for
 276                        additional filesystem operations to be batch
 277                        together with a synchronous write operation.
 278                        Since a synchronous write operation is going to
 279                        force a commit and then a wait for the I/O
 280                        complete, it doesn't cost much, and can be a
 281                        huge throughput win, we wait for a small amount
 282                        of time to see if any other transactions can
 283                        piggyback on the synchronous write.   The
 284                        algorithm used is designed to automatically tune
 285                        for the speed of the disk, by measuring the
 286                        amount of time (on average) that it takes to
 287                        finish committing a transaction.  Call this time
 288                        the "commit time".  If the time that the
 289                        transaction has been running is less than the
 290                        commit time, ext4 will try sleeping for the
 291                        commit time to see if other operations will join
 292                        the transaction.   The commit time is capped by
 293                        the max_batch_time, which defaults to 15000us
 294                        (15ms).   This optimization can be turned off
 295                        entirely by setting max_batch_time to 0.
 297min_batch_time=usec     This parameter sets the commit time (as
 298                        described above) to be at least min_batch_time.
 299                        It defaults to zero microseconds.  Increasing
 300                        this parameter may improve the throughput of
 301                        multi-threaded, synchronous workloads on very
 302                        fast disks, at the cost of increasing latency.
 304journal_ioprio=prio     The I/O priority (from 0 to 7, where 0 is the
 305                        highest priority) which should be used for I/O
 306                        operations submitted by kjournald2 during a
 307                        commit operation.  This defaults to 3, which is
 308                        a slightly higher priority than the default I/O
 309                        priority.
 311auto_da_alloc(*)        Many broken applications don't use fsync() when 
 312noauto_da_alloc         replacing existing files via patterns such as
 313                        fd = open("")/write(fd,..)/close(fd)/
 314                        rename("", "foo"), or worse yet,
 315                        fd = open("foo", O_TRUNC)/write(fd,..)/close(fd).
 316                        If auto_da_alloc is enabled, ext4 will detect
 317                        the replace-via-rename and replace-via-truncate
 318                        patterns and force that any delayed allocation
 319                        blocks are allocated such that at the next
 320                        journal commit, in the default data=ordered
 321                        mode, the data blocks of the new file are forced
 322                        to disk before the rename() operation is
 323                        committed.  This provides roughly the same level
 324                        of guarantees as ext3, and avoids the
 325                        "zero-length" problem that can happen when a
 326                        system crashes before the delayed allocation
 327                        blocks are forced to disk.
 329noinit_itable           Do not initialize any uninitialized inode table
 330                        blocks in the background.  This feature may be
 331                        used by installation CD's so that the install
 332                        process can complete as quickly as possible; the
 333                        inode table initialization process would then be
 334                        deferred until the next time the  file system
 335                        is unmounted.
 337init_itable=n           The lazy itable init code will wait n times the
 338                        number of milliseconds it took to zero out the
 339                        previous block group's inode table.  This
 340                        minimizes the impact on the system performance
 341                        while file system's inode table is being initialized.
 343discard                 Controls whether ext4 should issue discard/TRIM
 344nodiscard(*)            commands to the underlying block device when
 345                        blocks are freed.  This is useful for SSD devices
 346                        and sparse/thinly-provisioned LUNs, but it is off
 347                        by default until sufficient testing has been done.
 349nouid32                 Disables 32-bit UIDs and GIDs.  This is for
 350                        interoperability  with  older kernels which only
 351                        store and expect 16-bit values.
 353block_validity          This options allows to enables/disables the in-kernel
 354noblock_validity        facility for tracking filesystem metadata blocks
 355                        within internal data structures. This allows multi-
 356                        block allocator and other routines to quickly locate
 357                        extents which might overlap with filesystem metadata
 358                        blocks. This option is intended for debugging
 359                        purposes and since it negatively affects the
 360                        performance, it is off by default.
 362dioread_lock            Controls whether or not ext4 should use the DIO read
 363dioread_nolock          locking. If the dioread_nolock option is specified
 364                        ext4 will allocate uninitialized extent before buffer
 365                        write and convert the extent to initialized after IO
 366                        completes. This approach allows ext4 code to avoid
 367                        using inode mutex, which improves scalability on high
 368                        speed storages. However this does not work with
 369                        data journaling and dioread_nolock option will be
 370                        ignored with kernel warning. Note that dioread_nolock
 371                        code path is only used for extent-based files.
 372                        Because of the restrictions this options comprises
 373                        it is off by default (e.g. dioread_lock).
 375max_dir_size_kb=n       This limits the size of directories so that any
 376                        attempt to expand them beyond the specified
 377                        limit in kilobytes will cause an ENOSPC error.
 378                        This is useful in memory constrained
 379                        environments, where a very large directory can
 380                        cause severe performance problems or even
 381                        provoke the Out Of Memory killer.  (For example,
 382                        if there is only 512mb memory available, a 176mb
 383                        directory may seriously cramp the system's style.)
 385i_version               Enable 64-bit inode version support. This option is
 386                        off by default.
 388Data Mode
 390There are 3 different data modes:
 392* writeback mode
 393In data=writeback mode, ext4 does not journal data at all.  This mode provides
 394a similar level of journaling as that of XFS, JFS, and ReiserFS in its default
 395mode - metadata journaling.  A crash+recovery can cause incorrect data to
 396appear in files which were written shortly before the crash.  This mode will
 397typically provide the best ext4 performance.
 399* ordered mode
 400In data=ordered mode, ext4 only officially journals metadata, but it logically
 401groups metadata information related to data changes with the data blocks into a
 402single unit called a transaction.  When it's time to write the new metadata
 403out to disk, the associated data blocks are written first.  In general,
 404this mode performs slightly slower than writeback but significantly faster than journal mode.
 406* journal mode
 407data=journal mode provides full data and metadata journaling.  All new data is
 408written to the journal first, and then to its final location.
 409In the event of a crash, the journal can be replayed, bringing both data and
 410metadata into a consistent state.  This mode is the slowest except when data
 411needs to be read from and written to disk at the same time where it
 412outperforms all others modes.  Enabling this mode will disable delayed
 413allocation and O_DIRECT support.
 415/proc entries
 418Information about mounted ext4 file systems can be found in
 419/proc/fs/ext4.  Each mounted filesystem will have a directory in
 420/proc/fs/ext4 based on its device name (i.e., /proc/fs/ext4/hdc or
 421/proc/fs/ext4/dm-0).   The files in each per-device directory are shown
 422in table below.
 424Files in /proc/fs/ext4/<devname>
 426 File            Content
 427 mb_groups       details of multiblock allocator buddy cache of free blocks
 430/sys entries
 433Information about mounted ext4 file systems can be found in
 434/sys/fs/ext4.  Each mounted filesystem will have a directory in
 435/sys/fs/ext4 based on its device name (i.e., /sys/fs/ext4/hdc or
 436/sys/fs/ext4/dm-0).   The files in each per-device directory are shown
 437in table below.
 439Files in /sys/fs/ext4/<devname>
 440(see also Documentation/ABI/testing/sysfs-fs-ext4)
 442 File                         Content
 444 delayed_allocation_blocks    This file is read-only and shows the number of
 445                              blocks that are dirty in the page cache, but
 446                              which do not have their location in the
 447                              filesystem allocated yet.
 449 inode_goal                   Tuning parameter which (if non-zero) controls
 450                              the goal inode used by the inode allocator in
 451                              preference to all other allocation heuristics.
 452                              This is intended for debugging use only, and
 453                              should be 0 on production systems.
 455 inode_readahead_blks         Tuning parameter which controls the maximum
 456                              number of inode table blocks that ext4's inode
 457                              table readahead algorithm will pre-read into
 458                              the buffer cache
 460 lifetime_write_kbytes        This file is read-only and shows the number of
 461                              kilobytes of data that have been written to this
 462                              filesystem since it was created.
 464 max_writeback_mb_bump        The maximum number of megabytes the writeback
 465                              code will try to write out before move on to
 466                              another inode.
 468 mb_group_prealloc            The multiblock allocator will round up allocation
 469                              requests to a multiple of this tuning parameter if
 470                              the stripe size is not set in the ext4 superblock
 472 mb_max_to_scan               The maximum number of extents the multiblock
 473                              allocator will search to find the best extent
 475 mb_min_to_scan               The minimum number of extents the multiblock
 476                              allocator will search to find the best extent
 478 mb_order2_req                Tuning parameter which controls the minimum size
 479                              for requests (as a power of 2) where the buddy
 480                              cache is used
 482 mb_stats                     Controls whether the multiblock allocator should
 483                              collect statistics, which are shown during the
 484                              unmount. 1 means to collect statistics, 0 means
 485                              not to collect statistics
 487 mb_stream_req                Files which have fewer blocks than this tunable
 488                              parameter will have their blocks allocated out
 489                              of a block group specific preallocation pool, so
 490                              that small files are packed closely together.
 491                              Each large file will have its blocks allocated
 492                              out of its own unique preallocation pool.
 494 session_write_kbytes         This file is read-only and shows the number of
 495                              kilobytes of data that have been written to this
 496                              filesystem since it was mounted.
 498 reserved_clusters            This is RW file and contains number of reserved
 499                              clusters in the file system which will be used
 500                              in the specific situations to avoid costly
 501                              zeroout, unexpected ENOSPC, or possible data
 502                              loss. The default is 2% or 4096 clusters,
 503                              whichever is smaller and this can be changed
 504                              however it can never exceed number of clusters
 505                              in the file system. If there is not enough space
 506                              for the reserved space when mounting the file
 507                              mount will _not_ fail.
 513There is some Ext4 specific functionality which can be accessed by applications
 514through the system call interfaces. The list of all Ext4 specific ioctls are
 515shown in the table below.
 517Table of Ext4 specific ioctls
 519 Ioctl                        Description
 520 EXT4_IOC_GETFLAGS            Get additional attributes associated with inode.
 521                              The ioctl argument is an integer bitfield, with
 522                              bit values described in ext4.h. This ioctl is an
 523                              alias for FS_IOC_GETFLAGS.
 525 EXT4_IOC_SETFLAGS            Set 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_SETFLAGS.
 532                              Get the inode i_generation number stored for
 533                              each inode. The i_generation number is normally
 534                              changed only when new inode is created and it is
 535                              particularly useful for network filesystems. The
 536                              '_OLD' version of this ioctl is an alias for
 537                              FS_IOC_GETVERSION.
 541                              Set the inode i_generation number stored for
 542                              each inode. The '_OLD' version of this ioctl
 543                              is an alias for FS_IOC_SETVERSION.
 545 EXT4_IOC_GROUP_EXTEND        This ioctl has the same purpose as the resize
 546                              mount option. It allows to resize filesystem
 547                              to the end of the last existing block group,
 548                              further resize has to be done with resize2fs,
 549                              either online, or offline. The argument points
 550                              to the unsigned logn number representing the
 551                              filesystem new block count.
 553 EXT4_IOC_MOVE_EXT            Move the block extents from orig_fd (the one
 554                              this ioctl is pointing to) to the donor_fd (the
 555                              one specified in move_extent structure passed
 556                              as an argument to this ioctl). Then, exchange
 557                              inode metadata between orig_fd and donor_fd.
 558                              This is especially useful for online
 559                              defragmentation, because the allocator has the
 560                              opportunity to allocate moved blocks better,
 561                              ideally into one contiguous extent.
 563 EXT4_IOC_GROUP_ADD           Add a new group descriptor to an existing or
 564                              new group descriptor block. The new group
 565                              descriptor is described by ext4_new_group_input
 566                              structure, which is passed as an argument to
 567                              this ioctl. This is especially useful in
 568                              conjunction with EXT4_IOC_GROUP_EXTEND,
 569                              which allows online resize of the filesystem
 570                              to the end of the last existing block group.
 571                              Those two ioctls combined is used in userspace
 572                              online resize tool (e.g. resize2fs).
 574 EXT4_IOC_MIGRATE             This ioctl operates on the filesystem itself.
 575                              It converts (migrates) ext3 indirect block mapped
 576                              inode to ext4 extent mapped inode by walking
 577                              through indirect block mapping of the original
 578                              inode and converting contiguous block ranges
 579                              into ext4 extents of the temporary inode. Then,
 580                              inodes are swapped. This ioctl might help, when
 581                              migrating from ext3 to ext4 filesystem, however
 582                              suggestion is to create fresh ext4 filesystem
 583                              and copy data from the backup. Note, that
 584                              filesystem has to support extents for this ioctl
 585                              to work.
 587 EXT4_IOC_ALLOC_DA_BLKS       Force all of the delay allocated blocks to be
 588                              allocated to preserve application-expected ext3
 589                              behaviour. Note that this will also start
 590                              triggering a write of the data blocks, but this
 591                              behaviour may change in the future as it is
 592                              not necessary and has been done this way only
 593                              for sake of simplicity.
 595 EXT4_IOC_RESIZE_FS           Resize the filesystem to a new size.  The number
 596                              of blocks of resized filesystem is passed in via
 597                              64 bit integer argument.  The kernel allocates
 598                              bitmaps and inode table, the userspace tool thus
 599                              just passes the new number of blocks.
 601EXT4_IOC_SWAP_BOOT            Swap i_blocks and associated attributes
 602                              (like i_blocks, i_size, i_flags, ...) from
 603                              the specified inode with inode
 604                              EXT4_BOOT_LOADER_INO (#5). This is typically
 605                              used to store a boot loader in a secure part of
 606                              the filesystem, where it can't be changed by a
 607                              normal user by accident.
 608                              The data blocks of the previous boot loader
 609                              will be associated with the given inode.
 616kernel source:  <file:fs/ext4/>
 617                <file:fs/jbd2/>
 621useful links: