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. 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 for more information
 208                        about extended attributes.
 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 for more information
 215                        about acl.
 217bsddf           (*)     Make 'df' act like BSD.
 218minixdf                 Make 'df' act like Minix.
 220debug                   Extra debugging information is sent to syslog.
 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.
 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)
 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.
 238grpid                   Give objects the same group ID as their creator.
 241nogrpid         (*)     New objects have the group ID of their creator.
 244resgid=n                The group ID which may use the reserved blocks.
 246resuid=n                The user ID which may use the reserved blocks.
 248sb=n                    Use alternate superblock at this location.
 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                        (
 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                        (
 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.
 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.
 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.
 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.
 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.
 314auto_da_alloc(*)        Many broken applications don't use fsync() when 
 315noauto_da_alloc         replacing existing files via patterns such as
 316                        fd = open("")/write(fd,..)/close(fd)/
 317                        rename("", "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.
 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.
 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.
 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.
 352nouid32                 Disables 32-bit UIDs and GIDs.  This is for
 353                        interoperability  with  older kernels which only
 354                        store and expect 16-bit values.
 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.
 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).
 378i_version               Enable 64-bit inode version support. This option is
 379                        off by default.
 381Data Mode
 383There are 3 different data modes:
 385* writeback mode
 386In data=writeback mode, ext4 does not journal data at all.  This mode provides
 387a similar level of journaling as that of XFS, JFS, and ReiserFS in its default
 388mode - metadata journaling.  A crash+recovery can cause incorrect data to
 389appear in files which were written shortly before the crash.  This mode will
 390typically provide the best ext4 performance.
 392* ordered mode
 393In data=ordered mode, ext4 only officially journals metadata, but it logically
 394groups metadata information related to data changes with the data blocks into a
 395single unit called a transaction.  When it's time to write the new metadata
 396out to disk, the associated data blocks are written first.  In general,
 397this mode performs slightly slower than writeback but significantly faster than journal mode.
 399* journal mode
 400data=journal mode provides full data and metadata journaling.  All new data is
 401written to the journal first, and then to its final location.
 402In the event of a crash, the journal can be replayed, bringing both data and
 403metadata into a consistent state.  This mode is the slowest except when data
 404needs to be read from and written to disk at the same time where it
 405outperforms all others modes.  Enabling this mode will disable delayed
 406allocation and O_DIRECT support.
 408/proc entries
 411Information about mounted ext4 file systems can be found in
 412/proc/fs/ext4.  Each mounted filesystem will have a directory in
 413/proc/fs/ext4 based on its device name (i.e., /proc/fs/ext4/hdc or
 414/proc/fs/ext4/dm-0).   The files in each per-device directory are shown
 415in table below.
 417Files in /proc/fs/ext4/<devname>
 419 File            Content
 420 mb_groups       details of multiblock allocator buddy cache of free blocks
 423/sys entries
 426Information about mounted ext4 file systems can be found in
 427/sys/fs/ext4.  Each mounted filesystem will have a directory in
 428/sys/fs/ext4 based on its device name (i.e., /sys/fs/ext4/hdc or
 429/sys/fs/ext4/dm-0).   The files in each per-device directory are shown
 430in table below.
 432Files in /sys/fs/ext4/<devname>
 433(see also Documentation/ABI/testing/sysfs-fs-ext4)
 435 File                         Content
 437 delayed_allocation_blocks    This file is read-only and shows the number of
 438                              blocks that are dirty in the page cache, but
 439                              which do not have their location in the
 440                              filesystem allocated yet.
 442 inode_goal                   Tuning parameter which (if non-zero) controls
 443                              the goal inode used by the inode allocator in
 444                              preference to all other allocation heuristics.
 445                              This is intended for debugging use only, and
 446                              should be 0 on production systems.
 448 inode_readahead_blks         Tuning parameter which controls the maximum
 449                              number of inode table blocks that ext4's inode
 450                              table readahead algorithm will pre-read into
 451                              the buffer cache
 453 lifetime_write_kbytes        This file is read-only and shows the number of
 454                              kilobytes of data that have been written to this
 455                              filesystem since it was created.
 457 max_writeback_mb_bump        The maximum number of megabytes the writeback
 458                              code will try to write out before move on to
 459                              another inode.
 461 mb_group_prealloc            The multiblock allocator will round up allocation
 462                              requests to a multiple of this tuning parameter if
 463                              the stripe size is not set in the ext4 superblock
 465 mb_max_to_scan               The maximum number of extents the multiblock
 466                              allocator will search to find the best extent
 468 mb_min_to_scan               The minimum number of extents the multiblock
 469                              allocator will search to find the best extent
 471 mb_order2_req                Tuning parameter which controls the minimum size
 472                              for requests (as a power of 2) where the buddy
 473                              cache is used
 475 mb_stats                     Controls whether the multiblock allocator should
 476                              collect statistics, which are shown during the
 477                              unmount. 1 means to collect statistics, 0 means
 478                              not to collect statistics
 480 mb_stream_req                Files which have fewer blocks than this tunable
 481                              parameter will have their blocks allocated out
 482                              of a block group specific preallocation pool, so
 483                              that small files are packed closely together.
 484                              Each large file will have its blocks allocated
 485                              out of its own unique preallocation pool.
 487 session_write_kbytes         This file is read-only and shows the number of
 488                              kilobytes of data that have been written to this
 489                              filesystem since it was mounted.
 495There is some Ext4 specific functionality which can be accessed by applications
 496through the system call interfaces. The list of all Ext4 specific ioctls are
 497shown in the table below.
 499Table of Ext4 specific ioctls
 501 Ioctl                        Description
 502 EXT4_IOC_GETFLAGS            Get additional attributes associated with inode.
 503                              The ioctl argument is an integer bitfield, with
 504                              bit values described in ext4.h. This ioctl is an
 505                              alias for FS_IOC_GETFLAGS.
 507 EXT4_IOC_SETFLAGS            Set additional attributes associated with inode.
 508                              The ioctl argument is an integer bitfield, with
 509                              bit values described in ext4.h. This ioctl is an
 510                              alias for FS_IOC_SETFLAGS.
 514                              Get the inode i_generation number stored for
 515                              each inode. The i_generation number is normally
 516                              changed only when new inode is created and it is
 517                              particularly useful for network filesystems. The
 518                              '_OLD' version of this ioctl is an alias for
 519                              FS_IOC_GETVERSION.
 523                              Set the inode i_generation number stored for
 524                              each inode. The '_OLD' version of this ioctl
 525                              is an alias for FS_IOC_SETVERSION.
 527 EXT4_IOC_GROUP_EXTEND        This ioctl has the same purpose as the resize
 528                              mount option. It allows to resize filesystem
 529                              to the end of the last existing block group,
 530                              further resize has to be done with resize2fs,
 531                              either online, or offline. The argument points
 532                              to the unsigned logn number representing the
 533                              filesystem new block count.
 535 EXT4_IOC_MOVE_EXT            Move the block extents from orig_fd (the one
 536                              this ioctl is pointing to) to the donor_fd (the
 537                              one specified in move_extent structure passed
 538                              as an argument to this ioctl). Then, exchange
 539                              inode metadata between orig_fd and donor_fd.
 540                              This is especially useful for online
 541                              defragmentation, because the allocator has the
 542                              opportunity to allocate moved blocks better,
 543                              ideally into one contiguous extent.
 545 EXT4_IOC_GROUP_ADD           Add a new group descriptor to an existing or
 546                              new group descriptor block. The new group
 547                              descriptor is described by ext4_new_group_input
 548                              structure, which is passed as an argument to
 549                              this ioctl. This is especially useful in
 550                              conjunction with EXT4_IOC_GROUP_EXTEND,
 551                              which allows online resize of the filesystem
 552                              to the end of the last existing block group.
 553                              Those two ioctls combined is used in userspace
 554                              online resize tool (e.g. resize2fs).
 556 EXT4_IOC_MIGRATE             This ioctl operates on the filesystem itself.
 557                              It converts (migrates) ext3 indirect block mapped
 558                              inode to ext4 extent mapped inode by walking
 559                              through indirect block mapping of the original
 560                              inode and converting contiguous block ranges
 561                              into ext4 extents of the temporary inode. Then,
 562                              inodes are swapped. This ioctl might help, when
 563                              migrating from ext3 to ext4 filesystem, however
 564                              suggestion is to create fresh ext4 filesystem
 565                              and copy data from the backup. Note, that
 566                              filesystem has to support extents for this ioctl
 567                              to work.
 569 EXT4_IOC_ALLOC_DA_BLKS       Force all of the delay allocated blocks to be
 570                              allocated to preserve application-expected ext3
 571                              behaviour. Note that this will also start
 572                              triggering a write of the data blocks, but this
 573                              behaviour may change in the future as it is
 574                              not necessary and has been done this way only
 575                              for sake of simplicity.
 577 EXT4_IOC_RESIZE_FS           Resize the filesystem to a new size.  The number
 578                              of blocks of resized filesystem is passed in via
 579                              64 bit integer argument.  The kernel allocates
 580                              bitmaps and inode table, the userspace tool thus
 581                              just passes the new number of blocks.
 588kernel source:  <file:fs/ext4/>
 589                <file:fs/jbd2/>
 593useful links:
 597 kindly hosted by Redpill Linpro AS, provider of Linux consulting and operations services since 1995.