ovl: create ovl_need_index() helper

[linux/fpc-iii.git] / Documentation / filesystems / ext4.txt

Ext4 Filesystem
===============

Ext4 is an advanced level of the ext3 filesystem which incorporates
scalability and reliability enhancements for supporting large filesystems
(64 bit) in keeping with increasing disk capacities and state-of-the-art
feature requirements.

Mailing list:   linux-ext4@vger.kernel.org
Web site:       http://ext4.wiki.kernel.org


1. Quick usage instructions:
===========================

Note: More extensive information for getting started with ext4 can be
      found at the ext4 wiki site at the URL:
      http://ext4.wiki.kernel.org/index.php/Ext4_Howto

  - Compile and install the latest version of e2fsprogs (as of this
    writing version 1.41.3) from:

    http://sourceforge.net/project/showfiles.php?group_id=2406
        
        or

    https://www.kernel.org/pub/linux/kernel/people/tytso/e2fsprogs/

        or grab the latest git repository from:

    git://git.kernel.org/pub/scm/fs/ext2/e2fsprogs.git

  - Note that it is highly important to install the mke2fs.conf file
    that comes with the e2fsprogs 1.41.x sources in /etc/mke2fs.conf. If
    you have edited the /etc/mke2fs.conf file installed on your system,
    you will need to merge your changes with the version from e2fsprogs
    1.41.x.

  - Create a new filesystem using the ext4 filesystem type:

        # mke2fs -t ext4 /dev/hda1

    Or to configure an existing ext3 filesystem to support extents: 

        # tune2fs -O extents /dev/hda1

    If the filesystem was created with 128 byte inodes, it can be
    converted to use 256 byte for greater efficiency via:

        # tune2fs -I 256 /dev/hda1

    (Note: we currently do not have tools to convert an ext4
    filesystem back to ext3; so please do not do try this on production
    filesystems.)

  - Mounting:

        # mount -t ext4 /dev/hda1 /wherever

  - When comparing performance with other filesystems, it's always
    important to try multiple workloads; very often a subtle change in a
    workload parameter can completely change the ranking of which
    filesystems do well compared to others.  When comparing versus ext3,
    note that ext4 enables write barriers by default, while ext3 does
    not enable write barriers by default.  So it is useful to use
    explicitly specify whether barriers are enabled or not when via the
    '-o barriers=[0|1]' mount option for both ext3 and ext4 filesystems
    for a fair comparison.  When tuning ext3 for best benchmark numbers,
    it is often worthwhile to try changing the data journaling mode; '-o
    data=writeback' can be faster for some workloads.  (Note however that
    running mounted with data=writeback can potentially leave stale data
    exposed in recently written files in case of an unclean shutdown,
    which could be a security exposure in some situations.)  Configuring
    the filesystem with a large journal can also be helpful for
    metadata-intensive workloads.

2. Features
===========

2.1 Currently available

* ability to use filesystems > 16TB (e2fsprogs support not available yet)
* extent format reduces metadata overhead (RAM, IO for access, transactions)
* extent format more robust in face of on-disk corruption due to magics,
* internal redundancy in tree
* improved file allocation (multi-block alloc)
* lift 32000 subdirectory limit imposed by i_links_count[1]
* nsec timestamps for mtime, atime, ctime, create time
* inode version field on disk (NFSv4, Lustre)
* reduced e2fsck time via uninit_bg feature
* journal checksumming for robustness, performance
* persistent file preallocation (e.g for streaming media, databases)
* ability to pack bitmaps and inode tables into larger virtual groups via the
  flex_bg feature
* large file support
* inode allocation using large virtual block groups via flex_bg
* delayed allocation
* large block (up to pagesize) support
* efficient new ordered mode in JBD2 and ext4 (avoid using buffer head to force
  the ordering)

[1] Filesystems with a block size of 1k may see a limit imposed by the
directory hash tree having a maximum depth of two.

2.2 Candidate features for future inclusion

* online defrag (patches available but not well tested)
* reduced mke2fs time via lazy itable initialization in conjunction with
  the uninit_bg feature (capability to do this is available in e2fsprogs
  but a kernel thread to do lazy zeroing of unused inode table blocks
  after filesystem is first mounted is required for safety)

There are several others under discussion, whether they all make it in is
partly a function of how much time everyone has to work on them. Features like
metadata checksumming have been discussed and planned for a bit but no patches
exist yet so I'm not sure they're in the near-term roadmap.

The big performance win will come with mballoc, delalloc and flex_bg
grouping of bitmaps and inode tables.  Some test results available here:

 - http://www.bullopensource.org/ext4/20080818-ffsb/ffsb-write-2.6.27-rc1.html
 - http://www.bullopensource.org/ext4/20080818-ffsb/ffsb-readwrite-2.6.27-rc1.html

3. Options
==========

When mounting an ext4 filesystem, the following option are accepted:
(*) == default

ro                      Mount filesystem read only. Note that ext4 will
                        replay the journal (and thus write to the
                        partition) even when mounted "read only". The
                        mount options "ro,noload" can be used to prevent
                        writes to the filesystem.

journal_checksum        Enable checksumming of the journal transactions.
                        This will allow the recovery code in e2fsck and the
                        kernel to detect corruption in the kernel.  It is a
                        compatible change and will be ignored by older kernels.

journal_async_commit    Commit block can be written to disk without waiting
                        for descriptor blocks. If enabled older kernels cannot
                        mount the device. This will enable 'journal_checksum'
                        internally.

journal_path=path
journal_dev=devnum      When the external journal device's major/minor numbers
                        have changed, these options allow the user to specify
                        the new journal location.  The journal device is
                        identified through either its new major/minor numbers
                        encoded in devnum, or via a path to the device.

norecovery              Don't load the journal on mounting.  Note that
noload                  if the filesystem was not unmounted cleanly,
                        skipping the journal replay will lead to the
                        filesystem containing inconsistencies that can
                        lead to any number of problems.

data=journal            All data are committed into the journal prior to being
                        written into the main file system.  Enabling
                        this mode will disable delayed allocation and
                        O_DIRECT support.

data=ordered    (*)     All data are forced directly out to the main file
                        system prior to its metadata being committed to the
                        journal.

data=writeback          Data ordering is not preserved, data may be written
                        into the main file system after its metadata has been
                        committed to the journal.

commit=nrsec    (*)     Ext4 can be told to sync all its data and metadata
                        every 'nrsec' seconds. The default value is 5 seconds.
                        This means that if you lose your power, you will lose
                        as much as the latest 5 seconds of work (your
                        filesystem will not be damaged though, thanks to the
                        journaling).  This default value (or any low value)
                        will hurt performance, but it's good for data-safety.
                        Setting it to 0 will have the same effect as leaving
                        it at the default (5 seconds).
                        Setting it to very large values will improve
                        performance.

barrier=<0|1(*)>        This enables/disables the use of write barriers in
barrier(*)              the jbd code.  barrier=0 disables, barrier=1 enables.
nobarrier               This also requires an IO stack which can support
                        barriers, and if jbd gets an error on a barrier
                        write, it will disable again with a warning.
                        Write barriers enforce proper on-disk ordering
                        of journal commits, making volatile disk write caches
                        safe to use, at some performance penalty.  If
                        your disks are battery-backed in one way or another,
                        disabling barriers may safely improve performance.
                        The mount options "barrier" and "nobarrier" can
                        also be used to enable or disable barriers, for
                        consistency with other ext4 mount options.

inode_readahead_blks=n  This tuning parameter controls the maximum
                        number of inode table blocks that ext4's inode
                        table readahead algorithm will pre-read into
                        the buffer cache.  The default value is 32 blocks.

nouser_xattr            Disables Extended User Attributes.  See the
                        attr(5) manual page and http://acl.bestbits.at/
                        for more information about extended attributes.

noacl                   This option disables POSIX Access Control List
                        support. If ACL support is enabled in the kernel
                        configuration (CONFIG_EXT4_FS_POSIX_ACL), ACL is
                        enabled by default on mount. See the acl(5) manual
                        page and http://acl.bestbits.at/ for more information
                        about acl.

bsddf           (*)     Make 'df' act like BSD.
minixdf                 Make 'df' act like Minix.

debug                   Extra debugging information is sent to syslog.

abort                   Simulate the effects of calling ext4_abort() for
                        debugging purposes.  This is normally used while
                        remounting a filesystem which is already mounted.

errors=remount-ro       Remount the filesystem read-only on an error.
errors=continue         Keep going on a filesystem error.
errors=panic            Panic and halt the machine if an error occurs.
                        (These mount options override the errors behavior
                        specified in the superblock, which can be configured
                        using tune2fs)

data_err=ignore(*)      Just print an error message if an error occurs
                        in a file data buffer in ordered mode.
data_err=abort          Abort the journal if an error occurs in a file
                        data buffer in ordered mode.

grpid                   Give objects the same group ID as their creator.
bsdgroups

nogrpid         (*)     New objects have the group ID of their creator.
sysvgroups

resgid=n                The group ID which may use the reserved blocks.

resuid=n                The user ID which may use the reserved blocks.

sb=n                    Use alternate superblock at this location.

quota                   These options are ignored by the filesystem. They
noquota                 are used only by quota tools to recognize volumes
grpquota                where quota should be turned on. See documentation
usrquota                in the quota-tools package for more details
                        (http://sourceforge.net/projects/linuxquota).

jqfmt=<quota type>      These options tell filesystem details about quota
usrjquota=<file>        so that quota information can be properly updated
grpjquota=<file>        during journal replay. They replace the above
                        quota options. See documentation in the quota-tools
                        package for more details
                        (http://sourceforge.net/projects/linuxquota).

stripe=n                Number of filesystem blocks that mballoc will try
                        to use for allocation size and alignment. For RAID5/6
                        systems this should be the number of data
                        disks *  RAID chunk size in file system blocks.

delalloc        (*)     Defer block allocation until just before ext4
                        writes out the block(s) in question.  This
                        allows ext4 to better allocation decisions
                        more efficiently.
nodelalloc              Disable delayed allocation.  Blocks are allocated
                        when the data is copied from userspace to the
                        page cache, either via the write(2) system call
                        or when an mmap'ed page which was previously
                        unallocated is written for the first time.

max_batch_time=usec     Maximum amount of time ext4 should wait for
                        additional filesystem operations to be batch
                        together with a synchronous write operation.
                        Since a synchronous write operation is going to
                        force a commit and then a wait for the I/O
                        complete, it doesn't cost much, and can be a
                        huge throughput win, we wait for a small amount
                        of time to see if any other transactions can
                        piggyback on the synchronous write.   The
                        algorithm used is designed to automatically tune
                        for the speed of the disk, by measuring the
                        amount of time (on average) that it takes to
                        finish committing a transaction.  Call this time
                        the "commit time".  If the time that the
                        transaction has been running is less than the
                        commit time, ext4 will try sleeping for the
                        commit time to see if other operations will join
                        the transaction.   The commit time is capped by
                        the max_batch_time, which defaults to 15000us
                        (15ms).   This optimization can be turned off
                        entirely by setting max_batch_time to 0.

min_batch_time=usec     This parameter sets the commit time (as
                        described above) to be at least min_batch_time.
                        It defaults to zero microseconds.  Increasing
                        this parameter may improve the throughput of
                        multi-threaded, synchronous workloads on very
                        fast disks, at the cost of increasing latency.

journal_ioprio=prio     The I/O priority (from 0 to 7, where 0 is the
                        highest priority) which should be used for I/O
                        operations submitted by kjournald2 during a
                        commit operation.  This defaults to 3, which is
                        a slightly higher priority than the default I/O
                        priority.

auto_da_alloc(*)        Many broken applications don't use fsync() when 
noauto_da_alloc         replacing existing files via patterns such as
                        fd = open("foo.new")/write(fd,..)/close(fd)/
                        rename("foo.new", "foo"), or worse yet,
                        fd = open("foo", O_TRUNC)/write(fd,..)/close(fd).
                        If auto_da_alloc is enabled, ext4 will detect
                        the replace-via-rename and replace-via-truncate
                        patterns and force that any delayed allocation
                        blocks are allocated such that at the next
                        journal commit, in the default data=ordered
                        mode, the data blocks of the new file are forced
                        to disk before the rename() operation is
                        committed.  This provides roughly the same level
                        of guarantees as ext3, and avoids the
                        "zero-length" problem that can happen when a
                        system crashes before the delayed allocation
                        blocks are forced to disk.

noinit_itable           Do not initialize any uninitialized inode table
                        blocks in the background.  This feature may be
                        used by installation CD's so that the install
                        process can complete as quickly as possible; the
                        inode table initialization process would then be
                        deferred until the next time the  file system
                        is unmounted.

init_itable=n           The lazy itable init code will wait n times the
                        number of milliseconds it took to zero out the
                        previous block group's inode table.  This
                        minimizes the impact on the system performance
                        while file system's inode table is being initialized.

discard                 Controls whether ext4 should issue discard/TRIM
nodiscard(*)            commands to the underlying block device when
                        blocks are freed.  This is useful for SSD devices
                        and sparse/thinly-provisioned LUNs, but it is off
                        by default until sufficient testing has been done.

nouid32                 Disables 32-bit UIDs and GIDs.  This is for
                        interoperability  with  older kernels which only
                        store and expect 16-bit values.

block_validity(*)       These options enable or disable the in-kernel
noblock_validity        facility for tracking filesystem metadata blocks
                        within internal data structures.  This allows multi-
                        block allocator and other routines to notice
                        bugs or corrupted allocation bitmaps which cause
                        blocks to be allocated which overlap with
                        filesystem metadata blocks.

dioread_lock            Controls whether or not ext4 should use the DIO read
dioread_nolock          locking. If the dioread_nolock option is specified
                        ext4 will allocate uninitialized extent before buffer
                        write and convert the extent to initialized after IO
                        completes. This approach allows ext4 code to avoid
                        using inode mutex, which improves scalability on high
                        speed storages. However this does not work with
                        data journaling and dioread_nolock option will be
                        ignored with kernel warning. Note that dioread_nolock
                        code path is only used for extent-based files.
                        Because of the restrictions this options comprises
                        it is off by default (e.g. dioread_lock).

max_dir_size_kb=n       This limits the size of directories so that any
                        attempt to expand them beyond the specified
                        limit in kilobytes will cause an ENOSPC error.
                        This is useful in memory constrained
                        environments, where a very large directory can
                        cause severe performance problems or even
                        provoke the Out Of Memory killer.  (For example,
                        if there is only 512mb memory available, a 176mb
                        directory may seriously cramp the system's style.)

i_version               Enable 64-bit inode version support. This option is
                        off by default.

dax                     Use direct access (no page cache).  See
                        Documentation/filesystems/dax.txt.  Note that
                        this option is incompatible with data=journal.

Data Mode
=========
There are 3 different data modes:

* writeback mode
In data=writeback mode, ext4 does not journal data at all.  This mode provides
a similar level of journaling as that of XFS, JFS, and ReiserFS in its default
mode - metadata journaling.  A crash+recovery can cause incorrect data to
appear in files which were written shortly before the crash.  This mode will
typically provide the best ext4 performance.

* ordered mode
In data=ordered mode, ext4 only officially journals metadata, but it logically
groups metadata information related to data changes with the data blocks into a
single unit called a transaction.  When it's time to write the new metadata
out to disk, the associated data blocks are written first.  In general,
this mode performs slightly slower than writeback but significantly faster than journal mode.

* journal mode
data=journal mode provides full data and metadata journaling.  All new data is
written to the journal first, and then to its final location.
In the event of a crash, the journal can be replayed, bringing both data and
metadata into a consistent state.  This mode is the slowest except when data
needs to be read from and written to disk at the same time where it
outperforms all others modes.  Enabling this mode will disable delayed
allocation and O_DIRECT support.

/proc entries
=============

Information about mounted ext4 file systems can be found in
/proc/fs/ext4.  Each mounted filesystem will have a directory in
/proc/fs/ext4 based on its device name (i.e., /proc/fs/ext4/hdc or
/proc/fs/ext4/dm-0).   The files in each per-device directory are shown
in table below.

Files in /proc/fs/ext4/<devname>
..............................................................................
 File            Content
 mb_groups       details of multiblock allocator buddy cache of free blocks
..............................................................................

/sys entries
============

Information about mounted ext4 file systems can be found in
/sys/fs/ext4.  Each mounted filesystem will have a directory in
/sys/fs/ext4 based on its device name (i.e., /sys/fs/ext4/hdc or
/sys/fs/ext4/dm-0).   The files in each per-device directory are shown
in table below.

Files in /sys/fs/ext4/<devname>
(see also Documentation/ABI/testing/sysfs-fs-ext4)
..............................................................................
 File                         Content

 delayed_allocation_blocks    This file is read-only and shows the number of
                              blocks that are dirty in the page cache, but
                              which do not have their location in the
                              filesystem allocated yet.

 inode_goal                   Tuning parameter which (if non-zero) controls
                              the goal inode used by the inode allocator in
                              preference to all other allocation heuristics.
                              This is intended for debugging use only, and
                              should be 0 on production systems.

 inode_readahead_blks         Tuning parameter which controls the maximum
                              number of inode table blocks that ext4's inode
                              table readahead algorithm will pre-read into
                              the buffer cache

 lifetime_write_kbytes        This file is read-only and shows the number of
                              kilobytes of data that have been written to this
                              filesystem since it was created.

 max_writeback_mb_bump        The maximum number of megabytes the writeback
                              code will try to write out before move on to
                              another inode.

 mb_group_prealloc            The multiblock allocator will round up allocation
                              requests to a multiple of this tuning parameter if
                              the stripe size is not set in the ext4 superblock

 mb_max_to_scan               The maximum number of extents the multiblock
                              allocator will search to find the best extent

 mb_min_to_scan               The minimum number of extents the multiblock
                              allocator will search to find the best extent

 mb_order2_req                Tuning parameter which controls the minimum size
                              for requests (as a power of 2) where the buddy
                              cache is used

 mb_stats                     Controls whether the multiblock allocator should
                              collect statistics, which are shown during the
                              unmount. 1 means to collect statistics, 0 means
                              not to collect statistics

 mb_stream_req                Files which have fewer blocks than this tunable
                              parameter will have their blocks allocated out
                              of a block group specific preallocation pool, so
                              that small files are packed closely together.
                              Each large file will have its blocks allocated
                              out of its own unique preallocation pool.

 session_write_kbytes         This file is read-only and shows the number of
                              kilobytes of data that have been written to this
                              filesystem since it was mounted.

 reserved_clusters            This is RW file and contains number of reserved
                              clusters in the file system which will be used
                              in the specific situations to avoid costly
                              zeroout, unexpected ENOSPC, or possible data
                              loss. The default is 2% or 4096 clusters,
                              whichever is smaller and this can be changed
                              however it can never exceed number of clusters
                              in the file system. If there is not enough space
                              for the reserved space when mounting the file
                              mount will _not_ fail.
..............................................................................

Ioctls
======

There is some Ext4 specific functionality which can be accessed by applications
through the system call interfaces. The list of all Ext4 specific ioctls are
shown in the table below.

Table of Ext4 specific ioctls
..............................................................................
 Ioctl                        Description
 EXT4_IOC_GETFLAGS            Get additional attributes associated with inode.
                              The ioctl argument is an integer bitfield, with
                              bit values described in ext4.h. This ioctl is an
                              alias for FS_IOC_GETFLAGS.

 EXT4_IOC_SETFLAGS            Set additional attributes associated with inode.
                              The ioctl argument is an integer bitfield, with
                              bit values described in ext4.h. This ioctl is an
                              alias for FS_IOC_SETFLAGS.

 EXT4_IOC_GETVERSION
 EXT4_IOC_GETVERSION_OLD
                              Get the inode i_generation number stored for
                              each inode. The i_generation number is normally
                              changed only when new inode is created and it is
                              particularly useful for network filesystems. The
                              '_OLD' version of this ioctl is an alias for
                              FS_IOC_GETVERSION.

 EXT4_IOC_SETVERSION
 EXT4_IOC_SETVERSION_OLD
                              Set the inode i_generation number stored for
                              each inode. The '_OLD' version of this ioctl
                              is an alias for FS_IOC_SETVERSION.

 EXT4_IOC_GROUP_EXTEND        This ioctl has the same purpose as the resize
                              mount option. It allows to resize filesystem
                              to the end of the last existing block group,
                              further resize has to be done with resize2fs,
                              either online, or offline. The argument points
                              to the unsigned logn number representing the
                              filesystem new block count.

 EXT4_IOC_MOVE_EXT            Move the block extents from orig_fd (the one
                              this ioctl is pointing to) to the donor_fd (the
                              one specified in move_extent structure passed
                              as an argument to this ioctl). Then, exchange
                              inode metadata between orig_fd and donor_fd.
                              This is especially useful for online
                              defragmentation, because the allocator has the
                              opportunity to allocate moved blocks better,
                              ideally into one contiguous extent.

 EXT4_IOC_GROUP_ADD           Add a new group descriptor to an existing or
                              new group descriptor block. The new group
                              descriptor is described by ext4_new_group_input
                              structure, which is passed as an argument to
                              this ioctl. This is especially useful in
                              conjunction with EXT4_IOC_GROUP_EXTEND,
                              which allows online resize of the filesystem
                              to the end of the last existing block group.
                              Those two ioctls combined is used in userspace
                              online resize tool (e.g. resize2fs).

 EXT4_IOC_MIGRATE             This ioctl operates on the filesystem itself.
                              It converts (migrates) ext3 indirect block mapped
                              inode to ext4 extent mapped inode by walking
                              through indirect block mapping of the original
                              inode and converting contiguous block ranges
                              into ext4 extents of the temporary inode. Then,
                              inodes are swapped. This ioctl might help, when
                              migrating from ext3 to ext4 filesystem, however
                              suggestion is to create fresh ext4 filesystem
                              and copy data from the backup. Note, that
                              filesystem has to support extents for this ioctl
                              to work.

 EXT4_IOC_ALLOC_DA_BLKS       Force all of the delay allocated blocks to be
                              allocated to preserve application-expected ext3
                              behaviour. Note that this will also start
                              triggering a write of the data blocks, but this
                              behaviour may change in the future as it is
                              not necessary and has been done this way only
                              for sake of simplicity.

 EXT4_IOC_RESIZE_FS           Resize the filesystem to a new size.  The number
                              of blocks of resized filesystem is passed in via
                              64 bit integer argument.  The kernel allocates
                              bitmaps and inode table, the userspace tool thus
                              just passes the new number of blocks.

 EXT4_IOC_SWAP_BOOT           Swap i_blocks and associated attributes
                              (like i_blocks, i_size, i_flags, ...) from
                              the specified inode with inode
                              EXT4_BOOT_LOADER_INO (#5). This is typically
                              used to store a boot loader in a secure part of
                              the filesystem, where it can't be changed by a
                              normal user by accident.
                              The data blocks of the previous boot loader
                              will be associated with the given inode.

..............................................................................

References
==========

kernel source:  <file:fs/ext4/>
                <file:fs/jbd2/>

programs:       http://e2fsprogs.sourceforge.net/

useful links:   http://fedoraproject.org/wiki/ext3-devel
                http://www.bullopensource.org/ext4/
                http://ext4.wiki.kernel.org/index.php/Main_Page
                http://fedoraproject.org/wiki/Features/Ext4