ext4: Documentation updates.

[safe/jmp/linux-2.6] / Documentation / filesystems / ext4.txt

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

This is a development version of the ext4 filesystem, 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


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

  - Compile and install the latest version of e2fsprogs (as of this
    writing version 1.41) from:

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

    ftp://ftp.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

  - Create a new filesystem using the ext4dev filesystem type:

        # mke2fs -t ext4dev /dev/hda1

    Or configure an existing ext3 filesystem to support extents and set
    the test_fs flag to indicate that it's ok for an in-development
    filesystem to touch this filesystem:

        # tune2fs -O extents -E test_fs /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 ext4dev
    filesystem back to ext3; so please do not do try this on production
    filesystems.)

  - Mounting:

        # mount -t ext4dev /dev/hda1 /wherever

  - When comparing performance with other filesystems, remember that
    ext3/4 by default offers higher data integrity guarantees than most.
    So when comparing with a metadata-only journalling filesystem, such
    as ext3, use `mount -o data=writeback'.  And you might as well use
    `mount -o nobh' too along with it.  Making the journal larger than
    the mke2fs default often helps performance with 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 redunancy in tree
* improved file allocation (multi-block alloc, delayed alloc)
* fix 32000 subdirectory limit
* 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

2.2 Candidate features for future inclusion

* Online defrag (patches available but not well tested)
* reduced mke2fs time via lazy itable initialization in conjuction 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/ext4/20080530/ffsb-write-2.6.26-rc2.html
 - http://www.bullopensource.org/ext4/20080530/ffsb-readwrite-2.6.26-rc2.html

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

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

extents         (*)     ext4 will use extents to address file data.  The
                        file system will no longer be mountable by ext3.

noextents               ext4 will not use extents for newly created files

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=update          Update the ext4 file system's journal to the current
                        format.

journal=inum            When a journal already exists, this option is ignored.
                        Otherwise, it specifies the number of the inode which
                        will represent the ext4 file system's journal file.

journal_dev=devnum      When the external journal device's major/minor numbers
                        have changed, this option allows the user to specify
                        the new journal location.  The journal device is
                        identified through its new major/minor numbers encoded
                        in devnum.

noload                  Don't load the journal on mounting.

data=journal            All data are committed into the journal prior to being
                        written into the main file system.

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
                        the jbd code.  barrier=0 disables, barrier=1 enables.
                        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.

orlov           (*)     This enables the new Orlov block allocator. It is
                        enabled by default.

oldalloc                This disables the Orlov block allocator and enables
                        the old block allocator.  Orlov should have better
                        performance - we'd like to get some feedback if it's
                        the contrary for you.

user_xattr              Enables Extended User Attributes.  Additionally, you
                        need to have extended attribute support enabled in the
                        kernel configuration (CONFIG_EXT4_FS_XATTR).  See the
                        attr(5) manual page and http://acl.bestbits.at/ to
                        learn more about extended attributes.

nouser_xattr            Disables Extended User Attributes.

acl                     Enables POSIX Access Control Lists support.
                        Additionally, you need to have ACL support enabled in
                        the kernel configuration (CONFIG_EXT4_FS_POSIX_ACL).
                        See the acl(5) manual page and http://acl.bestbits.at/
                        for more information.

noacl                   This option disables POSIX Access Control List
                        support.

reservation

noreservation

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

check=none              Don't do extra checking of bitmaps on mount.
nocheck

debug                   Extra debugging information is sent to syslog.

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.

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
noquota
grpquota
usrquota

bh              (*)     ext4 associates buffer heads to data pages to
nobh                    (a) cache disk block mapping information
                        (b) link pages into transaction to provide
                            ordering guarantees.
                        "bh" option forces use of buffer heads.
                        "nobh" option tries to avoid associating buffer
                        heads (supported only for "writeback" mode).

mballoc         (*)     Use the multiple block allocator for block allocation
nomballoc               disabled multiple block allocator for block allocation.
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.

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 and 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.

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