ext4.txt

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

Ext4 is an 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

    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

  - 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,nobh' 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
* efficent 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 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/ext4/20080818-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=update		Update the ext4 file system's journal to the current
			format.

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.  Note that
                     	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.

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=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.

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.

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.
                        (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
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).

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	(*)	Deferring block allocation until write-out time.
nodelalloc		Disable delayed allocation. Blocks are allocation
			when data is copied from user to page cache.

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 priorty) 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.

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.  Currently ext4 does not have delayed
allocation support if this data journalling mode is selected.

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