Documentation/filesystems/ext3.txt - third_party/linux - Git at Google


 Ext3 Filesystem
 ===============

 Ext3 was originally released in September 1999. Written by Stephen Tweedie
 for the 2.2 branch, and ported to 2.4 kernels by Peter Braam, Andreas Dilger,
 Andrew Morton, Alexander Viro, Ted Ts'o and Stephen Tweedie.

 Ext3 is the ext2 filesystem enhanced with journalling capabilities.

 Options
 =======

 When mounting an ext3 filesystem, the following option are accepted:
 (*) == default

 ro			Mount filesystem read only. Note that ext3 will replay
 			the journal (and thus write to the partition) even when
 			mounted "read only". Mount options "ro,noload" can be
 			used to prevent writes to the filesystem.

 journal=update		Update the ext3 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 ext3 file system's journal file.

 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 this forces
 noload			mount of inconsistent filesystem, which can lead to
 			various 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	(*)	Ext3 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 ext3 mount options.

 user_xattr		Enables Extended User Attributes.  Additionally, you
 			need to have extended attribute support enabled in the
 			kernel configuration (CONFIG_EXT3_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_EXT3_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.
 			(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).

 Specification
 =============
 Ext3 shares all disk implementation with the ext2 filesystem, and adds
 transactions capabilities to ext2.  Journaling is done by the Journaling Block
 Device layer.

 Journaling Block Device layer
 -----------------------------
 The Journaling Block Device layer (JBD) isn't ext3 specific.  It was designed
 to add journaling capabilities to a block device.  The ext3 filesystem code
 will inform the JBD of modifications it is performing (called a transaction).
 The journal supports the transactions start and stop, and in case of a crash,
 the journal can replay the transactions to quickly put the partition back into
 a consistent state.

 Handles represent a single atomic update to a filesystem.  JBD can handle an
 external journal on a block device.

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

 * writeback mode
 In data=writeback mode, ext3 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 ext3 performance.

 * ordered mode
 In data=ordered mode, ext3 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 other modes.

 Compatibility
 -------------

 Ext2 partitions can be easily convert to ext3, with `tune2fs -j <dev>`.
 Ext3 is fully compatible with Ext2.  Ext3 partitions can easily be mounted as
 Ext2.


 External Tools
 ==============
 See manual pages to learn more.

 tune2fs: 	create a ext3 journal on a ext2 partition with the -j flag.
 mke2fs: 	create a ext3 partition with the -j flag.
 debugfs: 	ext2 and ext3 file system debugger.
 ext2online:	online (mounted) ext2 and ext3 filesystem resizer


 References
 ==========

 kernel source:	<file:fs/ext3/>
 		<file:fs/jbd/>

 programs: 	http://e2fsprogs.sourceforge.net/
 		http://ext2resize.sourceforge.net

 useful links:	http://www.ibm.com/developerworks/library/l-fs7/index.html
         http://www.ibm.com/developerworks/library/l-fs8/index.html

	Ext3 Filesystem
	===============

	Ext3 was originally released in September 1999. Written by Stephen Tweedie
	for the 2.2 branch, and ported to 2.4 kernels by Peter Braam, Andreas Dilger,
	Andrew Morton, Alexander Viro, Ted Ts'o and Stephen Tweedie.

	Ext3 is the ext2 filesystem enhanced with journalling capabilities.

	Options
	=======

	When mounting an ext3 filesystem, the following option are accepted:
	(*) == default

	ro Mount filesystem read only. Note that ext3 will replay
	the journal (and thus write to the partition) even when
	mounted "read only". Mount options "ro,noload" can be
	used to prevent writes to the filesystem.

	journal=update Update the ext3 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 ext3 file system's journal file.

	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 this forces
	noload mount of inconsistent filesystem, which can lead to
	various 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 (*) Ext3 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 ext3 mount options.

	user_xattr Enables Extended User Attributes. Additionally, you
	need to have extended attribute support enabled in the
	kernel configuration (CONFIG_EXT3_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_EXT3_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.
	(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).

	Specification
	=============
	Ext3 shares all disk implementation with the ext2 filesystem, and adds
	transactions capabilities to ext2. Journaling is done by the Journaling Block
	Device layer.

	Journaling Block Device layer
	-----------------------------
	The Journaling Block Device layer (JBD) isn't ext3 specific. It was designed
	to add journaling capabilities to a block device. The ext3 filesystem code
	will inform the JBD of modifications it is performing (called a transaction).
	The journal supports the transactions start and stop, and in case of a crash,
	the journal can replay the transactions to quickly put the partition back into
	a consistent state.

	Handles represent a single atomic update to a filesystem. JBD can handle an
	external journal on a block device.

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

	* writeback mode
	In data=writeback mode, ext3 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 ext3 performance.

	* ordered mode
	In data=ordered mode, ext3 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 other modes.

	Compatibility
	-------------

	Ext2 partitions can be easily convert to ext3, with `tune2fs -j <dev>`.
	Ext3 is fully compatible with Ext2. Ext3 partitions can easily be mounted as
	Ext2.


	External Tools
	==============
	See manual pages to learn more.

	tune2fs: create a ext3 journal on a ext2 partition with the -j flag.
	mke2fs: create a ext3 partition with the -j flag.
	debugfs: ext2 and ext3 file system debugger.
	ext2online: online (mounted) ext2 and ext3 filesystem resizer


	References
	==========

	kernel source: <file:fs/ext3/>
	<file:fs/jbd/>

	programs: http://e2fsprogs.sourceforge.net/
	http://ext2resize.sourceforge.net

	useful links: http://www.ibm.com/developerworks/library/l-fs7/index.html
	http://www.ibm.com/developerworks/library/l-fs8/index.html