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


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

 ext3 was originally released in September 1999. Written by Stephen Tweedie
 for 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 ext2 filesystem enhanced with journalling capabilities.

 Options
 =======

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

 jounal=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.

 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	(*)	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, the latest 5 seconds of work (your filesystem
 			will not be damaged though, thanks to 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 than leaving the default 5 sec.
 			Setting it to very large values will improve
 			performance.

 barrier=1		This enables/disables barriers. barrier=0 disables it,
 			barrier=1 enables it.

 orlov		(*)	This enables the new Orlov block allocator. It's 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 POSIX Extended Attributes. It's enabled by
 			default, however you need to confifure its support
 			(CONFIG_EXT3_FS_XATTR). This is neccesary if you want
 			to use POSIX Acces Control Lists support. You can visit
 			http://acl.bestbits.at to know more about POSIX Extended
 			attributes.

 nouser_xattr		Disables POSIX Extended Attributes.

 acl		(*)	Enables POSIX Access Control Lists support. This is
 			enabled by default, however you need to configure
 			its support (CONFIG_EXT3_FS_POSIX_ACL). If you want
 			to know more about ACLs visit http://acl.bestbits.at

 noacl			This option disables POSIX Access Control List support.

 reservation

 noreservation

 resize=

 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			Quota options are currently silently ignored.
 noquota			(see fs/ext3/super.c, line 594)
 grpquota
 usrquota


 Specification
 =============
 ext3 shares all disk implementation with ext2 filesystem, and add
 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
 design to add journaling capabilities on a block device.  The ext3
 filesystem code will inform the JBD of modifications it is performing
 (Call a transaction).  the journal support the transactions start and
 stop, and in case of crash, the journal can replayed the transactions
 to put the partition on a consistent state fastly.

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

 Data Mode
 ---------
 There's 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 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
 perform 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 outperform all others mode.

 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 know more.

 tune2fs: 	create a ext3 journal on a ext2 partition with the -j flags
 mke2fs: 	create a ext3 partition with the -j flags
 debugfs: 	ext2 and ext3 file system debugger

 References
 ==========

 kernel source:	file:/usr/src/linux/fs/ext3
 		file:/usr/src/linux/fs/jbd

 programs: 	http://e2fsprogs.sourceforge.net

 useful link:
 		http://www.zip.com.au/~akpm/linux/ext3/ext3-usage.html
 		http://www-106.ibm.com/developerworks/linux/library/l-fs7/
 		http://www-106.ibm.com/developerworks/linux/library/l-fs8/

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

	ext3 was originally released in September 1999. Written by Stephen Tweedie
	for 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 ext2 filesystem enhanced with journalling capabilities.

	Options
	=======

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

	jounal=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.

	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 (*) 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, the latest 5 seconds of work (your filesystem
	will not be damaged though, thanks to 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 than leaving the default 5 sec.
	Setting it to very large values will improve
	performance.

	barrier=1 This enables/disables barriers. barrier=0 disables it,
	barrier=1 enables it.

	orlov (*) This enables the new Orlov block allocator. It's 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 POSIX Extended Attributes. It's enabled by
	default, however you need to confifure its support
	(CONFIG_EXT3_FS_XATTR). This is neccesary if you want
	to use POSIX Acces Control Lists support. You can visit
	http://acl.bestbits.at to know more about POSIX Extended
	attributes.

	nouser_xattr Disables POSIX Extended Attributes.

	acl (*) Enables POSIX Access Control Lists support. This is
	enabled by default, however you need to configure
	its support (CONFIG_EXT3_FS_POSIX_ACL). If you want
	to know more about ACLs visit http://acl.bestbits.at

	noacl This option disables POSIX Access Control List support.

	reservation

	noreservation

	resize=

	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 Quota options are currently silently ignored.
	noquota (see fs/ext3/super.c, line 594)
	grpquota
	usrquota


	Specification
	=============
	ext3 shares all disk implementation with ext2 filesystem, and add
	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
	design to add journaling capabilities on a block device. The ext3
	filesystem code will inform the JBD of modifications it is performing
	(Call a transaction). the journal support the transactions start and
	stop, and in case of crash, the journal can replayed the transactions
	to put the partition on a consistent state fastly.

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

	Data Mode
	---------
	There's 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 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
	perform 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 outperform all others mode.

	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 know more.

	tune2fs: create a ext3 journal on a ext2 partition with the -j flags
	mke2fs: create a ext3 partition with the -j flags
	debugfs: ext2 and ext3 file system debugger

	References
	==========

	kernel source: file:/usr/src/linux/fs/ext3
	file:/usr/src/linux/fs/jbd

	programs: http://e2fsprogs.sourceforge.net

	useful link:
	http://www.zip.com.au/~akpm/linux/ext3/ext3-usage.html
	http://www-106.ibm.com/developerworks/linux/library/l-fs7/
	http://www-106.ibm.com/developerworks/linux/library/l-fs8/