[linux-2.6-block.git] / Documentation / filesystems / ext3.txt


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/
Commit	Line	Data
1da177e4 LT	1
	2	Ext3 Filesystem
	3	===============
	4
	5	ext3 was originally released in September 1999. Written by Stephen Tweedie
	6	for 2.2 branch, and ported to 2.4 kernels by Peter Braam, Andreas Dilger,
	7	Andrew Morton, Alexander Viro, Ted Ts'o and Stephen Tweedie.
	8
	9	ext3 is ext2 filesystem enhanced with journalling capabilities.
	10
	11	Options
	12	=======
	13
	14	When mounting an ext3 filesystem, the following option are accepted:
	15	(*) == default
	16
	17	jounal=update Update the ext3 file system's journal to the
	18	current format.
	19
	20	journal=inum When a journal already exists, this option is
	21	ignored. Otherwise, it specifies the number of
	22	the inode which will represent the ext3 file
	23	system's journal file.
	24
	25	noload Don't load the journal on mounting.
	26
	27	data=journal All data are committed into the journal prior
	28	to being written into the main file system.
	29
	30	data=ordered (*) All data are forced directly out to the main file
	31	system prior to its metadata being committed to
	32	the journal.
	33
	34	data=writeback Data ordering is not preserved, data may be
	35	written into the main file system after its
	36	metadata has been committed to the journal.
	37
	38	commit=nrsec (*) Ext3 can be told to sync all its data and metadata
	39	every 'nrsec' seconds. The default value is 5 seconds.
	40	This means that if you lose your power, you will lose,
	41	as much, the latest 5 seconds of work (your filesystem
	42	will not be damaged though, thanks to journaling). This
	43	default value (or any low value) will hurt performance,
	44	but it's good for data-safety. Setting it to 0 will
	45	have the same effect than leaving the default 5 sec.
	46	Setting it to very large values will improve
	47	performance.
	48
	49	barrier=1 This enables/disables barriers. barrier=0 disables it,
	50	barrier=1 enables it.
	51
	52	orlov (*) This enables the new Orlov block allocator. It's enabled
	53	by default.
	54
	55	oldalloc This disables the Orlov block allocator and enables the
	56	old block allocator. Orlov should have better performance,
	57	we'd like to get some feedback if it's the contrary for
	58	you.
	59
	60	user_xattr (*) Enables POSIX Extended Attributes. It's enabled by
	61	default, however you need to confifure its support
	62	(CONFIG_EXT3_FS_XATTR). This is neccesary if you want
	63	to use POSIX Acces Control Lists support. You can visit
	64	http://acl.bestbits.at to know more about POSIX Extended
65	attributes.
66
67	nouser_xattr Disables POSIX Extended Attributes.
68
69	acl (*) Enables POSIX Access Control Lists support. This is
70	enabled by default, however you need to configure
71	its support (CONFIG_EXT3_FS_POSIX_ACL). If you want
72	to know more about ACLs visit http://acl.bestbits.at
73
74	noacl This option disables POSIX Access Control List support.
75
76	reservation
77
78	noreservation
79
80	resize=
81
82	bsddf (*) Make 'df' act like BSD.
83	minixdf Make 'df' act like Minix.
84
85	check=none Don't do extra checking of bitmaps on mount.
86	nocheck
87
88	debug Extra debugging information is sent to syslog.
89
90	errors=remount-ro(*) Remount the filesystem read-only on an error.
91	errors=continue Keep going on a filesystem error.
92	errors=panic Panic and halt the machine if an error occurs.
93
94	grpid Give objects the same group ID as their creator.
95	bsdgroups
96
97	nogrpid (*) New objects have the group ID of their creator.
98	sysvgroups
99
100	resgid=n The group ID which may use the reserved blocks.
101
102	resuid=n The user ID which may use the reserved blocks.
103
104	sb=n Use alternate superblock at this location.
105
106	quota Quota options are currently silently ignored.
107	noquota (see fs/ext3/super.c, line 594)
108	grpquota
109	usrquota
110
111
112	Specification
113	=============
114	ext3 shares all disk implementation with ext2 filesystem, and add
115	transactions capabilities to ext2. Journaling is done by the
116	Journaling block device layer.
117
118	Journaling Block Device layer
119	-----------------------------
120	The Journaling Block Device layer (JBD) isn't ext3 specific. It was
121	design to add journaling capabilities on a block device. The ext3
122	filesystem code will inform the JBD of modifications it is performing
123	(Call a transaction). the journal support the transactions start and
124	stop, and in case of crash, the journal can replayed the transactions
125	to put the partition on a consistent state fastly.
126
127	handles represent a single atomic update to a filesystem. JBD can
128	handle external journal on a block device.
129
130	Data Mode
131	---------
132	There's 3 different data modes:
133
134	* writeback mode
135	In data=writeback mode, ext3 does not journal data at all. This mode
136	provides a similar level of journaling as XFS, JFS, and ReiserFS in its
137	default mode - metadata journaling. A crash+recovery can cause
138	incorrect data to appear in files which were written shortly before the
139	crash. This mode will typically provide the best ext3 performance.
140
141	* ordered mode
142	In data=ordered mode, ext3 only officially journals metadata, but it
143	logically groups metadata and data blocks into a single unit called a
144	transaction. When it's time to write the new metadata out to disk, the
145	associated data blocks are written first. In general, this mode
146	perform slightly slower than writeback but significantly faster than
147	journal mode.
148
149	* journal mode
150	data=journal mode provides full data and metadata journaling. All new
151	data is written to the journal first, and then to its final location.
152	In the event of a crash, the journal can be replayed, bringing both
153	data and metadata into a consistent state. This mode is the slowest
154	except when data needs to be read from and written to disk at the same
155	time where it outperform all others mode.
156
157	Compatibility
158	-------------
159
160	Ext2 partitions can be easily convert to ext3, with `tune2fs -j <dev>`.
161	Ext3 is fully compatible with Ext2. Ext3 partitions can easily be
162	mounted as Ext2.
163
164	External Tools
165	==============
166	see manual pages to know more.
167
168	tune2fs: create a ext3 journal on a ext2 partition with the -j flags
169	mke2fs: create a ext3 partition with the -j flags
170	debugfs: ext2 and ext3 file system debugger
171
172	References
173	==========
174
175	kernel source: file:/usr/src/linux/fs/ext3
176	file:/usr/src/linux/fs/jbd
177
178	programs: http://e2fsprogs.sourceforge.net
179
180	useful link:
181	http://www.zip.com.au/~akpm/linux/ext3/ext3-usage.html
182	http://www-106.ibm.com/developerworks/linux/library/l-fs7/
183	http://www-106.ibm.com/developerworks/linux/library/l-fs8/