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Linus Torvalds1da177e2005-04-16 15:20:36 -07001
2Ext3 Filesystem
3===============
4
5ext3 was originally released in September 1999. Written by Stephen Tweedie
6for 2.2 branch, and ported to 2.4 kernels by Peter Braam, Andreas Dilger,
7Andrew Morton, Alexander Viro, Ted Ts'o and Stephen Tweedie.
8
9ext3 is ext2 filesystem enhanced with journalling capabilities.
10
11Options
12=======
13
14When mounting an ext3 filesystem, the following option are accepted:
15(*) == default
16
17jounal=update Update the ext3 file system's journal to the
18 current format.
19
20journal=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
25noload Don't load the journal on mounting.
26
27data=journal All data are committed into the journal prior
28 to being written into the main file system.
29
30data=ordered (*) All data are forced directly out to the main file
31 system prior to its metadata being committed to
32 the journal.
33
34data=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
38commit=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
49barrier=1 This enables/disables barriers. barrier=0 disables it,
50 barrier=1 enables it.
51
52orlov (*) This enables the new Orlov block allocator. It's enabled
53 by default.
54
55oldalloc 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
60user_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
67nouser_xattr Disables POSIX Extended Attributes.
68
69acl (*) 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
74noacl This option disables POSIX Access Control List support.
75
76reservation
77
78noreservation
79
80resize=
81
82bsddf (*) Make 'df' act like BSD.
83minixdf Make 'df' act like Minix.
84
85check=none Don't do extra checking of bitmaps on mount.
86nocheck
87
88debug Extra debugging information is sent to syslog.
89
90errors=remount-ro(*) Remount the filesystem read-only on an error.
91errors=continue Keep going on a filesystem error.
92errors=panic Panic and halt the machine if an error occurs.
93
94grpid Give objects the same group ID as their creator.
95bsdgroups
96
97nogrpid (*) New objects have the group ID of their creator.
98sysvgroups
99
100resgid=n The group ID which may use the reserved blocks.
101
102resuid=n The user ID which may use the reserved blocks.
103
104sb=n Use alternate superblock at this location.
105
106quota Quota options are currently silently ignored.
107noquota (see fs/ext3/super.c, line 594)
108grpquota
109usrquota
110
111
112Specification
113=============
114ext3 shares all disk implementation with ext2 filesystem, and add
115transactions capabilities to ext2. Journaling is done by the
116Journaling block device layer.
117
118Journaling Block Device layer
119-----------------------------
120The Journaling Block Device layer (JBD) isn't ext3 specific. It was
121design to add journaling capabilities on a block device. The ext3
122filesystem code will inform the JBD of modifications it is performing
123(Call a transaction). the journal support the transactions start and
124stop, and in case of crash, the journal can replayed the transactions
125to put the partition on a consistent state fastly.
126
127handles represent a single atomic update to a filesystem. JBD can
128handle external journal on a block device.
129
130Data Mode
131---------
132There's 3 different data modes:
133
134* writeback mode
135In data=writeback mode, ext3 does not journal data at all. This mode
136provides a similar level of journaling as XFS, JFS, and ReiserFS in its
137default mode - metadata journaling. A crash+recovery can cause
138incorrect data to appear in files which were written shortly before the
139crash. This mode will typically provide the best ext3 performance.
140
141* ordered mode
142In data=ordered mode, ext3 only officially journals metadata, but it
143logically groups metadata and data blocks into a single unit called a
144transaction. When it's time to write the new metadata out to disk, the
145associated data blocks are written first. In general, this mode
146perform slightly slower than writeback but significantly faster than
147journal mode.
148
149* journal mode
150data=journal mode provides full data and metadata journaling. All new
151data is written to the journal first, and then to its final location.
152In the event of a crash, the journal can be replayed, bringing both
153data and metadata into a consistent state. This mode is the slowest
154except when data needs to be read from and written to disk at the same
155time where it outperform all others mode.
156
157Compatibility
158-------------
159
160Ext2 partitions can be easily convert to ext3, with `tune2fs -j <dev>`.
161Ext3 is fully compatible with Ext2. Ext3 partitions can easily be
162mounted as Ext2.
163
164External Tools
165==============
166see manual pages to know more.
167
168tune2fs: create a ext3 journal on a ext2 partition with the -j flags
169mke2fs: create a ext3 partition with the -j flags
170debugfs: ext2 and ext3 file system debugger
171
172References
173==========
174
175kernel source: file:/usr/src/linux/fs/ext3
176 file:/usr/src/linux/fs/jbd
177
178programs: http://e2fsprogs.sourceforge.net
179
180useful 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/