tjh.dev
Linux kernel mirror (for testing)
git.kernel.org/pub/scm/linux/kernel/git/torvalds/linux.git
kernel
os
linux
1
2Ext3 Filesystem
3===============
4
5Ext3 was originally released in September 1999. Written by Stephen Tweedie
6for the 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 the ext2 filesystem enhanced with journalling capabilities.
10
11Options
12=======
13
14When mounting an ext3 filesystem, the following option are accepted:
15(*) == default
16
17journal=update Update the ext3 file system's journal to the current
18 format.
19
20journal=inum When a journal already exists, this option is ignored.
21 Otherwise, it specifies the number of the inode which
22 will represent the ext3 file system's journal file.
23
24journal_dev=devnum When the external journal device's major/minor numbers
25 have changed, this option allows the user to specify
26 the new journal location. The journal device is
27 identified through its new major/minor numbers encoded
28 in devnum.
29
30noload Don't load the journal on mounting.
31
32data=journal All data are committed into the journal prior to being
33 written into the main file system.
34
35data=ordered (*) All data are forced directly out to the main file
36 system prior to its metadata being committed to the
37 journal.
38
39data=writeback Data ordering is not preserved, data may be written
40 into the main file system after its metadata has been
41 committed to the journal.
42
43commit=nrsec (*) Ext3 can be told to sync all its data and metadata
44 every 'nrsec' seconds. The default value is 5 seconds.
45 This means that if you lose your power, you will lose
46 as much as the latest 5 seconds of work (your
47 filesystem will not be damaged though, thanks to the
48 journaling). This default value (or any low value)
49 will hurt performance, but it's good for data-safety.
50 Setting it to 0 will have the same effect as leaving
51 it at the default (5 seconds).
52 Setting it to very large values will improve
53 performance.
54
55barrier=1 This enables/disables barriers. barrier=0 disables
56 it, barrier=1 enables it.
57
58orlov (*) This enables the new Orlov block allocator. It is
59 enabled by default.
60
61oldalloc This disables the Orlov block allocator and enables
62 the old block allocator. Orlov should have better
63 performance - we'd like to get some feedback if it's
64 the contrary for you.
65
66user_xattr Enables Extended User Attributes. Additionally, you
67 need to have extended attribute support enabled in the
68 kernel configuration (CONFIG_EXT3_FS_XATTR). See the
69 attr(5) manual page and http://acl.bestbits.at/ to
70 learn more about extended attributes.
71
72nouser_xattr Disables Extended User Attributes.
73
74acl Enables POSIX Access Control Lists support.
75 Additionally, you need to have ACL support enabled in
76 the kernel configuration (CONFIG_EXT3_FS_POSIX_ACL).
77 See the acl(5) manual page and http://acl.bestbits.at/
78 for more information.
79
80noacl This option disables POSIX Access Control List
81 support.
82
83reservation
84
85noreservation
86
87bsddf (*) Make 'df' act like BSD.
88minixdf Make 'df' act like Minix.
89
90check=none Don't do extra checking of bitmaps on mount.
91nocheck
92
93debug Extra debugging information is sent to syslog.
94
95errors=remount-ro(*) Remount the filesystem read-only on an error.
96errors=continue Keep going on a filesystem error.
97errors=panic Panic and halt the machine if an error occurs.
98
99grpid Give objects the same group ID as their creator.
100bsdgroups
101
102nogrpid (*) New objects have the group ID of their creator.
103sysvgroups
104
105resgid=n The group ID which may use the reserved blocks.
106
107resuid=n The user ID which may use the reserved blocks.
108
109sb=n Use alternate superblock at this location.
110
111quota
112noquota
113grpquota
114usrquota
115
116bh (*) ext3 associates buffer heads to data pages to
117nobh (a) cache disk block mapping information
118 (b) link pages into transaction to provide
119 ordering guarantees.
120 "bh" option forces use of buffer heads.
121 "nobh" option tries to avoid associating buffer
122 heads (supported only for "writeback" mode).
123
124
125Specification
126=============
127Ext3 shares all disk implementation with the ext2 filesystem, and adds
128transactions capabilities to ext2. Journaling is done by the Journaling Block
129Device layer.
130
131Journaling Block Device layer
132-----------------------------
133The Journaling Block Device layer (JBD) isn't ext3 specific. It was designed
134to add journaling capabilities to a block device. The ext3 filesystem code
135will inform the JBD of modifications it is performing (called a transaction).
136The journal supports the transactions start and stop, and in case of a crash,
137the journal can replay the transactions to quickly put the partition back into
138a consistent state.
139
140Handles represent a single atomic update to a filesystem. JBD can handle an
141external journal on a block device.
142
143Data Mode
144---------
145There are 3 different data modes:
146
147* writeback mode
148In data=writeback mode, ext3 does not journal data at all. This mode provides
149a similar level of journaling as that of XFS, JFS, and ReiserFS in its default
150mode - metadata journaling. A crash+recovery can cause incorrect data to
151appear in files which were written shortly before the crash. This mode will
152typically provide the best ext3 performance.
153
154* ordered mode
155In data=ordered mode, ext3 only officially journals metadata, but it logically
156groups metadata and data blocks into a single unit called a transaction. When
157it's time to write the new metadata out to disk, the associated data blocks
158are written first. In general, this mode performs slightly slower than
159writeback but significantly faster than journal mode.
160
161* journal mode
162data=journal mode provides full data and metadata journaling. All new data is
163written to the journal first, and then to its final location.
164In the event of a crash, the journal can be replayed, bringing both data and
165metadata into a consistent state. This mode is the slowest except when data
166needs to be read from and written to disk at the same time where it
167outperforms all other modes.
168
169Compatibility
170-------------
171
172Ext2 partitions can be easily convert to ext3, with `tune2fs -j <dev>`.
173Ext3 is fully compatible with Ext2. Ext3 partitions can easily be mounted as
174Ext2.
175
176
177External Tools
178==============
179See manual pages to learn more.
180
181tune2fs: create a ext3 journal on a ext2 partition with the -j flag.
182mke2fs: create a ext3 partition with the -j flag.
183debugfs: ext2 and ext3 file system debugger.
184ext2online: online (mounted) ext2 and ext3 filesystem resizer
185
186
187References
188==========
189
190kernel source: <file:fs/ext3/>
191 <file:fs/jbd/>
192
193programs: http://e2fsprogs.sourceforge.net/
194 http://ext2resize.sourceforge.net
195
196useful links: http://www.zip.com.au/~akpm/linux/ext3/ext3-usage.html
197 http://www-106.ibm.com/developerworks/linux/library/l-fs7/
198 http://www-106.ibm.com/developerworks/linux/library/l-fs8/