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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/ |