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 17ro Mount filesystem read only. Note that ext3 will replay 18 the journal (and thus write to the partition) even when 19 mounted "read only". Mount options "ro,noload" can be 20 used to prevent writes to the filesystem. 21 22journal=update Update the ext3 file system's journal to the current 23 format. 24 25journal=inum When a journal already exists, this option is ignored. 26 Otherwise, it specifies the number of the inode which 27 will represent the ext3 file system's journal file. 28 29journal_dev=devnum When the external journal device's major/minor numbers 30 have changed, this option allows the user to specify 31 the new journal location. The journal device is 32 identified through its new major/minor numbers encoded 33 in devnum. 34 35norecovery Don't load the journal on mounting. Note that this forces 36noload mount of inconsistent filesystem, which can lead to 37 various problems. 38 39data=journal All data are committed into the journal prior to being 40 written into the main file system. 41 42data=ordered (*) All data are forced directly out to the main file 43 system prior to its metadata being committed to the 44 journal. 45 46data=writeback Data ordering is not preserved, data may be written 47 into the main file system after its metadata has been 48 committed to the journal. 49 50commit=nrsec (*) Ext3 can be told to sync all its data and metadata 51 every 'nrsec' seconds. The default value is 5 seconds. 52 This means that if you lose your power, you will lose 53 as much as the latest 5 seconds of work (your 54 filesystem will not be damaged though, thanks to the 55 journaling). This default value (or any low value) 56 will hurt performance, but it's good for data-safety. 57 Setting it to 0 will have the same effect as leaving 58 it at the default (5 seconds). 59 Setting it to very large values will improve 60 performance. 61 62barrier=<0(*)|1> This enables/disables the use of write barriers in 63barrier the jbd code. barrier=0 disables, barrier=1 enables. 64nobarrier (*) This also requires an IO stack which can support 65 barriers, and if jbd gets an error on a barrier 66 write, it will disable again with a warning. 67 Write barriers enforce proper on-disk ordering 68 of journal commits, making volatile disk write caches 69 safe to use, at some performance penalty. If 70 your disks are battery-backed in one way or another, 71 disabling barriers may safely improve performance. 72 The mount options "barrier" and "nobarrier" can 73 also be used to enable or disable barriers, for 74 consistency with other ext3 mount options. 75 76user_xattr Enables Extended User Attributes. Additionally, you 77 need to have extended attribute support enabled in the 78 kernel configuration (CONFIG_EXT3_FS_XATTR). See the 79 attr(5) manual page and http://acl.bestbits.at/ to 80 learn more about extended attributes. 81 82nouser_xattr Disables Extended User Attributes. 83 84acl Enables POSIX Access Control Lists support. 85 Additionally, you need to have ACL support enabled in 86 the kernel configuration (CONFIG_EXT3_FS_POSIX_ACL). 87 See the acl(5) manual page and http://acl.bestbits.at/ 88 for more information. 89 90noacl This option disables POSIX Access Control List 91 support. 92 93reservation 94 95noreservation 96 97bsddf (*) Make 'df' act like BSD. 98minixdf Make 'df' act like Minix. 99 100check=none Don't do extra checking of bitmaps on mount. 101nocheck 102 103debug Extra debugging information is sent to syslog. 104 105errors=remount-ro Remount the filesystem read-only on an error. 106errors=continue Keep going on a filesystem error. 107errors=panic Panic and halt the machine if an error occurs. 108 (These mount options override the errors behavior 109 specified in the superblock, which can be 110 configured using tune2fs.) 111 112data_err=ignore(*) Just print an error message if an error occurs 113 in a file data buffer in ordered mode. 114data_err=abort Abort the journal if an error occurs in a file 115 data buffer in ordered mode. 116 117grpid Give objects the same group ID as their creator. 118bsdgroups 119 120nogrpid (*) New objects have the group ID of their creator. 121sysvgroups 122 123resgid=n The group ID which may use the reserved blocks. 124 125resuid=n The user ID which may use the reserved blocks. 126 127sb=n Use alternate superblock at this location. 128 129quota These options are ignored by the filesystem. They 130noquota are used only by quota tools to recognize volumes 131grpquota where quota should be turned on. See documentation 132usrquota in the quota-tools package for more details 133 (http://sourceforge.net/projects/linuxquota). 134 135jqfmt=<quota type> These options tell filesystem details about quota 136usrjquota=<file> so that quota information can be properly updated 137grpjquota=<file> during journal replay. They replace the above 138 quota options. See documentation in the quota-tools 139 package for more details 140 (http://sourceforge.net/projects/linuxquota). 141 142Specification 143============= 144Ext3 shares all disk implementation with the ext2 filesystem, and adds 145transactions capabilities to ext2. Journaling is done by the Journaling Block 146Device layer. 147 148Journaling Block Device layer 149----------------------------- 150The Journaling Block Device layer (JBD) isn't ext3 specific. It was designed 151to add journaling capabilities to a block device. The ext3 filesystem code 152will inform the JBD of modifications it is performing (called a transaction). 153The journal supports the transactions start and stop, and in case of a crash, 154the journal can replay the transactions to quickly put the partition back into 155a consistent state. 156 157Handles represent a single atomic update to a filesystem. JBD can handle an 158external journal on a block device. 159 160Data Mode 161--------- 162There are 3 different data modes: 163 164* writeback mode 165In data=writeback mode, ext3 does not journal data at all. This mode provides 166a similar level of journaling as that of XFS, JFS, and ReiserFS in its default 167mode - metadata journaling. A crash+recovery can cause incorrect data to 168appear in files which were written shortly before the crash. This mode will 169typically provide the best ext3 performance. 170 171* ordered mode 172In data=ordered mode, ext3 only officially journals metadata, but it logically 173groups metadata and data blocks into a single unit called a transaction. When 174it's time to write the new metadata out to disk, the associated data blocks 175are written first. In general, this mode performs slightly slower than 176writeback but significantly faster than journal mode. 177 178* journal mode 179data=journal mode provides full data and metadata journaling. All new data is 180written to the journal first, and then to its final location. 181In the event of a crash, the journal can be replayed, bringing both data and 182metadata into a consistent state. This mode is the slowest except when data 183needs to be read from and written to disk at the same time where it 184outperforms all other modes. 185 186Compatibility 187------------- 188 189Ext2 partitions can be easily convert to ext3, with `tune2fs -j <dev>`. 190Ext3 is fully compatible with Ext2. Ext3 partitions can easily be mounted as 191Ext2. 192 193 194External Tools 195============== 196See manual pages to learn more. 197 198tune2fs: create a ext3 journal on a ext2 partition with the -j flag. 199mke2fs: create a ext3 partition with the -j flag. 200debugfs: ext2 and ext3 file system debugger. 201ext2online: online (mounted) ext2 and ext3 filesystem resizer 202 203 204References 205========== 206 207kernel source: <file:fs/ext3/> 208 <file:fs/jbd/> 209 210programs: http://e2fsprogs.sourceforge.net/ 211 http://ext2resize.sourceforge.net 212 213useful links: http://www.ibm.com/developerworks/library/l-fs7/index.html 214 http://www.ibm.com/developerworks/library/l-fs8/index.html 215