| Ext3 Filesystem | |
| =============== | |
| Ext3 was originally released in September 1999. Written by Stephen Tweedie | |
| for the 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 the ext2 filesystem enhanced with journalling capabilities. | |
| Options | |
| ======= | |
| When mounting an ext3 filesystem, the following option are accepted: | |
| (*) == default | |
| ro Mount filesystem read only. Note that ext3 will replay | |
| the journal (and thus write to the partition) even when | |
| mounted "read only". Mount options "ro,noload" can be | |
| used to prevent writes to the filesystem. | |
| journal=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. | |
| journal_dev=devnum When the external journal device's major/minor numbers | |
| have changed, this option allows the user to specify | |
| the new journal location. The journal device is | |
| identified through its new major/minor numbers encoded | |
| in devnum. | |
| norecovery Don't load the journal on mounting. Note that this forces | |
| noload mount of inconsistent filesystem, which can lead to | |
| various problems. | |
| 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 as the latest 5 seconds of work (your | |
| filesystem will not be damaged though, thanks to the | |
| 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 as leaving | |
| it at the default (5 seconds). | |
| Setting it to very large values will improve | |
| performance. | |
| barrier=<0(*)|1> This enables/disables the use of write barriers in | |
| barrier the jbd code. barrier=0 disables, barrier=1 enables. | |
| nobarrier (*) This also requires an IO stack which can support | |
| barriers, and if jbd gets an error on a barrier | |
| write, it will disable again with a warning. | |
| Write barriers enforce proper on-disk ordering | |
| of journal commits, making volatile disk write caches | |
| safe to use, at some performance penalty. If | |
| your disks are battery-backed in one way or another, | |
| disabling barriers may safely improve performance. | |
| The mount options "barrier" and "nobarrier" can | |
| also be used to enable or disable barriers, for | |
| consistency with other ext3 mount options. | |
| orlov (*) This enables the new Orlov block allocator. It is | |
| 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 Extended User Attributes. Additionally, you | |
| need to have extended attribute support enabled in the | |
| kernel configuration (CONFIG_EXT3_FS_XATTR). See the | |
| attr(5) manual page and http://acl.bestbits.at/ to | |
| learn more about extended attributes. | |
| nouser_xattr Disables Extended User Attributes. | |
| acl Enables POSIX Access Control Lists support. | |
| Additionally, you need to have ACL support enabled in | |
| the kernel configuration (CONFIG_EXT3_FS_POSIX_ACL). | |
| See the acl(5) manual page and http://acl.bestbits.at/ | |
| for more information. | |
| noacl This option disables POSIX Access Control List | |
| support. | |
| reservation | |
| noreservation | |
| 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. | |
| (These mount options override the errors behavior | |
| specified in the superblock, which can be | |
| configured using tune2fs.) | |
| data_err=ignore(*) Just print an error message if an error occurs | |
| in a file data buffer in ordered mode. | |
| data_err=abort Abort the journal if an error occurs in a file | |
| data buffer in ordered mode. | |
| 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 These options are ignored by the filesystem. They | |
| noquota are used only by quota tools to recognize volumes | |
| grpquota where quota should be turned on. See documentation | |
| usrquota in the quota-tools package for more details | |
| (http://sourceforge.net/projects/linuxquota). | |
| jqfmt=<quota type> These options tell filesystem details about quota | |
| usrjquota=<file> so that quota information can be properly updated | |
| grpjquota=<file> during journal replay. They replace the above | |
| quota options. See documentation in the quota-tools | |
| package for more details | |
| (http://sourceforge.net/projects/linuxquota). | |
| bh (*) ext3 associates buffer heads to data pages to | |
| nobh (a) cache disk block mapping information | |
| (b) link pages into transaction to provide | |
| ordering guarantees. | |
| "bh" option forces use of buffer heads. | |
| "nobh" option tries to avoid associating buffer | |
| heads (supported only for "writeback" mode). | |
| Specification | |
| ============= | |
| Ext3 shares all disk implementation with the ext2 filesystem, and adds | |
| 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 designed | |
| to add journaling capabilities to a block device. The ext3 filesystem code | |
| will inform the JBD of modifications it is performing (called a transaction). | |
| The journal supports the transactions start and stop, and in case of a crash, | |
| the journal can replay the transactions to quickly put the partition back into | |
| a consistent state. | |
| Handles represent a single atomic update to a filesystem. JBD can handle an | |
| external journal on a block device. | |
| Data Mode | |
| --------- | |
| There are 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 that of 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 performs 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 | |
| outperforms all other modes. | |
| 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 learn more. | |
| tune2fs: create a ext3 journal on a ext2 partition with the -j flag. | |
| mke2fs: create a ext3 partition with the -j flag. | |
| debugfs: ext2 and ext3 file system debugger. | |
| ext2online: online (mounted) ext2 and ext3 filesystem resizer | |
| References | |
| ========== | |
| kernel source: <file:fs/ext3/> | |
| <file:fs/jbd/> | |
| programs: http://e2fsprogs.sourceforge.net/ | |
| http://ext2resize.sourceforge.net | |
| useful links: http://www.ibm.com/developerworks/library/l-fs7.html | |
| http://www.ibm.com/developerworks/library/l-fs8.html |