| |
| 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. |
| |
| noload Don't load the journal on mounting. Note that this forces |
| 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=1 This enables/disables barriers. barrier=0 disables |
| it, barrier=1 enables it. |
| |
| 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 |
| noquota |
| grpquota |
| usrquota |
| |
| 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 |