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Ext3 Filesystem
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===============
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ext3 was originally released in September 1999. Written by Stephen Tweedie
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for 2.2 branch, and ported to 2.4 kernels by Peter Braam, Andreas Dilger,
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Andrew Morton, Alexander Viro, Ted Ts'o and Stephen Tweedie.
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ext3 is ext2 filesystem enhanced with journalling capabilities.
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Options
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=======
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When mounting an ext3 filesystem, the following option are accepted:
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(*) == default
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jounal=update Update the ext3 file system's journal to the
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current format.
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journal=inum When a journal already exists, this option is
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ignored. Otherwise, it specifies the number of
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the inode which will represent the ext3 file
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system's journal file.
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journal_dev=devnum When the external journal device's major/minor numbers
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have changed, this option allows to specify the new
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journal location. The journal device is identified
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through its new major/minor numbers encoded in devnum.
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noload Don't load the journal on mounting.
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data=journal All data are committed into the journal prior
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to being written into the main file system.
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data=ordered (*) All data are forced directly out to the main file
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system prior to its metadata being committed to
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the journal.
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data=writeback Data ordering is not preserved, data may be
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written into the main file system after its
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metadata has been committed to the journal.
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commit=nrsec (*) Ext3 can be told to sync all its data and metadata
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every 'nrsec' seconds. The default value is 5 seconds.
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This means that if you lose your power, you will lose,
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as much, the latest 5 seconds of work (your filesystem
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will not be damaged though, thanks to journaling). This
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default value (or any low value) will hurt performance,
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but it's good for data-safety. Setting it to 0 will
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have the same effect than leaving the default 5 sec.
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Setting it to very large values will improve
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performance.
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barrier=1 This enables/disables barriers. barrier=0 disables it,
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barrier=1 enables it.
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orlov (*) This enables the new Orlov block allocator. It's enabled
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by default.
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oldalloc This disables the Orlov block allocator and enables the
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old block allocator. Orlov should have better performance,
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we'd like to get some feedback if it's the contrary for
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you.
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user_xattr Enables Extended User Attributes. Additionally, you need
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to have extended attribute support enabled in the kernel
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configuration (CONFIG_EXT3_FS_XATTR). See the attr(5)
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manual page and http://acl.bestbits.at to learn more
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about extended attributes.
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nouser_xattr Disables Extended User Attributes.
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acl Enables POSIX Access Control Lists support. Additionally,
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you need to have ACL support enabled in the kernel
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configuration (CONFIG_EXT3_FS_POSIX_ACL). See the acl(5)
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manual page and http://acl.bestbits.at for more
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information.
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noacl This option disables POSIX Access Control List support.
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reservation
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noreservation
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resize=
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bsddf (*) Make 'df' act like BSD.
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minixdf Make 'df' act like Minix.
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check=none Don't do extra checking of bitmaps on mount.
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nocheck
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debug Extra debugging information is sent to syslog.
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errors=remount-ro(*) Remount the filesystem read-only on an error.
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errors=continue Keep going on a filesystem error.
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errors=panic Panic and halt the machine if an error occurs.
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grpid Give objects the same group ID as their creator.
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bsdgroups
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nogrpid (*) New objects have the group ID of their creator.
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sysvgroups
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resgid=n The group ID which may use the reserved blocks.
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resuid=n The user ID which may use the reserved blocks.
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sb=n Use alternate superblock at this location.
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quota Quota options are currently silently ignored.
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noquota (see fs/ext3/super.c, line 594)
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grpquota
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usrquota
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Specification
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=============
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ext3 shares all disk implementation with ext2 filesystem, and add
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transactions capabilities to ext2. Journaling is done by the
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Journaling block device layer.
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Journaling Block Device layer
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-----------------------------
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The Journaling Block Device layer (JBD) isn't ext3 specific. It was
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design to add journaling capabilities on a block device. The ext3
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filesystem code will inform the JBD of modifications it is performing
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(Call a transaction). the journal support the transactions start and
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stop, and in case of crash, the journal can replayed the transactions
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to put the partition on a consistent state fastly.
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handles represent a single atomic update to a filesystem. JBD can
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handle external journal on a block device.
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Data Mode
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---------
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There's 3 different data modes:
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* writeback mode
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In data=writeback mode, ext3 does not journal data at all. This mode
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provides a similar level of journaling as XFS, JFS, and ReiserFS in its
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default mode - metadata journaling. A crash+recovery can cause
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incorrect data to appear in files which were written shortly before the
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crash. This mode will typically provide the best ext3 performance.
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* ordered mode
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In data=ordered mode, ext3 only officially journals metadata, but it
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logically groups metadata and data blocks into a single unit called a
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transaction. When it's time to write the new metadata out to disk, the
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associated data blocks are written first. In general, this mode
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perform slightly slower than writeback but significantly faster than
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journal mode.
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* journal mode
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data=journal mode provides full data and metadata journaling. All new
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data is written to the journal first, and then to its final location.
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In the event of a crash, the journal can be replayed, bringing both
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data and metadata into a consistent state. This mode is the slowest
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except when data needs to be read from and written to disk at the same
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time where it outperform all others mode.
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Compatibility
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-------------
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Ext2 partitions can be easily convert to ext3, with `tune2fs -j <dev>`.
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Ext3 is fully compatible with Ext2. Ext3 partitions can easily be
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mounted as Ext2.
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External Tools
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==============
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see manual pages to know more.
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tune2fs: create a ext3 journal on a ext2 partition with the -j flags
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mke2fs: create a ext3 partition with the -j flags
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debugfs: ext2 and ext3 file system debugger
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References
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==========
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kernel source: file:/usr/src/linux/fs/ext3
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file:/usr/src/linux/fs/jbd
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programs: http://e2fsprogs.sourceforge.net
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useful link:
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http://www.zip.com.au/~akpm/linux/ext3/ext3-usage.html
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http://www-106.ibm.com/developerworks/linux/library/l-fs7/
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http://www-106.ibm.com/developerworks/linux/library/l-fs8/
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