ade1a29e16
This patch adds "-o bh" option to force use of buffer_heads. This option is needed when we make "nobh" as default - and if we run into problems. Signed-off-by: Badari Pulavarty <pbadari@us.ibm.com> Signed-off-by: Andrew Morton <akpm@osdl.org> Signed-off-by: Linus Torvalds <torvalds@osdl.org>
198 lines
6.7 KiB
Text
198 lines
6.7 KiB
Text
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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 the 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 the 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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journal=update Update the ext3 file system's journal to the current
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format.
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journal=inum When a journal already exists, this option is ignored.
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Otherwise, it specifies the number of the inode which
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will represent the ext3 file 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 the user to specify
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the new journal location. The journal device is
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identified through its new major/minor numbers encoded
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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 to being
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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 the
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journal.
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data=writeback Data ordering is not preserved, data may be written
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into the main file system after its metadata has been
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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 as the latest 5 seconds of work (your
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filesystem will not be damaged though, thanks to the
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journaling). This default value (or any low value)
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will hurt performance, but it's good for data-safety.
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Setting it to 0 will have the same effect as leaving
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it at the default (5 seconds).
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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
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it, barrier=1 enables it.
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orlov (*) This enables the new Orlov block allocator. It is
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enabled by default.
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oldalloc This disables the Orlov block allocator and enables
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the old block allocator. Orlov should have better
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performance - we'd like to get some feedback if it's
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the contrary for you.
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user_xattr Enables Extended User Attributes. Additionally, you
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need to have extended attribute support enabled in the
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kernel configuration (CONFIG_EXT3_FS_XATTR). See the
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attr(5) manual page and http://acl.bestbits.at/ to
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learn more about extended attributes.
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nouser_xattr Disables Extended User Attributes.
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acl Enables POSIX Access Control Lists support.
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Additionally, you need to have ACL support enabled in
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the kernel configuration (CONFIG_EXT3_FS_POSIX_ACL).
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See the acl(5) manual page and http://acl.bestbits.at/
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for more information.
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noacl This option disables POSIX Access Control List
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support.
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reservation
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noreservation
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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
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noquota
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grpquota
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usrquota
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bh (*) ext3 associates buffer heads to data pages to
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nobh (a) cache disk block mapping information
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(b) link pages into transaction to provide
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ordering guarantees.
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"bh" option forces use of buffer heads.
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"nobh" option tries to avoid associating buffer
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heads (supported only for "writeback" mode).
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Specification
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=============
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Ext3 shares all disk implementation with the ext2 filesystem, and adds
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transactions capabilities to ext2. Journaling is done by the Journaling Block
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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 design to
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add journaling capabilities on a block device. The ext3 filesystem code will
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inform the JBD of modifications it is performing (called a transaction). The
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journal supports the transactions start and stop, and in case of crash, the
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journal can replayed the transactions to put the partition back in a
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consistent state fast.
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Handles represent a single atomic update to a filesystem. JBD can handle an
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external journal on a block device.
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Data Mode
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---------
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There are 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 provides
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a similar level of journaling as that of XFS, JFS, and ReiserFS in its default
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mode - metadata journaling. A crash+recovery can cause incorrect data to
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appear in files which were written shortly before the crash. This mode will
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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 logically
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groups metadata and data blocks into a single unit called a transaction. When
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it's time to write the new metadata out to disk, the associated data blocks
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are written first. In general, this mode performs slightly slower than
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writeback but significantly faster than journal mode.
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* journal mode
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data=journal mode provides full data and metadata journaling. All new data is
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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 data and
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metadata into a consistent state. This mode is the slowest except when data
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needs to be read from and written to disk at the same time where it
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outperforms all others modes.
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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 mounted as
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Ext2.
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External Tools
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==============
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See manual pages to learn more.
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tune2fs: create a ext3 journal on a ext2 partition with the -j flag.
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mke2fs: create a ext3 partition with the -j flag.
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debugfs: ext2 and ext3 file system debugger.
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ext2online: online (mounted) ext2 and ext3 filesystem resizer
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References
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==========
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kernel source: <file:fs/ext3/>
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<file:fs/jbd/>
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programs: http://e2fsprogs.sourceforge.net/
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http://ext2resize.sourceforge.net
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useful links: 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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