![Badari Pulavarty](/assets/img/avatar_default.png)
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>
199 lines
6.7 KiB
Text
199 lines
6.7 KiB
Text
|
|
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
|
|
|
|
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.
|
|
|
|
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.
|
|
|
|
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 design to
|
|
add journaling capabilities on 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 crash, the
|
|
journal can replayed the transactions to put the partition back in a
|
|
consistent state fast.
|
|
|
|
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 others 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.zip.com.au/~akpm/linux/ext3/ext3-usage.html
|
|
http://www-106.ibm.com/developerworks/linux/library/l-fs7/
|
|
http://www-106.ibm.com/developerworks/linux/library/l-fs8/
|