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Ext3 Filesystem
===============
ext3 was originally released in September 1999. Written by Stephen Tweedie
for 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 ext2 filesystem enhanced with journalling capabilities.
Options
=======
When mounting an ext3 filesystem, the following option are accepted:
(*) == default
jounal=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.
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, the latest 5 seconds of work (your filesystem
will not be damaged though, thanks to 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 than leaving the default 5 sec.
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's 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 POSIX Extended Attributes. It's enabled by
default, however you need to confifure its support
(CONFIG_EXT3_FS_XATTR). This is neccesary if you want
to use POSIX Acces Control Lists support. You can visit
http://acl.bestbits.at to know more about POSIX Extended
attributes.
nouser_xattr Disables POSIX Extended Attributes.
acl (*) Enables POSIX Access Control Lists support. This is
enabled by default, however you need to configure
its support (CONFIG_EXT3_FS_POSIX_ACL). If you want
to know more about ACLs visit http://acl.bestbits.at
noacl This option disables POSIX Access Control List support.
reservation
noreservation
resize=
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 Quota options are currently silently ignored.
noquota (see fs/ext3/super.c, line 594)
grpquota
usrquota
Specification
=============
ext3 shares all disk implementation with ext2 filesystem, and add
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
(Call a transaction). the journal support the transactions start and
stop, and in case of crash, the journal can replayed the transactions
to put the partition on a consistent state fastly.
handles represent a single atomic update to a filesystem. JBD can
handle external journal on a block device.
Data Mode
---------
There's 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 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
perform 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 outperform all others mode.
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 know more.
tune2fs: create a ext3 journal on a ext2 partition with the -j flags
mke2fs: create a ext3 partition with the -j flags
debugfs: ext2 and ext3 file system debugger
References
==========
kernel source: file:/usr/src/linux/fs/ext3
file:/usr/src/linux/fs/jbd
programs: http://e2fsprogs.sourceforge.net
useful link:
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/
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