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 Documentation/filesystems/ext3.txt

Signed-off-by: Jesper Juhl <jesper.juhl@gmail.com>
Signed-off-by: Andrew Morton <akpm@osdl.org>
Signed-off-by: Linus Torvalds <torvalds@osdl.org>
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Jesper Juhl authored and Linus Torvalds committed Jan 10, 2006
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178 changes: 90 additions & 88 deletions Documentation/filesystems/ext3.txt
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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,
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 ext2 filesystem enhanced with journalling capabilities.
Ext3 is the 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=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=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 to specify the new
journal location. The journal device is identified
through its new major/minor numbers encoded in devnum.
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=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.
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.
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.
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.
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.
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.
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.
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.
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.
noacl This option disables POSIX Access Control List
support.

reservation

Expand All @@ -88,7 +90,7 @@ bsddf (*) Make 'df' act like BSD.
minixdf Make 'df' act like Minix.

check=none Don't do extra checking of bitmaps on mount.
nocheck
nocheck

debug Extra debugging information is sent to syslog.

Expand All @@ -97,7 +99,7 @@ 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
bsdgroups

nogrpid (*) New objects have the group ID of their creator.
sysvgroups
Expand All @@ -108,81 +110,81 @@ 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)
quota
noquota
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.
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
(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.
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 external journal on a block device.
Handles represent a single atomic update to a filesystem. JBD can handle an
external journal on a block device.

Data Mode
---------
There's 3 different data modes:
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 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.
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
perform slightly slower than writeback but significantly faster than
journal 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 outperform all others 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.
Ext3 is fully compatible with Ext2. Ext3 partitions can easily be mounted as
Ext2.


External Tools
==============
see manual pages to know more.
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.

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
kernel source: <file:fs/ext3/>
<file:fs/jbd/>

programs: http://e2fsprogs.sourceforge.net
programs: http://e2fsprogs.sourceforge.net/

useful link:
http://www.zip.com.au/~akpm/linux/ext3/ext3-usage.html
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/

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