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kernel-ark/Documentation/filesystems/ext3.txt
Eric Sandeen cf7eff4666 ext3: allow specifying external journal by pathname mount option 
It's always been a hassle that if an external journal's
device number changes, the filesystem won't mount.
And since boot-time enumeration can change, device number
changes aren't unusual.

The current mechanism to update the journal location is by
passing in a mount option w/ a new devnum, but that's a hassle;
it's a manual approach, fixing things after the fact.

Adding a mount option, "-o journal_path=/dev/$DEVICE" would
help, since then we can do i.e.

# mount -o journal_path=/dev/disk/by-label/$JOURNAL_LABEL ...

and it'll mount even if the devnum has changed, as shown here:

# losetup /dev/loop0 journalfile
# mke2fs -L mylabel-journal -O journal_dev /dev/loop0
# mkfs.ext3 -L mylabel -J device=/dev/loop0 /dev/sdb1

Change the journal device number:

# losetup -d /dev/loop0
# losetup /dev/loop1 journalfile

And today it will fail:

# mount /dev/sdb1 /mnt/test
mount: wrong fs type, bad option, bad superblock on /dev/sdb1,
       missing codepage or helper program, or other error
       In some cases useful info is found in syslog - try
       dmesg | tail  or so

# dmesg | tail -n 1
[17343.240702] EXT3-fs (sdb1): error: couldn't read superblock of external journal

But with this new mount option, we can specify the new path:

# mount -o journal_path=/dev/loop1 /dev/sdb1 /mnt/test
#

(which does update the encoded device number, incidentally):

# umount /dev/sdb1
# dumpe2fs -h /dev/sdb1 | grep "Journal device"
dumpe2fs 1.41.12 (17-May-2010)
Journal device:	          0x0701

But best of all we can just always mount by journal-path, and
it'll always work:

# mount -o journal_path=/dev/disk/by-label/mylabel-journal /dev/sdb1 /mnt/test
#

So the journal_path option can be specified in fstab, and as long as
the disk is available somewhere, and findable by label (or by UUID),
we can mount.

Signed-off-by: Eric Sandeen <sandeen@redhat.com>
Signed-off-by: Jan Kara <jack@suse.cz>
2013-07-31 22:11:15 +02:00

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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
ro Mount filesystem read only. Note that ext3 will replay
the journal (and thus write to the partition) even when
mounted "read only". Mount options "ro,noload" can be
used to prevent writes to the filesystem.
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_path=path
journal_dev=devnum When the external journal device's major/minor numbers
have changed, these options allow the user to specify
the new journal location. The journal device is
identified through either its new major/minor numbers
encoded in devnum, or via a path to the device.
norecovery Don't load the journal on mounting. Note that this forces
noload mount of inconsistent filesystem, which can lead to
various problems.
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=<0|1(*)> This enables/disables the use of write barriers in
barrier (*) the jbd code. barrier=0 disables, barrier=1 enables.
nobarrier This also requires an IO stack which can support
barriers, and if jbd gets an error on a barrier
write, it will disable again with a warning.
Write barriers enforce proper on-disk ordering
of journal commits, making volatile disk write caches
safe to use, at some performance penalty. If
your disks are battery-backed in one way or another,
disabling barriers may safely improve performance.
The mount options "barrier" and "nobarrier" can
also be used to enable or disable barriers, for
consistency with other ext3 mount options.
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.
(These mount options override the errors behavior
specified in the superblock, which can be
configured using tune2fs.)
data_err=ignore(*) Just print an error message if an error occurs
in a file data buffer in ordered mode.
data_err=abort Abort the journal if an error occurs in a file
data buffer in ordered mode.
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 These options are ignored by the filesystem. They
noquota are used only by quota tools to recognize volumes
grpquota where quota should be turned on. See documentation
usrquota in the quota-tools package for more details
(http://sourceforge.net/projects/linuxquota).
jqfmt=<quota type> These options tell filesystem details about quota
usrjquota=<file> so that quota information can be properly updated
grpjquota=<file> during journal replay. They replace the above
quota options. See documentation in the quota-tools
package for more details
(http://sourceforge.net/projects/linuxquota).
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 designed
to add journaling capabilities to 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 a crash,
the journal can replay the transactions to quickly put the partition back into
a consistent state.
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 other 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.ibm.com/developerworks/library/l-fs7/index.html
http://www.ibm.com/developerworks/library/l-fs8/index.html
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