mirror of
https://github.com/torvalds/linux.git
synced 2024-11-17 17:41:44 +00:00
cf7eff4666
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>
216 lines
7.9 KiB
Plaintext
216 lines
7.9 KiB
Plaintext
|
|
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
|