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Re: Separate common journal device



On Apr 20, 2004  23:56 -0500, Vijayan Prabhakaran wrote:
> Is it possible to use a separate journal device (one on a separate
> drive or a partition) shared among more than 1 Ext3 file systems ?

It is possible now to use an external block device for a single filesystem.
The on-disk format is designed to allow multiple filesystems to share the
same device, but that has never been fully implemented.

At one point I had implemented a patch to mount a "jbd" filesystem with the
journal device as the first step of having a shared journal device.  Having
the "jbd" device in /etc/fstab (before filesystems that use it) allows e2fsck
to do journal replay on all of the filesystems before the journal starts to
be used, or alternately dumps the journal data to an external file for later
replay (e.g. if block devices are not available when e2fsck is run on the
jbd device).  It also allows the jbd code to configure the in-core code to
be ready for external filesystems to connect to it.  Finally, it also marks
the block device as in-use so it is less likely that it will be overwritten
accidentally.

See the following email for the (ancient) patch.  Most of the comments
and a large fraction of the code in that email are still relevant, with
the exception that all of the UUID handling already exists as libblkid
in e2fsprogs, and it doesn't say what kernel version this is for (I'd
suspect 2.3, but I'm not totally sure.  Sadly, nobody commented on it
at the time and it was lost in the mists of antiquity.

> Subject: [PATCH][RFC] mountable journal devices
> To: Ext2 development mailing list <ext2-devel lists sourceforge net>
> Date: Wed, 8 Aug 2001 02:08:23 -0600 (MDT)
http://marc.theaimsgroup.com/?l=ext2-devel&m=99725819513803

And the thread starting at discusses shared external journal devices:
https://listman.redhat.com/archives/ext3-users/2001-November/msg00182.html

Cheers, Andreas
--
Andreas Dilger
http://sourceforge.net/projects/ext2resize/
http://www-mddsp.enel.ucalgary.ca/People/adilger/



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