[Lustre-discuss] external journal raid1 vs. single disk ext journal + hot spare on raid6

[Andreas Dilger]

On May 14, 2009  23:37 -0400, Robin Humble wrote:
> one problem we came across was that ext3/ldiskfs hard-codes the device
> name of the external journal (eg. /dev/md5 or /dev/sdc1 or whatever)
> into the filesystem. 
> that means that when you failover OSS's it will look for /dev/whatever
> on the failed-over node, and won't mount if it can't find it.
> so you need non-intersecting namespaces of journal devices within an OSS
> pair, so that each regular and failed-over RAID5/6 can always find its
> correct journal device.
> I didn't manage to get ext3/ldiskfs to be sane and use UUID's instead of
> hardcoded device names :-/

There is a "journal_device" mount option for this.  We'd like to make
mount.lustre find this device automatically, but it hasn't been fixed
yet.  See bug 16861.

> presumably you could also tune2fs to rename or delete the external
> journal as part of a failover, but that's a horrible hack.  

No, that will potentially lose some data, since ext3 considers data written
to the journal as "safe"

Cheers, Andreas
--
Andreas Dilger
Sr. Staff Engineer, Lustre Group
Sun Microsystems of Canada, Inc.