[Ocfs2-users] OCFS2 and Snapshots

[Joel Becker]

On Thu, Jul 20, 2006 at 07:54:57PM +0200, Andre Brinkmann wrote:
> Similar to OCFS2, XFS relies on the uniqueness of a serial number. But 
> with XFS, I am able to change this uid with a user space admin tool. I 
> hope to find (or implement) similar features in OCFS2 to be able to 
> support snapshots. I think the possibility to change the uuid is import 
> for most storage system. Even hardware RAIDs with integrated snapshots 
> will have similar problems.

	I find it interesting that you use a modification of the uuid to
handle snapshotting.  Certainly would work for a snapshot of
OCFS2...however, it's also dangerous.  Two OCFS2 nodes absolutely rely
on the uuid to ensure they are heartbeating for the same thing.  So we
cannot safely change the uuid if there is any chance another node is
running on the filesystem (let alone our own node :-).
	Now, your snapshot won't be mounted anywhere, sure, but our
tools won't know a snapshot vs a live filesystem, and will need to make
sure.  That just requires some locking.
	So, I think it is something we can look into as a feature for
tunefs.ocfs2, but it isn't a one-liner and requires some correctness.
	Would you prefer to _set_ the uuid, or just generate a new one?
I'd personally go with the latter (all it needs to do is differ from the
non-snapshot volume), but I wanted to ask your opinion.

Joel

-- 

Life's Little Instruction Book #20

	"Be forgiving of yourself and others."

Joel Becker
Principal Software Developer
Oracle
E-mail: joel.becker at oracle.com
Phone: (650) 506-8127