[Xen-users] RE: iSCSI and LVM

["James Harper" <james.harper@xxxxxxxxxxxxxxxx>]

> "James Harper" <james.harper@xxxxxxxxxxxxxxxx> writes:
> 
> >> You can use live migration in such setup, even safely if you back
it
> >> by clvm.  You can even live without clvm if you deactivate your VG
on
> >> all but a single dom0 before changing the LVM metadata in any way.
A
> >> non-clustered VG being active on multiple dom0s isn't a problem in
> >> itself and makes live migration possible, but you'd better
understand
> >> what you're doing.
> >
> > You can't snapshot though. I tried that once years ago and it made a
> > horrible mess.
> 
> Even if done after deactivating the VG on all but a single node?  That
> would be a bug.  According to my understanding, it should work.  I
never
> tried, though, as snapshotting isn't my preferred way of making
backups.
> On the other hand I run domUs on snapshots of local LVs without any
> problem.  And an LV being "local" is a concept beyond LVM in the above
> setting, so it can't matter...

A snapshot is copy-on-write. Every time the 'source' is written to, a
copy of the original block is saved to the snapshot (I may have that the
wrong way around). This allows snapshots to be pretty much instant as
very little data is manipulated. It also saves a lot of space. Doing
that though involves a remapping of the snapshot every time the source
is written to (eg block x isn't in the 'source' anymore, so storage is
allocated to it etc) which involves a metadata update.

So if the VG remained deactivated on all nodes for the life of the
snapshot then it may work, and maybe this is what you meant in which
case you are correct. If the activated the VG on the other nodes after
creating the snapshot though, then problems may (will) arise!

James




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