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Re: [Xen-users] Re: iSCSI and LVM

On Tuesday 15 June 2010 16:34:35 Ferenc Wagner wrote:
> "James Harper" <james.harper@xxxxxxxxxxxxxxxx> writes:
> >> "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).
> It's a little bit more complicated, but the basic idea is this.
> > 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.
> No, operation of the snapshot doesn't involve continuous *LVM* metadata
> updates, even though the chunk mapping is really metadata with respect
> to the block devices themselves.
> > 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.
> Yes, I didn't elaborate, but this is my advice.
> > If the activated the VG on the other nodes after creating the snapshot
> > though, then problems may (will) arise!
> Only if you access data in the same LV from different hosts (metadata
> updates are also excluded, of course).  From this point of view, the
> origin and the snapshot LVs (and the cow device) must be considered the
> "same" LV.  Basically, this is why clvm does not support snapshots.  And
> of course I didn't consider cluster filesystems and similar above.
> I think we're pretty much on the same page.

I would like to especially for Jonathan add that snapshotting of virtual  
machines does not provide a safe way of backing them up, unless they are shut 
down first. 


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