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Re: [Xen-users] iscsi vs nfs for xen VMs

On Sun, Jan 30, 2011 at 11:14:02AM +1100, James Harper wrote:
> > > It's widely used.
> > >
> > > Citrix XenServer (and XCP) do it like that, and also the
> > > Xen based VirtualIron did it like that.
> > >
> > 
> > Forgot to add this:
> > 
> > In Citrix XenServer it's the XAPI toolstack that's
> > taking care of LVM locking, so only the "pool master"
> > is executing LVM commands.
> > 
> > As long as you know you're executing LVM commands
> > only from a single node, you're good. No need for CLVM.
> So it still does have locking then.


> > 
> > I guess you also need to refresh all the other nodes
> > after executing LVM commands on the primary/master node.
> > 
> That was my point though. Snapshot works by copy-on-write. Every time a block 
> in primary volume is written to for the first time since the snapshot was 
> taken, the data needs to be copied to the snapshot. Same when the snapshot is 
> written to. That involves a metadata update so I don't understand how it can 
> work without a major performance hit as you lock and unlock everything with 
> (potentially) every write.

Hmm.. if the toolstack makes sure each LV is only used from a single node
at a time (which it does), isn't it enough to just have locking
when you *create* the snapshot from the same node? snapshot reserve
gets allocated then etc.

Ie. the toolstack makes sure the node that actually is accessing
the volume/snapshot is always the same node, and it's in sync? 

-- Pasi

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