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

To: Pasi Kärkkäinen <pasik@xxxxxx>
Subject: RE: [Xen-users] iscsi vs nfs for xen VMs
From: "James Harper" <james.harper@xxxxxxxxxxxxxxxx>
Date: Sun, 30 Jan 2011 11:14:02 +1100
Cc: Christian Zoffoli <czoffoli@xxxxxxxxxxx>, Jia Rao <rickenrao@xxxxxxxxx>, xen-users@xxxxxxxxxxxxxxxxxxx
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Thread-topic: [Xen-users] iscsi vs nfs for xen VMs
> > 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.


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