[Engine-devel] Question about CloneVMFromSnapshot feature in context of shared disks and direct LUNs-based disks

[Mike Kolesnik]

----- Original Message -----
> 
> 
> ----- Original Message -----
> > From: "Livnat Peer" <lpeer at redhat.com>
> > To: "Yair Zaslavsky" <yzaslavs at redhat.com>, "Mike Kolesnik"
> > <mkolesni at redhat.com>
> > Cc: engine-devel at ovirt.org
> > Sent: Thursday, January 19, 2012 9:19:52 AM
> > Subject: Re: [Engine-devel] Question about CloneVMFromSnapshot
> > feature in context of shared disks and direct
> > LUNs-based disks
> > 
> > On 19/01/12 08:38, Yair Zaslavsky wrote:
> > > Hi all,
> > > Following the upstream meeting dated Wednesday January 18th, 2012
> > > -
> > > I presented the clone VM from snpashot feature and we discussed
> > > the
> > > feature behaviour.
> > > 
> > > Two issues that were raised are the behaviour of the feature in
> > > context
> > > of shared disks and direct LUNs-based disks -
> > > On one hand, if we copy&collapse such images - this may yield to
> > > data
> > > corruption (think of a scenario where the source and destination
> > > VMs use
> > > the same disk).
> > > On the other hand - if we decide not to copy&collapse  - the
> > > target
> > > VM
> > > will have missing VM and its state will not totally reflect the
> > > logical
> > > state.
> > > One of the solution raises is to mark such disks (at the
> > > destination) as
> > > unplugged, allowing the administrator the ability to plug them
> > > (understanding of course the consequences).
> > > 
> > > I would like to receive inputs on this issue
> > > 
> > > 
> > > Kind regards,
> > > 
> > > Yair
> > 
> > Hi Yair,
> > 
> > Some clarifications on the above issue.
> > Currently when taking a snapshot on a VM with shared disks or
> > direct
> > LUN
> > disk there are 3 optional behaviors:
> > 
> > 1. Blocking the snapshot action. (User can not take a snapshot of
> > the
> > VM
> > if it has plugged shared or direct LUN disks)
> > 
> > 2. Taking the snapshot and marking the shared disk and direct LUN
> > disks
> > as unplugged (in the VM snapshot configuration) and marking the
> > snapshot
> > state as partial.
> > 
> > 3. Taking the snapshot of the VM as is, leaving the VM
> > configuration
> > with plugged disks.
> > 
> > 
> > The issue with including these disks in the snapshot is that they
> > are
> > not really being snapshotted, they are not capturing the point in
> > time
> > we are trying to achieve in the snapshot.
> > 
> > Enabling the snapshot action in such a state is a bit misleading to
> > the
> > user.
> > 
> > If we do allow taking the snapshot we should mark the snapshot as
> > partial to indicate that the snapshot did not capture the point in
> > time
> > as the user intended.
> > 
> > I have no preference with regards to the second and third approach,
> > the
> > second approach is a bit more safe, we basically force the user to
> > plug
> > the disks and be sure that he knows what he is doing and the third
> > approach is less safe and less annoying to the user (he took the
> > snapshot, cloned it and wants to start the VM - don't require extra
> > actions)
> > 
> > Kolesnik - please note when starting VM in a preview mode we should
> > mount the disks in read-only mode (if supported).

I don't understand this, can you please elaborate why and in which case? 
  The disk is plugged/unplugged? 
  What happens when you commit? It becomes r/w?

> > 
> > 
> > Livnat
> > 
> > 
> > 
> 
> +1 for option 3
> 

+1 for option 3 as well (also good with option 1, but I think this will hinder usability).


Regards,
Mike