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

[Omer Frenkel]

----- 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).
> 
> 
> Livnat
> 
> 
> 
> 
> 
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> 

+1 for option 3