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

[Livnat Peer]

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