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

[Yair Zaslavsky]

On 01/19/2012 10:32 AM, Mike Kolesnik wrote:
> 
> 
> ----- 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).
I agree with Mike - I think option one is too "aggressive" in protecting
us, this is why I prefer 3.  +1 on option 3
> 
> 
> Regards,
> Mike