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From Kelven Yang <kelven.y...@citrix.com>
Subject Re: [DISCUSS] pause VM
Date Tue, 09 Apr 2013 00:19:27 GMT


On 4/8/13 4:19 PM, "Marcus Sorensen" <shadowsor@gmail.com> wrote:

>Oh yes, I know these things, but here's my point. You said:
>
>"When CloudStack takes a snapshot of volume, it already utilizes snapshot
>feature provided by hypervisor to allow taking point of time storage image
>in a safe manner from operating system point of view."
>
>What about five volumes? Can we ensure they are all snapshotted at that
>same point in time without also pausing the VM, in addition to using those
>hypervisor-provided features?  For example, let's say the VM is using a
>volume manager and striping data across five data disks, we absolutely
>have
>to take all data disk snapshots at the same point in such a scenario. We
>of
>course have to rely on the user to tell us that they want this, but we
>should have the capability.
>
>If we already have a VM snapshot feature that does every connected disk
>simultaneously while the VM is suspended, or the hypervisor can do it for
>us by quickly pausing/snapshotting/resuming seamlessly then fine, that's
>great. It just didn't seem like that functionality existed now, so I was
>wondering if anyone is working on it.

I'm not sure using the tactic to pause a VM for the purpose of ensuring
full data integrity is the right answer for the use case you pointed. As
far as I know for VMware hypervisors, taking a VM snapshot does all the
things your mentioned above (quiesce the guest file system, snapshot
memory etc). Xen Server may be a little different, for crash consistency,
I think we ask user to stop a VM(instead of pausing it) before taking a
volume snapshot.

So the question is, can "pausing VM" offer what we are looking for or is
it a half-completed solution? Forcely-pausing a VM to offer a not ensured
solution to customer is what I'm worrying about.

-Kelven  

>
>That aside, it seems like all of the hypervisors CS controls provide their
>own UI to pause a VM. Is it something that has been discussed?
>
>
>
>
>On Mon, Apr 8, 2013 at 4:16 PM, Kelven Yang <kelven.yang@citrix.com>
>wrote:
>
>>
>>
>> On 4/8/13 1:39 PM, "Marcus Sorensen" <shadowsor@gmail.com> wrote:
>>
>> >Has anyone put together a suggestion for adding a 'paused' state to
>> >virtual
>> >machines? Or is anyone working on it? This seems fairly
>>straightforward,
>> >and I think we may need the functionality going forward. Even if people
>> >rarely use it themselves, if storage solutions are going to offer
>> >snapshots
>> >they may want to pause the VM in order to get an atomic snapshot if
>>the VM
>> >has multiple disks. So at least internally it may be a useful utility.
>>
>> Pause VM itself can not ensure data integrity. True data integrity can't
>> be ensured without participant of guest OS and related application (for
>> example, Microsoft offers Volume Shadow Copy Service to orchestrate and
>> allow applications to participate the process via a set of tools and API
>> interaction to achieve that).
>>
>> When CloudStack takes a snapshot of volume, it already utilizes snapshot
>> feature provided by hypervisor to allow taking point of time storage
>>image
>> in a safe manner from operating system point of view. The ability of
>> pausing VM is not mandatory in this case, although pausing/resuming VM
>>may
>> be useful in certain use case, it does not seem to be in any urgency or
>> fit naturally in general cloud-operations.
>>
>> -Kelven
>>
>> >
>>
>>


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