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Date: Mon, 8 Apr 2013 20:11:08 -0600
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Subject: Re: [DISCUSS] pause VM
From: Marcus Sorensen <shadowsor@gmail.com>
To: dev@cloudstack.apache.org
Cc: cloudstack-dev@incubator.apache.org
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Perhaps. I think there are separate issues here. There is volume snapshot
and vm snapshot. If the hypervisor is capable of making volumes consistent
, but VM snapshot is more about saving running state, then the two may not
serve the same purpose when leveraging san-based snapshots. Its absolutely
possible to pause the VM, quiesce the disks, and then snapshot them
consistently utilizing underlying storage capabilities.

But we are getting off in the weeds here with a single example, when what I
want to discuss is pausing a VM in general.
On Apr 8, 2013 6:19 PM, "Kelven Yang" <kelven.yang@citrix.com> wrote:

>
>
> 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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