[openstack-dev] [nova] a question about instance snapshot

Jay Pipes jaypipes at gmail.com
Tue Mar 11 19:15:48 UTC 2014

On Tue, 2014-03-11 at 06:35 +0000, Bohai (ricky) wrote:
> > -----Original Message-----
> > From: Jay Pipes [mailto:jaypipes at gmail.com]
> > Sent: Tuesday, March 11, 2014 3:20 AM
> > To: openstack-dev at lists.openstack.org
> > Subject: Re: [openstack-dev] [nova] a question about instance snapshot
> >
> > On Mon, 2014-03-10 at 12:13 -0400, Shawn Hartsock wrote:
> > > We have very strong interest in pursing this feature in the VMware
> > > driver as well. I would like to see the revert instance feature
> > > implemented at least.
> > >
> > > When I used to work in multi-discipline roles involving operations it
> > > would be common for us to snapshot a vm, run through an upgrade
> > > process, then revert if something did not upgrade smoothly. This
> > > ability alone can be exceedingly valuable in long-lived virtual
> > > machines.
> > >
> > > I also have some comments from parties interested in refactoring how
> > > the VMware drivers handle snapshots but I'm not certain how much that
> > > plays into this "live snapshot" discussion.
> >
> > I think the reason that there isn't much interest in doing this kind of thing is
> > because the worldview that VMs are pets is antithetical to the worldview that
> > VMs are cattle, and Nova tends to favor the latter (where DRS/DPM on
> > vSphere tends to favor the former).
> >
> > There's nothing about your scenario above of being able to "revert" an instance
> > to a particular state that isn't possible with today's Nova.
> > Snapshotting an instance, doing an upgrade of software on the instance, and
> > then restoring from the snapshot if something went wrong (reverting) is
> > already fully possible to do with the regular Nova snapshot and restore
> > operations. The only difference is that the "live-snapshot"
> > stuff would include saving the memory view of a VM in addition to its disk state.
> > And that, at least in my opinion, is only needed when you are treating VMs like
> > pets and not cattle.
> >
> Hi Jay,
> I read every words in your reply and respect what you said.
> But i can't agree with you that memory snapshot is a feature for pat not for cattle.
> I think it's a feature whatever what do you look the instance as.
> The world doesn't care about what we look the instance as, in fact, currently almost all the
> mainstream hypervisors have supported the memory snapshot.
> If it's just a dispensable feature and no users need it, I can't understand why
> the hypervisors provide it without exception.
> In the document " OPENSTACK OPERATIONS GUIDE" section " Live snapshots" has the
> below words:
> " To ensure that important services have written their contents to disk (such as, databases),
> we recommend you read the documentation for those applications to determine what commands
> to issue to have them sync their contents to disk. If you are unsure how to do this,
>  the safest approach is to simply stop these running services normally.
> "
> This just pushes all the responsibility to guarantee the consistency of the instance to the end user.
> It's absolutely not convenient and I doubt whether it's appropriate.

Hi Ricky,

I guess we will just have to disagree about the relative usefulness of
this kind of thing for users of the cloud (and not users of traditional
managed hosting) :) Like I said, if it does not affect the performance
of other tenants' instances, I'm fine with adding the functionality in a
way that is generic (not hypervisor-specific).


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