Re: LAST_CHECKPOINT and save/restore/migrate compatibility (was Re: [Xen-devel] [PATCH 3 of 4] libxc: provide notification of final checkpoint to restore end)

[Shriram Rajagopalan <rshriram@xxxxxxxxx>]

On Sat, Apr 2, 2011 at 1:13 AM, Ian Campbell <Ian.Campbell@xxxxxxxxxxxxx> wrote:

On Sat, 2011-04-02 at 04:57 +0100, Shriram Rajagopalan wrote:
> On 2011-04-01, at 6:58 AM, Ian Campbell <Ian.Campbell@xxxxxxxxxxxxx> wrote:

> > Is it the case that Remus only cares about checkpointing between like
> > versions of the toolstack?
> >
> Can you please elaborate this statement ?

For standard suspend/resume or migration we support migrating from
version N to version N+1 (but not vice versa), to support upgrades.

In the case of Remus though are we interested in supporting a rolling
checkpoint from a version N system to a version N+1 fallback system? Or
is Remus only interested in supporting checkpoints between systems
running the same version of Xen?

N->N+1 should work, for Remus even now, without the LAST_CHECKPOINT patch.
 But as you said, failback from N+1 -> N wont work.

I agree with George's suggestion on MORE_CHECKPOINTS, for backwards compatibility
wrt live migration. But for HA, it doest make much sense if a user is able to do HA only
one way and cannot failback. This is not a upgrade scenario. So, that would require some
exception to be thrown when a version incompatibility is detected. IMO, its better to
let the user handle this limitation than letting him/her do the N->N+1 HA and then finding out
that they cannot failback.


shriram

Bear in mind that if you did support N->N+1 checkpoints you wouldn't be
able to migrate the VM back after a failover...

FWIW I think George got to the bottom of the specific issue he was
seeing and that the LAST_CHECKPOINT thing was a red-herring, although
flipping the protocol to use a MORE_CHECKPOINTS schema perhaps makes
sense anyway.

Ian.

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