This is an archived copy of the Xen.org mailing list, which we have preserved to ensure that existing links to archives are not broken. The live archive, which contains the latest emails, can be found at http://lists.xen.org/
Home Products Support Community News


Re: LAST_CHECKPOINT and save/restore/migrate compatibility (was Re: [Xen

To: Ian Campbell <Ian.Campbell@xxxxxxxxxxxxx>
Subject: 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)
From: Shriram Rajagopalan <rshriram@xxxxxxxxx>
Date: Sat, 2 Apr 2011 07:03:02 -0700
Cc: xen-devel@xxxxxxxxxxxxxxxxxxx
Delivery-date: Sat, 02 Apr 2011 07:04:36 -0700
Envelope-to: www-data@xxxxxxxxxxxxxxxxxxx
In-reply-to: <1301731990.3516.6.camel@xxxxxxxxxxxxxxxxxxxxx>
List-help: <mailto:xen-devel-request@lists.xensource.com?subject=help>
List-id: Xen developer discussion <xen-devel.lists.xensource.com>
List-post: <mailto:xen-devel@lists.xensource.com>
List-subscribe: <http://lists.xensource.com/mailman/listinfo/xen-devel>, <mailto:xen-devel-request@lists.xensource.com?subject=subscribe>
List-unsubscribe: <http://lists.xensource.com/mailman/listinfo/xen-devel>, <mailto:xen-devel-request@lists.xensource.com?subject=unsubscribe>
References: <AANLkTik=XfbVbDzEGb3KL-UyN3oWVBvkrj4t+G9Fj_hf@xxxxxxxxxxxxxx> <1301659093.27123.227.camel@xxxxxxxxxxxxxxxxxxxxxx> <068F4087-A4CD-4EB7-B752-780A663D1BC1@xxxxxxxxx> <1301731990.3516.6.camel@xxxxxxxxxxxxxxxxxxxxx>
Reply-to: rshriram@xxxxxxxxx
Sender: xen-devel-bounces@xxxxxxxxxxxxxxxxxxx
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.

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.


Xen-devel mailing list