Re: [Xen-devel] What runtime states to be preserved across save / restore?

[Wei Liu <wei.liu2@xxxxxxxxxx>]

On Tue, May 06, 2014 at 10:32:09AM +0100, George Dunlap wrote:
> On Tue, Apr 29, 2014 at 11:42 AM, Wei Liu <wei.liu2@xxxxxxxxxx> wrote:
> > On Tue, Apr 29, 2014 at 12:40:15PM +0200, Daniel Kiper wrote:
> >> On Tue, Apr 29, 2014 at 10:05:26AM +0100, Wei Liu wrote:
> >> > On Tue, Apr 29, 2014 at 09:29:10AM +0200, Daniel Kiper wrote:
> >> > > On Mon, Apr 28, 2014 at 03:16:17PM +0100, Ian Campbell wrote:
> >> > > > On Mon, 2014-04-28 at 14:57 +0100, Wei Liu wrote:
> >> > > >
> >> > > > > > Other things I can think of right now:
> >> > > > > >
> >> > > > > >       * The devid of each device -- for some classes these can be
> >> > > > > >         automatically assigned, I'm not really sure if that needs
> >> > > > > >         preserving or not.
> >> > > > >
> >> > > > > I don't think so. The receiving end should make its own decision.
> >> > > >
> >> > > > Having eth0 become eth1 over migrate would be surprising though,
> >> > > > wouldn't it?
> >> > > >
> >> > > > > >       * Hotplug of devices more generally
> >> > > > > >
> >> > > > >
> >> > > > > Scripts? If so, they, with the things you mentioned below ...
> >> > > >
> >> > > > Not script, I meant e.g. xl block-attach and stuff like that.
> >> > >
> >> > > Do not forget about memory hotplug and in general memory != maxmem.
> >> > > This stuff is also important in migration case.
> >> > >
> >> >
> >> > I think max memory and target memory are easy -- I can retrieve them
> >> > from xenstore. But where can I get information about memory hotplug?
> >> > If the relevant bits are missing in libxl then I think we need another
> >> > series to address this problem.
> >>
> >> I think that target and max memory should be sufficient. I mentioned
> >
> > Cool!
> >
> >> about that because when I tested migration with xm a few years ago
> >> I was not able to migrate machine which had hotplugged memory. IIRC
> >> it happened because migration process refered to config file instead
> >> of checking relevant values in xenstore. Acording to my knowledge save
> >> and restore machinery is (or was) used for migration. Correct me
> >> if I am wrong.
> >>
> >
> > You're right about this. ;-)
> 
> Hmm, speaking of which, you may want to check the Xen-enforced limit
> as well.  Some toolstacks (such as xapi, and I believe xl) will
> optionally lower a VM's memory limit in Xen, so that a VM cannot
> balloon back up once it's ballooned down.  However, other toolstacks
> (such as Oracle's, as I understand it) allow the VMs to balloon
> themselves up if they want, and so don't set the limit.  Whatever the
> limit is should probably be copied across by default.
> 

I grep'ed libxl code. If I'm not mistaken, the "Xen-enforced limit" is
done by setting max memory target, which is already preserved in current
scheme.

Wei.

>  -George

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