Re: Re-reading domain configs on domain restart

[Ian Campbell <Ian.Campbell@citrix.com>]

On Fri, 2012-03-02 at 10:04 +0000, George Dunlap wrote:
> On Thu, Feb 23, 2012 at 3:16 PM, Ian Campbell <Ian.Campbell@citrix.com> wrote:
> > On Thu, 2012-02-23 at 11:17 +0000, Ian Campbell wrote:
> >> As another (hopefully simple) idea how about a "xl dom-set-config" or
> >> similar which (only) updates the config stashed in the userinfo to be
> >> used on reboot? I would specifically exclude the possibility of this
> >> reconfiguring the running domain for simplicity.
> >
> > I've only lightly tested the following, but it seemed to do what I
> > expected (I used it to change memory from 512 to 1024 for a Windows VM
> > on reboot).
> >
> > I think there might be a better name, that better reflects the fact that
> > it doesn't actually change the config right now, any ideas?
> > "domain-config-override"?
> >
> > Thoughts on the general concept?
> 
> Is the config used for anything other than a reboot?  If so, it might
> lead to unexpected behavior, if for example, I want to change config
> option X on reboot, but it's read (and acted on) before I get done
> with my various changes.  It would be good if the user could be sure
> that none of the changes would have any effect until the next reboot.

The option takes a whole new configuration file so you are either using
the old or new configuration. There should be no intermediate state.

AFAIK this file is used for any domain "restart", which includes reboot,
save+restore and migration.

I think that any such changes which you might want to apply on reboot
you would also want to apply post migration or restore too.

I think the right long term solution to this is	
	libxl_running_domain_to_config(ctx, domid, &libxl_domain_config)
but that isn't going to happen for 4.2 at least and this is a good
bandaid plus a useful generic facility.

> I think the concept of having a "config-on-reboot" is a good one.  For
> convenience, would it make sense to add a parameter to "xl reboot" to
> take an argument (-c for example) that will do the config override
> before issuing the reboot command?

We have this for migrate and restore so I think extending that to reboot
is a sensible idea.

Ian.