Re: [Qemu-devel] Re: KVM call agenda for Apr 27

[Anthony Liguori]

On 04/27/2010 03:53 AM, Kevin Wolf wrote:
> Am 27.04.2010 00:36, schrieb Anthony Liguori:
>    
> > On 04/26/2010 05:12 PM, Chris Wright wrote:
> >      
> > > * Anthony Liguori (address@hidden) wrote:
> > >
> > >        
> > > > On 04/26/2010 12:26 PM, Chris Wright wrote:
> > > >
> > > >          
> > > > > Please send in any agenda items you are interested in covering.
> > > > >
> > > > > While I don't expect it to be the case this week, if we have a
> > > > > lack of agenda items I'll cancel the week's call.
> > > > >
> > > > >            
> > > > - qemu management interface (and libvirt)
> > > > - stable tree policy (push vs. pull and call for stable volunteers)
> > > >
> > > >          
> > > block plug in (follow-on from qmp block watermark)
> > >
> > >        
> > A few comments:
> >
> > 1) The problem was not block watermark itself but generating a
> > notification on the watermark threshold.  It's a heuristic and should be
> > implemented based on polling block stats.  Otherwise, we'll be adding
> > tons of events to qemu that we'll struggle to maintain.
> >      
> Polling just feels completely wrong. You're almost guaranteed to poll in
> the wrong intervals because depending on what the guest is doing you
> might need it every couple of seconds (installation) or you may not need
> it in days (just working on a fully allocated image).
>    

The event basically boils down to: when some threshold is reached, raise 
an event.  What statistics go into this computation and what algorithm 
is used to compute the threshold depends on the management tool.

The memory ballooning daemon we're developing basically does nothing but 
this.  It monitors stats from the guest to generate events when we've 
reached a certain memory condition that will likely require some action 
(like ballooning).  Why would we do the block watermarking in qemu and 
not do the ballooning heuristics too?

I know of quite a few tools that all do some form of this.

> > 2) A block plugin doesn't solve the problem if it's just at the
> > BlockDriverState level because it can't interact with qcow2.
> >      
> There's no interaction with qcow2 needed, it would just need to remember
> the highest offset it was requested to read or write. But I'd really
> hate to stick another protocol between qcow2 and file.
>
> It doesn't solve the problem nicely though because it still needs to
> generate some event which is not present in a normal qemu without the
> plugin.
>    

Polling is really the right solution.  It gives the management tool 
ultimate flexibility in tweaking the heuristics as they see fit.

Regards,

Anthony Liguori

> Kevin
>