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

[Daniel P. Berrange]

On Tue, Apr 27, 2010 at 04:15:54PM +0300, Gleb Natapov wrote:
> On Tue, Apr 27, 2010 at 02:11:46PM +0100, Daniel P. Berrange wrote:
> > On Tue, Apr 27, 2010 at 08:03:42AM -0500, Anthony Liguori wrote:
> > > On 04/27/2010 03:14 AM, Avi Kivity wrote:
> > > >On 04/27/2010 01:36 AM, Anthony Liguori wrote:
> > > >>
> > > >>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. 
> > > >
> > > >Polling for an event that never happens is bad engineering.  What 
> > > >frequency do you poll?  you're forcing the user to make a lose-lose 
> > > >tradeoff.
> > > >
> > > >>Otherwise, we'll be adding tons of events to qemu that we'll struggle 
> > > >>to maintain.
> > > >
> > > >That's not a valid reason to reject a user requirement.  We may argue 
> > > >the requirement is bogus, or that the suggested implementation is 
> > > >wrong and point in a different direction, but saying that we may have 
> > > >to add more code in the future due to other requirements is ... well I 
> > > >can't find a word for it.
> > > 
> > > Polling is the best solution because it offers the most flexibility.  
> > > Baking the heuristic into qemu just removes flexibility for all consumers.
> > 
> > Polling as the added advantage that you can recover better if the
> > app talking to QMP is offline for a period. eg if libvirt were 
> > disconnected from QMP at the time the high watermark event were
> > triggered, the next you'll know is a ENOSPACE event. If the app
> > were able to poll on the allocation value, then it could immediately
> > see the watermark had been passed the first time it polled after
> > libvirt reconnected to QMP. As you say its also more flexible because
> > you can invent a usage where you have 2 or 3 watermarks where you
> > could try harder to get more space as you pass each watermark.
> > 
> When libvirt reconnects it should poll once and then wait for
> notification. If you want to have several watermarks configure
> first one and after getting notification about it configure
> second one and so on.

So regardless of whether polling or events are 'best', we need to have the
pollable QMP command implemented to get rid of the potential for a missed 
event to a watermark threshold that has already past. The same race problem 
exists with updating the thresholds on the fly as one is passed.

Daniel
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