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

[Daniel P. Berrange]

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.


Daniel
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