On Tue, Apr 27, 2010 at 02:38:17PM +0100, Daniel P. Berrange wrote: > 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. > Of course. Polling command is needed in any case. -- Gleb. -- To unsubscribe from this list: send the line "unsubscribe kvm" in the body of a message to majordomo@xxxxxxxxxxxxxxx More majordomo info at http://vger.kernel.org/majordomo-info.html
