On Thu, 26 Feb 2015, Bryn M. Reeves wrote: > On Thu, Feb 26, 2015 at 11:49:28AM -0500, Mikulas Patocka wrote: > > We have already dm-statistics that counts various events - see > > Documentation/device-mapper/statistics.txt. It counts the nubmer of > > requests and the time spent servicing each request, thus you can > > calculate average latency from these values. > > Right: average service time (as reported by iostat etc.) is easily derived > from the existing stats. > > Does the separate latency accounting buy anything additional? > > > Please look at dm-statistics to see if it fits your purpose. If you need > > additional information not provided by dm-statistics, it would be better > > to extend the statistics code rather than introduce new "latency" > > infrastructure. > > Agreed; I'm working on userspace support for dm-statistics at the moment > and if there is a need for these additional measurements I would greatly > prefer to consume them as additional fields in the existing dm-stats > counter set. > > This also has the advantage of benefiting from the existing step and > area support allowing a device to be subdivided into discrete stats > regions. > > Regards, > Bryn. Coly's paper (http://blog.coly.li/docs/osc13-coly.pdf) shows that they take histogram of latencies and use it to predict disk failure. That could be easily added to dm-statistics. Average latency alone can't be used to predict disk failure because average latency depends on the type of workload (for example - sequantial or nearly sequential requests have much lower latency than random requests). I'd like to know if we need separate histogram per region, or if it is sufficient to have a histogram per device. dm-latency has no regions, it has a histogram for the whole device. The histogram-per-region would consume more memory, I'm interested if there is some reasonable use case for that. Mikulas -- dm-devel mailing list dm-devel@xxxxxxxxxx https://www.redhat.com/mailman/listinfo/dm-devel
