[dm-devel] [PATCH 0/4] dm-latency: Introduction

[Mikulas Patocka]

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