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

[Bryn M. Reeves]

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.