per-entity statistics

[Jose R. Santos]

Martin Peschke wrote:

>Hi,
>
>I am interested in SCSI related statitics. I have been looking
>through available tapsets. I have got some questions and I
>would appriciate your thoughts.
>
>First, certain data, like request latencies, is most useful
>if being provided per device, or per LUN respectively, I guess.
>My question is what kind of systemtap variable might suit this
>requirement best.
>
>Tapsets like http://sourceware.org/ml/systemtap/2005-q3/msg00507.html
>use systemtap's assoziative arrays for similar purposes.
>I could imagine an array in order to be able to store data for each
>LUN separately.
>
>Would this approach work if I want to maintain more than a single
>counter per LUN? Looks like going this path might result in an
>array of arrays, say, an array of latency histograms. Is this
>feasible?
>
>Would it be better to maintain separate global variables for
>each LUN? Something like:
>
>global latencies_lun1
>global latencies_lun2
>global latencies_lun3
>etc.
>
>But this would require the script, i.e. the number of global arrays,
>to be adapted for each measurement. Has someone played with doing this
>in an automated fashion? A number of entities as input, and out comes
>an appropriate script.
>
>This leads to my other questions, which are about selecting LUNs
>one wants to do measurements for. The scripts I have come across
>so far and which are capable of providing per-entity statistics,
>e.g. per-process statistics, measure all entities, or all processes
>respectively.
>
>Assuming I want to measure 3 LUNs attached through adapter A, but
>I don't want to strain my system measuring all the other 23 LUNs
>attached through other adapters, how could I do that? I guess,
>my probe would need to check everytime it has fired whether
>it's being active for a LUN to be measured before continueing.
>
>I assume the LUNs to be measured would be described using some array.
>Then my probe would try to match LUNs using that array.
>That wouldn't buy much efficiency, would it? Any other idea?
>
>Besides, is there a good way to pass LUN selections to a
>systemtap script? Or would it be best, as mentioned above,
>to regenerate a script with the selection stuff being
>built in?
>
>Martin
>  
>
Hi Martin,

This is one of the scenarios we are designing our trace tool for.  While 
it does not meet you requirement to just probe a selected number of 
devices, something like that would be easy to hack  into the current 
implementation since we already take SCSI host number, channel, lun and 
id.  We also have hooks for the IO schedulers and system call and have 
plans to put hooks into SCSI drivers.  For some of the workloads that we 
are want use this tool, we need to be able to measure latencies from the 
moment the IO was submitted by the application.

-JRS