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SubjectRe: [Patch 0/5] I/O statistics through request queues
Would this be a good candidate to implement using kprobes?  I was under 
the impression that basing instrumentation on kprobes would be a good
thing since you can load the instrumentation code only when needed, then
unload it.

Martin Peschke wrote:
> This patch set makes the block layer maintain statistics for request
> queues. Resulting data closely resembles the actual I/O traffic to a
> device, as the block layer takes hints from block device drivers when a
> request is being issued as well as when it is about to complete.
>
> It is crucial (for us) to be able to look at such kernel level data in
> case of customer situations. It allows us to determine what kind of
> requests might be involved in performance situations. This information
> helps to understand whether one faces a device issue or a Linux issue.
> Not being able to tap into performance data is regarded as a big minus
> by some enterprise customers, who are reluctant to use Linux SCSI
> support or Linux.
>
> Statistics data includes:
> - request sizes (read + write),
> - residual bytes of partially completed requests (read + write),
> - request latencies (read + write),
> - request retries (read + write),
> - request concurrency,
>
> For sample data please have a look at the SCSI stack patch or the DASD
> driver patch respectively. This data is only gathered if statistics have
> been enabled by users at run time (default is off).
>
> The advantage of instrumenting request queues is that we can cover a
> broad range of devices, including SCSI tape devices.
> Having the block layer maintain such statistics on behalf of drivers
> provides for comparability through a common set of statistics.
>
> A previous approach was to put all the code into the SCSI stack:
> http://marc.theaimsgroup.com/?l=linux-kernel&m=115928678420835&w=2
> I gathered from feedback that moving that stuff to the block layer
> might be preferable.
>
> These patches use the statistics component described in
> Documentation/statistics.txt.
>
> Patches are against 2.6.19-rc2-mm2.
>
> [Patch 1/5] I/O statistics through request queues: timeval_to_us()
> [Patch 2/5] I/O statistics through request queues: queue instrumentation
> [Patch 3/5] I/O statistics through request queues: small SCSI cleanup
> [Patch 4/5] I/O statistics through request queues: SCSI
> [Patch 5/5] I/O statistics through request queues: DASD
>
> Martin
>

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