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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:
    > 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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