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    SubjectRe: [Patch 0/5] I/O statistics through request queues
    On Wed, Oct 25 2006, Martin Peschke wrote:
    > >>>I have to say it's news to
    > >>>me that it's performance intensive, tests I did with Alan Brunelle a
    > >>>year or so ago showed it to be quite low impact.
    > >>I found some discussions on linux-btrace (Feburary 2006).
    > >>There is little information on how the alleged 2 percent impact has
    > >>been determined. Test cases seem to comprise formatting disks ...hmm.
    > >
    > >It may sound strange, but formatting a large drive generates a huge
    > >flood of block layer events from lots of io queued and merged. So it's
    > >not a bad benchmark for this type of thing. And it's easy to test :-)
    >
    > Just wondering to what degree this might resemble I/O workloads run
    > by customers in their data centers.

    It wont of course, the point is to generate a flood of events to put as
    much pressure on blktrace logging as possible. Dirtying tons of data
    does that.

    > >>>You'd be silly to locally store traces, send them out over the network.
    > >>Will try this next and post complaints, if any, along with numbers.
    > >
    > >Thanks! Also note that you do not need to log every event, just register
    > >a mask of interesting ones to decrease the output logging rate. We could
    > >so with some better setup for that though, but at least you should be
    > >able to filter out some unwanted events.
    >
    > ...and consequently try to scale down relay buffers, reducing the risk of
    > memory constraints caused by blktrace activation.

    Pretty pointless, unless you are tracing lots of disks. 4x128kb gone
    wont be a showstopper for anyone.

    > >>However, a fast network connection plus a second system for blktrace
    > >>data processing are serious requirements. Think of servers secured
    > >>by firewalls. Reading some counters in debugfs, sysfs or whatever
    > >>might be more appropriate for some one who has noticed an unexpected
    > >>I/O slowdown and needs directions for further investigation.
    > >
    > >It's hard to make something that will suit everybody. Maintaining some
    > >counters in sysfs is of course less expensive when your POV is cpu
    > >cycles.
    >
    > Counters are also cheaper with regard to memory consumption. Counters
    > are probably cause less side effects, but are less flexible than
    > full-blown traces.

    And the counters are special cases and extremely inflexible.

    --
    Jens Axboe

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