Messages in this thread | | | Date | Tue, 1 Aug 2006 21:39:48 +0200 | From | Jens Axboe <> | Subject | Re: Accuracy of disk statistics IO counter |
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On Tue, Aug 01 2006, Mark Seger wrote: > > >>Specifically, I wrote a 1GB file with a blocksize of 1MB, which would > >>result in 1000 writes at the application level. What I believe then > >>happens is that each write turns into 8 128KB requests to the driver, > >>which should result in 8000 actual writes. Toss in metadata operations > >>and who knows what else and the actual number should be a little > >>higher. What I've see after repeating the tests a number of times on > >>2.6.16-27 is numbers ranging from 6800-7000 writes which feels like a > >>big enough difference to at least point out. > >> > >> > > > >Install http://brick.kernel.dk/snaps/blktrace-git-20060723022503.tar.gz > >and blktrace your disk for the duration of the test and compare the io > >numbers. Requires 2.6.17 or later, though. > > > > > I had problems getting blktrace going and posted a note to > linux-btrace@vger.kernel.org at Alan Brunelle's suggestion and he also
Documentation issues, sorry about that. It hasn't been updated for the debugfs change.
> said he'd take a closer look at it himself. On the other hand he > pointed me to 'stap' and I was able to use it to get the details I was > looking for - SystemTAP really rocks for this type of analysis! As it > turns out my assumption about driver blocksize was wrong (sorry about > that). It turns out that the size of requests from the driver to the > disk is 160mb and so the I/O count was smaller than I had anticipated.
160KiB, I'm assuming? :-)
-- Jens Axboe
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