Re: Accuracy of disk statistics IO counter

[Jens Axboe]

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@xxxxxxxxxxxxxxx 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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