Re: Accuracy of disk statistics IO counter

[Jens Axboe <axboe@suse.de>]

On Fri, Jul 28 2006, Mark Seger wrote:
> Awhile back I had suggested moving the place in the block driver logic 
> where stats get updated to more accurately reflect what was happening at 
> the time they were actually sent to the drivers and I believe they were 
> indeed made in the 2.6.15 timeframe.  I've recently started taking 
> closer look at the numbers and while the sector counts look correct I'm 
> not sure I'd agree with the number of I/Os being reported and believe 
> they can be off by maybe 15% or more.
> 
> 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.

-- 
Jens Axboe