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
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