Messages in this thread |  | | | Date | Sat, 29 Jul 2006 12:42:32 +0200 | | From | Jens Axboe <> | | Subject | Re: Accuracy of disk statistics IO counter |
| |
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
- To unsubscribe from this list: send the line "unsubscribe linux-kernel" in the body of a message to majordomo@vger.kernel.org More majordomo info at http://vger.kernel.org/majordomo-info.html Please read the FAQ at http://www.tux.org/lkml/
|  |