On Tue, Feb 02 2010, Jens Axboe wrote: > On Tue, Feb 02 2010, Bart Van Assche wrote: > > On Tue, Feb 2, 2010 at 9:19 AM, Jens Axboe <jens.axboe@xxxxxxxxxx> wrote: > > > On Tue, Feb 02 2010, Bart Van Assche wrote: > > >> The reason I started running such silly tests is because I noticed > > >> that tests with dd and a small block size complete in a shorter time > > >> than tests with fio for a fast storage device (e.g. remote RAM disk > > >> accessed via SRP or iSER). Do the two tests below trigger similar > > >> system calls ? The ratio of fio time / dd time is about 1.50 for block > > >> size 512 and about 1.15 for block size 4096. > > > > > > Fio definitely has more overhead than a simple read() to buf, write buf > > > to /dev/null. If you switch off the stat calculations, it'll drop > > > somewhat (use --gtod_reduce=1). But even then it's going to be slower > > > than dd. Fio is modular and supports different IO engines etc, so the IO > > > path is going to be a lot longer than with dd. The flexibility of fio > > > does come at a cost. If you time(1) fio and dd, you'll most likely see a > > > lot more usr time in fio. > > > > > > That said, it is probably time to do some profiling and make sure that > > > fio is as fast as it can be. > > > > That would definitely be appreciated. I would like to switch from dd > > to fio for storage system benchmarking, something I can't do yet > > because of the different results reported by the two tools. > > So the first thing I noticed is that you get an lseek() because fio > doesn't track the sequential nature of that job. How close do you get > for bs=512 with using --gtod_reduce=1 and commenting out the lseek() in > engines/sync.c:fio_syncio_prep()? I committed a fix for this, it'll do it automatically now if you use latest -git. A quick test here on my desktop machine: fio --bs=4k --size=64G --buffered=1 --rw=write --verify=0 --name=/dev/null --gtod_reduce=1 --disk_util=0 10378MB/s fio --bs=512 --size=8G --buffered=1 --rw=write --verify=0 --name=/dev/null --gtod_reduce=1 --disk_util=0 1306MB/s dd if=/dev/zero of=/dev/null bs=4k count=16M 68719476736 bytes (69 GB) copied, 9,71235 s, 7,1 GB/s dd if=/dev/zero of=/dev/null bs=512 count=8M 4294967296 bytes (4,3 GB) copied, 3,3574 s, 1,3 GB/s So the same for 512b buffers, fio is much quicker for 64k (must be due to proper aligning). I'll boot the big box and see what that says. -- Jens Axboe -- To unsubscribe from this list: send the line "unsubscribe fio" in the body of a message to majordomo@xxxxxxxxxxxxxxx More majordomo info at http://vger.kernel.org/majordomo-info.html
