On 2012-04-01 12:31, Hoppetauet wrote: > Hello > > I'm running some benchmarks on virtual machines > I made a script that runs fio N times, with the following job file > > [seqwrite] > rw=write > size=${SIZE} > directory=${DIRECTORY} > bs=${BS} > overwrite=1 > refill_buffers > > The first run gives about 30MB/s, which is what dd tells me is correct > for the disk at hand > however, from the second to last runs, I get about double that, which > suggests some sort of caching effect > > Is the data that's written to the file not random? I thought > refill-buffers and overwrite would ensure that It is completely random data, and it's reseeded for every run. So with the above job, there shouldn't be any chance to de-dupe or compress anything. Maybe it's the layout? Fio defaults to using the same sequence of random offsets everytime, to make a given run repeatable. You can set randrepeat=0 to turn that off. That'll cause fio to random seedly the IO offset generator as well, making the written patterns different from run to run as well. -- 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