Sequential write problems

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

The virtual machines are ubuntu oneiric 11.10, and my fio version is 2.06

Thanks

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