On Mon, Feb 13, 2012 at 11:39 AM, Prateek Sharma <prateek3...@gmail.com> wrote: > On Mon, Feb 13, 2012 at 4:53 PM, Stefan Hajnoczi <stefa...@gmail.com> wrote: >> On Sat, Feb 11, 2012 at 9:57 AM, Prateek Sharma <prateek3...@gmail.com> >> wrote: >>> $QEMU -cpu core2duo,+vmx -drive file=$VM_PATH,if=virtio,aio=native >>> -drive file=viotest.img,if=virtio,index=2 >> >> -drive cache=none is typically used for good performance when the >> image is on a local disk. Try that and I think you'll see an >> improvement. >> >> Stefan > > Hi Stefan, > I did try setting cache=none in one of the runs, and saw a small > performance *drop* for sequential reads. Could it be because of the > host page-cache read-ahead and other factors? > In any case, i just wanted to know what the current qemu > virtio-blk numbers are, and whether i have misconfigured things badly. > What is the "fastest" way to do IO in qemu? virtio-blk, vhost-blk, > virtio-dataplane, something else?
The fastest support way on local disks tends to be if=virtio,cache=none,aio=native. You are right that a pure read benchmark will "benefit" from read-ahead. cache=none helps for writes (compared to the default cache=writethrough) and has less complicated performance behavior when there is a lot of I/O going on (because it bypasses the page cache). It would be interesting to compare the block I/O requests during a bare metal run with your guest run. Normally they should be identical for the benchmark to be fair. I'm not sure whether the I/O request pattern is identical in your case (I haven't looked what hdparm -tT does exactly). Stefan
