On Wed, Oct 12, 2011 at 1:21 PM, benoit ROUSSELLE <brousselle@xxxxxxxxx> wrote: > I am doing some disk io performance testing on the following environment: > Dell AMD R515 with: > - debian6 (2.6.32) on host and guests. > - raid1 mirror on a perc h700 > - lvm used to create virtual disks volumes > - virtio enabled per default on 2.6.32 and used for nic and disk drivers > > For my tests i use the following command: > dd bs=6M count=500 if=/dev/zero of=toto.txt > > On local host i get 500MB/s > On 1 guest 150MB/s > On 2 guests in parallel 70MB/s dd performs buffered I/O by default. That means it just writes to the page cache and the kernel decides when to write out dirty pages. So your host probably has a bunch more RAM than the guest - dd write(2) calls are simply dirtying memory. Your guest has less RAM and needs to do actual block I/O. That's why the results are so different. You are not measuring virtio-blk performance here. Use dd oflag=direct to bypass the page cache and actually do block I/O. Stefan -- To unsubscribe from this list: send the line "unsubscribe kvm" in the body of a message to majordomo@xxxxxxxxxxxxxxx More majordomo info at http://vger.kernel.org/majordomo-info.html
