On Thu, Oct 13, 2011 at 4:27 PM, Stefan Hajnoczi <stefanha@xxxxxxxxx> wrote: > 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. You are right the change is impressive, but still there is a big difference: dd oflag=direct bs=6M count=1000 if=/dev/zero of=titi.txt i get on the host: 6291456000 bytes (6.3 GB) copied, 29.8403 s, 211 MB/s and in the vm: 6291456000 bytes (6.3 GB) copied, 51.3302 s, 123 MB/s Thanks, Benoit. -- 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
