On Wed, Feb 16, 2011 at 12:50 PM, Thomas Broda <thomas@xxxxxxxxxxxxx> wrote: > On Tue, 15 Feb 2011 15:50:00 +0000, Stefan Hajnoczi > <stefanha@xxxxxxxxx> wrote: > > On Tue, Feb 15, 2011 at 10:15 AM, Thomas Broda <thomas@xxxxxxxxxxxxx> > wrote: > Using O_DIRECT, performance went down to 11 MB/s on the hypervisor... > > > Hmm...can you restate that as: > > host X MB/s > guest Y MB/s > > Trying dd with "oflag=direct" an "of=/dev/vg0/lvtest" (directly on the > KVM hypervisor) yielded a result of 11MB/s. > > If I try this on the guest with /dev/vda1 as output device, results are > between 1.9MB/s and 7.7MB/s, usually around 3.5MB/s. > > To sum it up: > > Host: 11 MB/s > Guest: 3.5 MB/s > > I've checked the RAID controller in the meantime. It's a HP Smart Array > P400. Write Caching is switched off since the contoller has no BBU > (yet). > > Could it be related to this? The disabled write cache will result in slow writes so your host benchmark result is low on an absolute scale. However, the relative Guest/Host performance is very poor here (3.5/11 = 31%). A number of performance improvements have been made to KVM and Centos 5.5 does not contain them because it is too old. If you want to see a more current reflection of KVM performance, you could try Fedora 14 host and guest. The components that matter are: host kernel, qemu-kvm userspace, and guest kernel. 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
