There are quite a few options for backing store for kvm virtual
machines. I’ve started benchmarking just a few of the possible
combinations. For starters, I installed a stock headless ubuntu
precise system from the mini network installer. It was running
the generic kernel, with no x-windows and only the virtualization
host package set selected through tasksel. I ran all VMs with
1024M ram, vnc, and the user network – which is fine as I was
only running kernbench.
The specific test I ran was the phoronix pts/build-linux-kernel
test, which runs 3 compiles and gives you the average time and
standard error and standard deviation.
The only thing I varied was the backing store. I used a raw
partition, and lvm partition, and raw, qcow2, and qed image
files (all un-allocated, that is, sparse). The qcow2 and qed
images I tried both on ext4 and xfs filesystems. The VM in
all cases was partitioned with the default ext4 fs.
I ran the test (with three iterations) twice with each setup.
| drive option | Average time | Standard error | Standard deviation |
| -drive file=raw.img,if=virtio,cache=none (raw.img on ext4) | 1004.07 | 1.24 | 0.21% |
| -drive file=raw.img,if=virtio,cache=none (raw.img on ext4) | 1003.54 | 2.48 | 0.43% |
| -hda /dev/sda7 | 1003.77 | 0.93 | 0.16% |
| -hda /dev/sda7 | 1006.18 | 1.21 | 0.21% |
| -drive file=/dev/sda7,if=virtio,cache=none | 1003.77 | 0.93 | 0.16% |
| -drive file=/dev/sda7,if=virtio,cache=none | 1005.66 | 2.08 | 0.36% |
| -drive file=/dev/schroot/kvm,if=virtio,cache=none | 1002.84 | 1.39 | 0.24% |
| -drive file=/dev/schroot/kvm,if=virtio,cache=none | 1008.85 | 1.51 | 0.26% |
| -drive file=/dev/schroot/kvm,if=virtio,cache=writethrough | 1002.21 | 2.05 | 0.36% |
| -drive file=/dev/schroot/kvm,if=virtio,cache=writethrough | 1004.07 | 1.31 | 0.23% |
| -drive file=/dev/qcow2.img,if=virtio,cache=none (qcow2.img on ext4) | 1007.84 | 1.29 | 0.22% |
| -drive file=/dev/qcow2.img,if=virtio,cache=none (qcow2.img on ext4) | 1011.28 | 1.17 | 0.20% |
| -drive file=/dev/qed.img,if=virtio,cache=none (qed.img on ext4) | 1018.18 | 0.86 | 0.15% |
| -drive file=/dev/qed.img,if=virtio,cache=none (qed.img on ext4) | 1012.69 | 1.14 | 0.20% |
| -drive file=/dev/qcow2.img,if=virtio,cache=none (qcow2.img on xfs) | 1006.70 | 1.53 | 0.26% |
| -drive file=/dev/qcow2.img,if=virtio,cache=none (qcow2.img on xfs) | 1009.96 | 1.38 | 0.24% |
| -drive file=/dev/qed.img,if=virtio,cache=none (qed.img on xfs) | 1005.70 | 0.80 | 0.14% |
| -drive file=/dev/qed.img,if=virtio,cache=none (qed.img on xfs) | 1009.56 | 1.14 | 0.20% |
| -drive file=/dev/qcow2.img,if=virtio,cache=writethrough (qcow2.img on xfs) | 1002.05 | 1.91 | 0.33% |
| -drive file=/dev/qcow2.img,if=virtio,cache=writethrough (qcow2.img on xfs) | 1003.56 | 1.85 | 0.32% |
I had a few snafus with reinstallations, so hopefully next time I can do
this faster, but overall this took me about a week on one laptop with an
intel core2 duo T6670 @ 2.20Ghz. I’m hoping to have two of these in a few
weeks so I can do more tests simultaneously.
I’ll leave analysis out of this post for now, and just dump these numbers.
I know, I’d rather report mean+/-95%CI, for each set of 6, but the numbers
I’m reporting were nicely provided (as a .png) as is by the testsuite,
and I was short on time…
