[Bug 550409] Re: Slow disk IO for all guests with all disc modes and types.

[Mathias Gug]

On Tue, Mar 30, 2010 at 03:42:56PM -0000, Ben Selinger wrote:
> 
>  2. start the guests
> Guests are normally started with virt-manager, which results in the following command:
> /usr/bin/kvm -S -M pc-0.12 -cpu qemu32 -enable-kvm -m 512 -smp 1 -name winxp -uuid eb0416e2-54d2-d4cb-936d-331edbe443c0 -chardev socket,id=monitor,path=/var/lib/libvirt/qemu/winxp.monitor,server,nowait -monitor chardev:monitor -localtime -boot c -drive file=/datahauz/storage/software/Windows/EN_WINDOWS_XP_PRO_WITH_SP2.ISO,if=ide,media=cdrom,index=2 -drive file=/datahauz/storage/kvm/tinyxp.img,if=ide,index=0,boot=on,format=raw -net nic,macaddr=52:54:00:0d:11:23,vlan=0,name=nic.0 -net tap,fd=37,vlan=0,name=tap.0 -chardev pty,id=serial0 -serial chardev:serial0 -parallel none -usb -usbdevice tablet -vnc 127.0.0.1:0 -k en-us -vga cirrus
>

Thanks. Could you also paste the kvm command used for the linux guest.
 
>  3. run the benchmarks
> Several benchmarks were used.  Under windows I used IOmeter, and also timed file copies while observing both host and guest disc access.  I did the following file copy tests.
> 1. Scp a 2GB file from the hosts disc, to the guests disc.
> 2. Scp a 2GB file from the hosts memory to the guests disc.
> 3. Scp a 2GB file from the guests disc to the hosts disc.
> 4. Scp a 2GB file from the guests disc to the hosts memory.
> Then I put the guest disc image in /dev/shm and tried the same tests again.
> With the guest disc hosted on /dev/shm, I get slightly better disc speeds (15MBps windows, 30MBps Linux)
>

Using Scp means you're including the network stack in the benchmark. I'd
recommend to use a benchmark running in the guest only (like bonnie+) to test
the IO layer.

Could you also paste the command line used for benchmarking?
 
> Another point to note, is that when scping to the Linux guest disc, I get about 20MBps, but to the guest memory (/dev/shm) I get 38MBps.  Either way, Linux guests do better on both disc and memory throughput, but are still much slower (1/4) The speed I would expect.
> My expectations are based on performance results under VMware, Virtualbox, and Xen, which are all quite similar (~40-70MBps disc throughput)

Have these benchmarks been run on the same hardware? With the same benchmarking
tools?

-- 
Mathias Gug
Ubuntu Developer  http://www.ubuntu.com

-- 
Slow disk IO for all guests with all disc modes and types.
https://bugs.launchpad.net/bugs/550409
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