Raw with Write Passthrough Cache disabled on sata for best perfomance?

dman777 · 08-22-2011, 11:33 PM

I am going to run a small Qemu KVM linux guest(10 gigs) on my linux host...all running on my ssd drive. The guest will never grow in size so I was thinking about using raw instead of qcow2 for best performance. I read with raw it is best performance wise to disable write pass through cache. But this test was done on a IDE. I am using sata with AHCI divers. Would this still hold true?

macemoneta · 08-23-2011, 12:33 AM

I don't think you will notice any performance difference between preallocated qcow2 and raw. However, virtio will make a significant difference. For example:

Code:

/usr/bin/qemu-kvm -m 2048 -smp 2 -vga vmware \
                  -drive file=Disk.img,if=virtio,index=0,media=disk,boot=on \
                  -net nic,model=virtio,macaddr=52:54:00:12:34:56 -net user

dman777 · 08-23-2011, 05:20 AM

http://www.ilsistemista.net/index.ph...6.html?start=5 shows that with only WT enabled on qcow2 it improved performance. But with WT disabled the perfomance was better without VirtIO. What is your opinion on that?

macemoneta · 08-23-2011, 10:12 AM

Lower bars are better in that chart, and virtio performance is also a function of the kernel (generally, newer=better).

dman777 · 08-23-2011, 02:03 PM

ya, but by that chart only when WT was enabled virtio offered an improvement. without WT, virtio did not help at all.

macemoneta · 08-23-2011, 02:22 PM

Unless you are planning on running Windows Server 2008 R2 Foundation under RHEL 6 that chart does not apply to your workload, but even there a slight performance improvement occurred. Virtualization performance varies with each host/guest combination, as well as with each host kernel/guest driver.