[fedora-virt] Has something happened to virtio cache modes?

[Cole Robinson]

On 01/27/2013 04:21 PM, Tom Horsley wrote:
> Back around fedora 16 I tried all the different virtio
> cache modes and experimentally discovered that
> cache=none was nearly infinitely faster than cache=default
> and cache=writeback was even a little faster than cache=none.
> So I set all my VMs to cache=writeback.
> 
> I've been copying my virtual machine defs to each new
> fedora since then, but now it seems to be dog slow
> with disk IO again.
> 
> I'm planning to experiment with different settings again,
> but I have to wait for my Windows XP virtual machine
> to finish doing a windows update which may take all night
> at the rate it is writing to disk :-(.
> 
> Just thought I'd ask if there is some known difference
> in the virtio modes in fedora 18?

I know there was some fiddling with the cache mode at the qemu level,
particularly the default was changed from writethrough to writeback. Maybe
something regressed.

Though my understanding was that cache=none should be the faster option. Does
that setting produce expected performance? Of course we should still figure
out why things slowed down so much with your settings.

- Cole