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

[Kevin Wolf]

Am 27.01.2013 22:21, schrieb Tom Horsley:
> 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?

One thing that is relatively new, and might not have been in F17, is
that virtio-blk automagically enables a writethrough mode if the guest
doesn't advertise that it supports flushes. If this is the case, you
actually get cache=directsync instead of cache=none, which obviously
hurts performance, but ensures that your image doesn't get corrupted on
a crash.

I don't think there is an option to override this magic, except maybe
cache=unsafe.

Kevin