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

[Paolo Bonzini]

Il 28/01/2013 11:01, Kevin Wolf ha scritto:
> 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.

Yes, cache=unsafe will override it.

Paolo