Re: [Qemu-devel] [PATCH] sheepdog: implement direct write semantics

[Paolo Bonzini]

Il 08/01/2013 14:18, Liu Yuan ha scritto:
> Maybe not for a second thought. See following combination:
> 
>    cache flags            WCE toggled and resulting behavior
>    writethrough           writethrough
>    writeback              writetrhough (writeback + flush as expected)
> 
> cache flags means specify 'cache=xxx' at startup and WCE toggled on the
> fly in the guest (supose guest kernel support WCE control)
> 
> So the result is *not* broken. If we set cache=writethrough for
> sheepdog, then WCE won't take any effect because 'flush' request will be
> ignored by Sheepdog driver. And with cache=writeback, WCE does disable
> the writecache and actually turns it to a writethrough cache by sending
> flush req every time for write.
> 
> To conclude, let Sheepdog interpret cache flags won't cause trouble even
> with current Guest WCE feature, the different is that if we set
> cache=writethrough, guest can't change it via WCE toggling. Is this
> behavior acceptable?

But why is it useful to force-disable writeback caching?  Do you have
any performance numbers?

Paolo