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

[Kevin Wolf]

Am 09.01.2013 13:07, schrieb Liu Yuan:
> Besides performance, I think backward compatibility is more important:
>   1 if we run a old kernel host (quite possible for a long running
> server) which doesn't support WCE, then we will never have a chance to
> choose writethrough cache for guest OS against new QEMU (most users tend
> to update user space tools to exclude bugs)

How does the host kernel even play a role in this context?

>   2 The upper layer software which relies on the 'cache=xxx' to choose
> cache mode will fail its assumption against new QEMU.

Which assumptions do you mean? As far as I can say the behaviour hasn't
changed, except possibly for the performance.

> We can see that the writeback behavior for Guest-WCE toggling is the
> same as expected. The difference is that if we set cache=writethrough,
> guest can't change it via WCE toggling.

Then you need to make sure to communicate this to the guest. I'm not
convinced that adding extra logic to each device for this is a good idea.

Kevin