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

[Liu Yuan <namei.unix@gmail.com>]

On 01/09/2013 08:42 PM, Kevin Wolf wrote:
> 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?
> 

Oops, my mind was broken. Okay, guest OS which doesn't support WCE can't
change cache type as we discussed.

>>   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.
> 

When users set 'cache=writethrough' to export only a writethrough cache
to Guest, but with new QEMU, it will actually get a writeback cache as
default.

>> 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.
> 

We don't need to communicate to the guest. I think 'cache=xxx' means
what kind of cache the users *expect* to export to Guest OS. So if
cache=writethrough set, Guest OS couldn't turn it to writeback cache
magically. This is like I bought a disk with 'writethrough' cache
built-in, I didn't expect that it turned to be a disk with writeback
cache under the hood which could possible lose data when power outage
happened.

So Guest-WCE only works when users provide us a writeback capable disk.

Thanks,
Yuan