Re: [Qemu-devel] bs->enable_write_cache and the guest ABI

[Christoph Hellwig <hch@lst.de>]

On Sun, Mar 07, 2010 at 04:42:18PM +0200, Avi Kivity wrote:
> This means that if I start a guest with cache=writethrough and then 
> restart (or live migrate) it with cache=none, then the guest will see a 
> change, even though the user only changed the drive's backing, not 
> something guest visible.  In the case of live migration, the guest will 
> not even notice the change and we may be at risk of data loss.
> 
> For 0.13 I propose setting enable_write_cache to true unconditionally.  
> For 0.12 the question is more difficult, since we'll be changing the 
> guest ABI.  Given that guests are unlikely not to be able to cope with 
> write caches, and that the alternative is data loss, I believe that's 
> also the right solution there.

Setting it to true unconditionally will cause performance degradation
for cache=writethrough devices, as we now have to drain the queue in
the guest for no reason at all.

I think the better option would be to move the cache setting to qdev
property on the block device at it's a device visible setting.