Re: [Qemu-devel] [PATCH] block-raw: Make cache=off default again

[Kevin Wolf <kwolf@redhat.com>]

Avi Kivity schrieb:
> On 06/22/2009 03:32 PM, Kevin Wolf wrote:
>>> Were your refcount-combining patches merged?  I don't see them, and
>>> performance will suck without them.
>>>      
>> They were merged some weeks ago, at least if a980c98c is what you mean.
>> I don't have any patches for inclusion that aren't merged yet (except
>> for this one, obviously).
>>    
> 
> I meant that, yes.  Missed it going in.
> 
> We still have a read-modify-write when extending an image, but I guess 
> we're pretty close now, so it's worthwhile to try a guest install with 
> cache=off.

As long as we don't have overlapping requests the RMW is basically a
memset, so at least on IDE this shouldn't hurt too much.

What happens with virtio I still need to understand. Obviously, as soon
as virtio decides to fall back to 4k requests, performance becomes
terrible. And if it doesn't, we still can get concurrent requests to the
same cluster, resulting in a real RMW (however, if it's only one RMW
remaining, it's kind of okay - last week's patches have removed some
more of them in the cluster allocation path...)

Kevin