[Qemu-devel] Re: [PATCH +stable] block: don't attempt to merge overlapping requests

[Stefan Hajnoczi]

On Wed, May 19, 2010 at 10:06 AM, Avi Kivity <address@hidden> wrote:
>> In the cache=writeback case the virtio-blk guest driver does:
>>
>> blk_queue_ordered(q, QUEUE_ORDERED_DRAIN_FLUSH, ...)
>>
>
> I don't follow.  What's the implication?

I was wondering whether the queue is incorrectly set to a mode where
overlapping write requests aren't being ordered.  Anyway, Christoph
says overlapping write requests can be issued so my theory is broken.

> btw I really dislike how the cache attribute (which I see as a pure host
> choice) is exposed to the guest.  It means we can't change caching mode on
> the fly (for example after live migration), or that changing caching mode
> during a restart may expose a previously hidden guest bug.

Christoph has mentioned wanting to make the write cache switchable
from inside the guest.

Stefan