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

[Stefan Hajnoczi]

On Tue, May 18, 2010 at 8:41 PM, Michael Tokarev <address@hidden> wrote:
> But actually I don't quite see where that dependency is: guest
> does not know how its data is cached on host...

I was thinking of this bit in linux-2.6:drivers/block/virtio_blk.c:

/* If barriers are supported, tell block layer that queue is ordered */
if (virtio_has_feature(vdev, VIRTIO_BLK_F_FLUSH))
    blk_queue_ordered(q, QUEUE_ORDERED_DRAIN_FLUSH,
                                  virtblk_prepare_flush);
else if (virtio_has_feature(vdev, VIRTIO_BLK_F_BARRIER))
    blk_queue_ordered(q, QUEUE_ORDERED_TAG, NULL);

I was wondering whether we have something wrong, causing the guest to
perform overlapping write requests without draining the queue or
flushing.

Stefan