Am Donnerstag 20 August 2009 22:56:16 schrieb Christoph Hellwig: > Currently virtio-blk doesn't set any QUEUE_ORDERED_ flag by default, which > means it does not allow filesystems to use barriers. But the typical use > case for virtio-blk is to use a backed that uses synchronous I/O, and in > that case we can simply set QUEUE_ORDERED_DRAIN to make the block layer > drain the request queue around barrier I/O and provide the semantics that > the filesystems need. This is what the SCSI disk driver does for disks > that have the write cache disabled. > > With this patch we incorrectly advertise barrier support if someone > configure qemu with write back caching. While this displays wrong > information in the guest there is nothing that guest could have done > even if we rightfully told it that we do not support any barriers. > > Signed-off-by: Christoph Hellwig <hch@xxxxxx> Make sense to me. Reviewed-by: Christian Borntraeger <borntraeger@xxxxxxxxxx> [...] > - /* If barriers are supported, tell block layer that queue is ordered */ > + /* > + * If barriers are supported, tell block layer that queue is ordered. > + * > + * If no barriers are supported assume the host uses synchronous > + * writes and just drain the the queue before and after the barrier. > + */ > if (virtio_has_feature(vdev, VIRTIO_BLK_F_BARRIER)) > blk_queue_ordered(vblk->disk->queue, QUEUE_ORDERED_TAG, NULL); > + else > + blk_queue_ordered(vblk->disk->queue, QUEUE_ORDERED_DRAIN, NULL); [...] -- To unsubscribe from this list: send the line "unsubscribe kvm" in the body of a message to majordomo@xxxxxxxxxxxxxxx More majordomo info at http://vger.kernel.org/majordomo-info.html
