Re: [PATCH] virtio-blk: set QUEUE_ORDERED_DRAIN by default

[Christian Borntraeger <borntraeger@de.ibm.com>]

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@lst.de>

Make sense to me.
Reviewed-by: Christian Borntraeger <borntraeger@de.ibm.com>

[...]
> -	/* 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);
[...]