linux-kernel - Re: [PATCH] virtio_blk: make virtio blks as non-rotational devices

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Date:   Mon, 29 Mar 2021 17:23:20 +0100
From:   Stefan Hajnoczi <stefanha@...hat.com>
To:     Sochin Jiang <jiangxiaoqing.sochin@...edance.com>
Cc:     mst@...hat.com, jasowang@...hat.com, pbonzini@...hat.com,
        lizefan.x@...edance.com, linux-kernel@...r.kernel.org,
        Christoph Hellwig <hch@...radead.org>
Subject: Re: [PATCH] virtio_blk: make virtio blks as non-rotational devices

On Fri, Mar 26, 2021 at 11:39:13AM +0800, Sochin Jiang wrote:
> This confuses some users seeing one rotational block device
> in the guest(/sys/block/vdx/queue/rotational), let's make
> virtio blks as virtual block devices, just like xen blks,
>  and as we known, QUEUE_FLAG_VIRT is defined as QUEUE_FLAG_NONROT
> actually. See also rbd and nbd block devices.
> 
> Signed-off-by: Sochin Jiang <jiangxiaoqing.sochin@...edance.com>
> ---
>  drivers/block/virtio_blk.c | 1 +
>  1 file changed, 1 insertion(+)

I would like to make this change because it seems consistent and often
the disk really is non-rotational. However, a justification and
performance results are needed especially since this has been reverted
previously. Please see commit f8b12e513b953aebf30f8ff7d2de9be7e024dbbe
("virtio_blk: revert QUEUE_FLAG_VIRT addition").

That was in 2009, so maybe things have changed now. Please explain what
has changed in the commit description.

This flag can affect performance. Please include performance data.

> diff --git a/drivers/block/virtio_blk.c b/drivers/block/virtio_blk.c
> index b9fa3ef5b57c..31a978a4dab5 100644
> --- a/drivers/block/virtio_blk.c
> +++ b/drivers/block/virtio_blk.c
> @@ -822,6 +822,7 @@ static int virtblk_probe(struct virtio_device *vdev)
>  		max_size = min(max_size, v);
> 
>  	blk_queue_max_segment_size(q, max_size);
> +	blk_queue_flag_set(QUEUE_FLAG_VIRT, q);
> 
>  	/* Host can optionally specify the block size of the device */
>  	err = virtio_cread_feature(vdev, VIRTIO_BLK_F_BLK_SIZE,
> --
> 2.11.0
> 

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