linux-kernel - Re: [PATCH V3 3/3] virtio-blk: Add bio-based IO path for virtio-blk

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Date:	Mon, 30 Jul 2012 12:43:12 +0800
From:	Asias He <asias@...hat.com>
To:	Paolo Bonzini <pbonzini@...hat.com>
CC:	kvm@...r.kernel.org, linux-kernel@...r.kernel.org,
	virtualization@...ts.linux-foundation.org,
	"Michael S. Tsirkin" <mst@...hat.com>,
	Christoph Hellwig <hch@....de>,
	Minchan Kim <minchan.kim@...il.com>,
	Rusty Russell <rusty@...tcorp.com.au>
Subject: Re: [PATCH V3 3/3] virtio-blk: Add bio-based IO path for virtio-blk

On 07/28/2012 02:42 PM, Paolo Bonzini wrote:
>>> I'm not sure what the correct behavior for bio & cacheflush is, if
>>> any.
>>
>> REQ_FLUSH is not supported in the bio path.
>
> Ouch, that's correct:
>
> @@ -414,7 +529,7 @@ static void virtblk_update_cache_mode(struct virtio_device *vdev)
>   	u8 writeback = virtblk_get_cache_mode(vdev);
>   	struct virtio_blk *vblk = vdev->priv;
>
> -	if (writeback)
> +	if (writeback && !use_bio)
>   		blk_queue_flush(vblk->disk->queue, REQ_FLUSH);
>   	else
>   		blk_queue_flush(vblk->disk->queue, 0);
>
> then it is not safe against power losses.

Yes. Something like this:

   qemu -drive file=foo.img,cache=writeback/unsafe

is not safe against power losses also?

I think we can add REQ_FLUSH & REQ_FUA support to bio path and that 
deserves another patch.

-- 
Asias
--
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