linux-kernel - Re: [PATCH V5 4/4] virtio-blk: Add REQ_FLUSH and REQ

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Date:	Mon, 6 Aug 2012 17:17:31 +0200
From:	Christoph Hellwig <hch@....de>
To:	Asias He <asias@...hat.com>
Cc:	Christoph Hellwig <hch@....de>, linux-kernel@...r.kernel.org,
	Rusty Russell <rusty@...tcorp.com.au>,
	Jens Axboe <axboe@...nel.dk>, Tejun Heo <tj@...nel.org>,
	Shaohua Li <shli@...nel.org>,
	"Michael S. Tsirkin" <mst@...hat.com>, kvm@...r.kernel.org,
	virtualization@...ts.linux-foundation.org
Subject: Re: [PATCH V5 4/4] virtio-blk: Add REQ_FLUSH and REQ_FUA support
	to bio path

On Thu, Aug 02, 2012 at 02:43:04PM +0800, Asias He wrote:
>> Even if it has a payload waiting is highly suboptimal and it should
>> use a non-blocking sequencing like it is done in the request layer.
>
> So, for REQ_FLUSH, what we need is that send out the VIRTIO_BLK_T_FLUSH and 
> not to wait.

If it's REQ_FLUSH without data a VIRTIO_BLK_T_FLUSH should be sent out only,
if it's a REQ_FLUSH that has data a VIRTIO_BLK_T_FLUSH should be sent out,
but instead of waiting for it to finish the I/O completion handler should
then submit the actual write.

> We still need to wait until the actual write is finished here?
> Like,
>
> REQ_FUA is emulated by:
> 1. Send the actual write
> 2. Wait until the actual write is finished
> 3. Send VIRTIO_BLK_T_FLUSH to device
> 4. Signal the end of the write to upper layer

Remove step 2 and run step 3 from the I/O completion handler.

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