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

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Date:	Thu, 2 Aug 2012 08:27:42 +0200
From:	Christoph Hellwig <hch@....de>
To:	Asias He <asias@...hat.com>
Cc:	linux-kernel@...r.kernel.org,
	Rusty Russell <rusty@...tcorp.com.au>,
	Jens Axboe <axboe@...nel.dk>, Christoph Hellwig <hch@....de>,
	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 02:25:56PM +0800, Asias He wrote:
> We need to support both REQ_FLUSH and REQ_FUA for bio based path since
> it does not get the sequencing of REQ_FUA into REQ_FLUSH that request
> based drivers can request.
> 
> REQ_FLUSH is emulated by:
> 1. Send VIRTIO_BLK_T_FLUSH to device
> 2. Wait until the flush is finished

There is no need to wait for the flush to finish if the REQ_FLUSH
request has no data payload.

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.

> 
> 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. Wait until the flush is finished
> 5. Signal the end of the write to upper layer

The same comment about not blocking applies here as well.

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