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

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Date:	Thu, 02 Aug 2012 14:43:04 +0800
From:	Asias He <asias@...hat.com>
To:	Christoph Hellwig <hch@....de>
CC:	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 08/02/2012 02:27 PM, Christoph Hellwig wrote:
> 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.

So, for REQ_FLUSH, what we need is that send out the VIRTIO_BLK_T_FLUSH 
and not to wait.

>>
>> 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.

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
-- 
Asias
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