Re: [PATCH V5 4/4] virtio-blk: Add REQ_FLUSH and REQ_FUA support to bio path

[Asias He <asias@xxxxxxxxxx>]

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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