Re: [PATCH, RFC 2/2] dm: support REQ_FLUSH directly

["Jun'ichi Nomura" <j-nomura@ce.jp.nec.com>]

Hi Christoph,

(08/04/10 17:54), Christoph Hellwig wrote:
> On Wed, Aug 04, 2010 at 01:57:37PM +0900, Kiyoshi Ueda wrote:
>>> -		if (unlikely(dm_rq_is_flush_request(rq))) {
>>> +		if (rq->cmd_flags & REQ_FLUSH) {
>>>  			BUG_ON(md->flush_request);
>>>  			md->flush_request = rq;
>>>  			blk_start_request(rq);
>>
>> Current request-based device-mapper's flush code depends on
>> the block-layer's barrier behavior which dispatches only one request
>> at a time when flush is needed.
>> In other words, current request-based device-mapper can't handle
>> other requests while a flush request is in progress.
>>
>> I'll take a look how I can fix the request-based device-mapper to
>> cope with it.  I think it'll take time for carefull investigation.
> 
> Given that request based device mapper doesn't even look at the
> block numbers from what I can see just removing any special casing
> for REQ_FLUSH should probably do it.

Special casing is necessary because device-mapper may have to
send multiple copies of REQ_FLUSH request to multiple
targets, while normal request is just sent to single target.

Thanks,
-- 
Jun'ichi Nomura, NEC Corporation