linux-kernel - RE: Question about synchronous write on SSD

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Message-ID: <002f01c872db$69c854c0$1fa9580a@swcenter.sec.samsung.co.kr>
Date:	Tue, 19 Feb 2008 18:39:43 +0900
From:	"Kyungmin Park" <kmpark@...radead.org>
To:	"'Thomas Petazzoni'" <thomas.petazzoni@...e-electrons.com>
Cc:	"'Linux Filesystem Mailing List'" <linux-fsdevel@...r.kernel.org>,
	<linux-kernel@...r.kernel.org>
Subject: RE: Question about synchronous write on SSD

> Le Tue, 19 Feb 2008 14:48:18 +0900,
> "Kyungmin Park" <kmpark@...radead.org> a écrit :
> 
> > +               /* Write synchronous */
> > +               bio->bi_rw |= (1 << BIO_RW_SYNC);
> 
> Adding BIO_RW_SYNC doesn't make generic_make_request() synchronous as
> in "generic_make_request() returns only after write completion".
> BIO_RW_SYNC only asks the I/O layer to unplug immediatly. But
> generic_make_request() still returns before the completion of the I/O,
> and the completion is notified asynchronously.
> 

Agree, however see the following sequence.

__generic_make_request call q->make_request_fn(q, bio);
It was set by blk_init_queue_node with __make_request.
There are two ways in __make_request.
Case 1, get_rq
Case 2, out or merged (otherwise you mean unplug case)

In case 1, if the BIO_RW_SYNC is set, the request gets the REQ_RW_SYNC
And REQ_RW_SYNC says 
"include/linux/blkdev.h":112: __REQ_RW_SYNC,          /* request is sync (O_DIRECT) */
It means it acts as O_DIRECT flag. Is it right?
And it also is same as case 2. Unplug the device.
So next time it hasn't chance to merge???

Is it right?

BR,
Kyungmin Park

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