linux-kernel - Re: [PATCH 1/3] block_write_full_page: Use synchronous writes for WBC_SYNC

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Message-Id: <20090406235052.1ea47513.akpm@linux-foundation.org>
Date:	Mon, 6 Apr 2009 23:50:52 -0700
From:	Andrew Morton <akpm@...ux-foundation.org>
To:	"Theodore Ts'o" <tytso@....edu>,
	Linux Kernel Developers List <linux-kernel@...r.kernel.org>,
	Ext4 Developers List <linux-ext4@...r.kernel.org>,
	jack@...e.cz, Jens Axboe <jens.axboe@...cle.com>
Subject: Re: [PATCH 1/3] block_write_full_page: Use synchronous writes for
 WBC_SYNC_ALL writebacks

On Mon, 6 Apr 2009 23:21:41 -0700 Andrew Morton <akpm@...ux-foundation.org> wrote:

> I mean, let's graph it:
> 
> WRITE_SYNC -> WRITE_SYNC_PLUG -> BIO_RW_SYNCIO -> bio_sync() -> REQ_RW_SYNC -> rw_is_sync() -> does something mysterious in get_request()
>                                                                             -> rq_is_sync() -> does something mysterious in IO schedulers
>                               -> BIO_RW_NOIDLE -> bio_noidle() -> REQ_NOIDLE -> rq_noidle() -> does something mysterious in cfq-iosched only
>            -> BIO_RW_UNPLUG   -> bio_unplug() -> REQ_UNPLUG -> OK, the cognoscenti know what this is supposed to do, but it is unused!

whoop, I found a use of bio_unplug() in __make_request().

So it appears that the intent of your patch is to cause an unplug after
submission of each WB_SYNC_ALL block?

But what about all the other stuff which WRITE_SYNC might or might not
do?  What does WRITE_SYNC _actually_ do, and what are the actual
effects of this change??

And what effect will this large stream of unplugs have upon merging?
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