linux-kernel - Re: [PATCH v6 0/4] ext4: Coordinate data-only flush requests sent by fsync

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Message-ID: <20101130164301.GB30858@lst.de>
Date:	Tue, 30 Nov 2010 17:43:01 +0100
From:	Christoph Hellwig <hch@....de>
To:	Neil Brown <neilb@...e.de>
Cc:	"Darrick J. Wong" <djwong@...ibm.com>,
	Jens Axboe <axboe@...nel.dk>, "Theodore Ts'o" <tytso@....edu>,
	Andreas Dilger <adilger.kernel@...ger.ca>,
	Alasdair G Kergon <agk@...hat.com>, Jan Kara <jack@...e.cz>,
	Mike Snitzer <snitzer@...hat.com>,
	linux-kernel <linux-kernel@...r.kernel.org>,
	linux-raid@...r.kernel.org, Keith Mannthey <kmannth@...ibm.com>,
	dm-devel@...hat.com, Mingming Cao <cmm@...ibm.com>,
	Tejun Heo <tj@...nel.org>, linux-ext4@...r.kernel.org,
	Ric Wheeler <rwheeler@...hat.com>,
	Christoph Hellwig <hch@....de>, Josef Bacik <josef@...hat.com>
Subject: Re: [PATCH v6 0/4] ext4: Coordinate data-only flush requests sent by fsync

On Tue, Nov 30, 2010 at 11:39:06AM +1100, Neil Brown wrote:
> Would it not be sufficient to simply gather flushes while a flush is pending.
> i.e
>   - if no flush is pending, set the 'flush pending' flag, submit a flush,
>     then clear the flag.
>   - if a flush is pending, then wait for it to complete, and then submit a
>     single flush on behalf of all pending flushes.
>
> That way when flush is fast, you do a flush every time, and when it is slow
> you gather multiple flushes together.

We can even optimize the second flush away if no other I/O has
completed since the first flush has started.  That will always be the
case for SATA devices as the cache flush command is not queued.

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