linux-kernel - Re: [RFC]block: don't mark flush request as SOFTBARRIER

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Message-ID: <20110911025128.GM29319@htj.dyndns.org>
Date:	Sun, 11 Sep 2011 11:51:28 +0900
From:	Tejun Heo <htejun@...il.com>
To:	Shaohua Li <shaohua.li@...el.com>
Cc:	lkml <linux-kernel@...r.kernel.org>,
	Jens Axboe <jaxboe@...ionio.com>,
	Vivek Goyal <vgoyal@...hat.com>
Subject: Re: [RFC]block: don't mark flush request as SOFTBARRIER

Hello,

On Thu, Sep 08, 2011 at 09:29:54AM +0800, Shaohua Li wrote:
> I'm wondering why flush request should be marked as
> SOFTBARRIER. Current flush sequence guarantees the correctness of
> flush even the request is reordered by ioscheduler.

The primary reason would be because when those flush data requests are
put on the dispatch queue, it doesn't go through elv_dispatch_sort().
SOFTBARRIERs function as dispatch sort boundaries and queueing flush
data request at the head w/o SOFTBARRIER would confuse dispatch
sorting, and we want to put it at the head of dispatch queue to
decrease latency for flush sequence.

Thanks.

-- 
tejun
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