linux-kernel - Re: REQ_HARDBARRIER and REQ

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Message-ID: <20070224124206.GG22586@kernel.dk>
Date:	Sat, 24 Feb 2007 13:42:06 +0100
From:	Jens Axboe <jens.axboe@...cle.com>
To:	V P <upathiyayan@...il.com>
Cc:	linux-kernel@...r.kernel.org
Subject: Re: REQ_HARDBARRIER and REQ_SOFTBARRIER

On Sat, Feb 24 2007, V P wrote:
> Hi,
> 
> I'm trying to modify the ordering of I/O requests in Linux kernel, and
> came across the barrier flags REQ_HARDBARRIER and REQ_SOFTBARRIER.
> 
> One thing I noticed (which might be wrong) is that all the requests
> have both these flags set. What is the significance of these flags? Is

Uhm? That's definitely not true.

> it a must for a request to have this? Can I reorder two requests with
> these flags set? How is SOFTBARRIER different from HARDBARRIER?

Both imply a reordering barrier in the io scheduler, and the hard
barrier further implies a barrier at the drive/driver side. So you may
not reorder across a barrier.

-- 
Jens Axboe

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