linux-kernel - Re: What does "ordering by draining" mean?

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Message-ID: <20071210103753.GB5008@kernel.dk>
Date:	Mon, 10 Dec 2007 11:37:56 +0100
From:	Jens Axboe <jens.axboe@...cle.com>
To:	jidong xiao <jidong.xiao@...il.com>
Cc:	linux-kernel@...r.kernel.org
Subject: Re: What does "ordering by draining" mean?

On Mon, Dec 10 2007, jidong xiao wrote:
> Hi,
> 
> I am reading the source code of block I/O layer, and now seeing there
> saying "ordering by draining" or "ordering by tag" in
> include/blkdev.h,and the comments in that file says: "Hardbarrier is
> supported with one of the following methods", but what do these two
> methods exactly mean?

ordering by draining means that you empty the pending queue of requests
and then flush the cache to prevent a later issue of a new request to
reach the platter before the older ones. order by tag means that the hw
has a way of being told not to reorder beyond the barrier, in which case
you don't have to drain the queue.

-- 
Jens Axboe

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