Date: Mon, 10 Dec 2007 11:37:56 +0100
From: Jens Axboe <jens.axboe@oracle.com>
To: jidong xiao <jidong.xiao@gmail.com>
Cc: linux-kernel@vger.kernel.org
Subject: Re: What does "ordering by draining" mean?
Message-ID: <20071210103753.GB5008@kernel.dk>
References: <4104961b0712100149h69b1e1fas355b770f93affd03@mail.gmail.com>
MIME-Version: 1.0
Content-Type: text/plain; charset=us-ascii
Content-Disposition: inline
In-Reply-To: <4104961b0712100149h69b1e1fas355b770f93affd03@mail.gmail.com>
Sender: linux-kernel-owner@vger.kernel.org
Content-Length: 927
Lines: 23

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

--
To unsubscribe from this list: send the line "unsubscribe linux-kernel" in
the body of a message to majordomo@vger.kernel.org
More majordomo info at  http://vger.kernel.org/majordomo-info.html
Please read the FAQ at  http://www.tux.org/lkml/