linux-kernel - Re: [PATCH 1/7] Make DISCARD_BARRIER and DISCARD

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Date:	Thu, 03 Sep 2009 18:52:20 +0100
From:	David Woodhouse <dwmw2@...radead.org>
To:	Christoph Hellwig <hch@...radead.org>
Cc:	linux-scsi@...r.kernel.org, linux-ide@...r.kernel.org,
	linux-kernel@...r.kernel.org, liml@....ca, jens.axboe@...cle.com,
	matthew@....cx, Matthew Wilcox <willy@...ux.intel.com>
Subject: Re: [PATCH 1/7] Make DISCARD_BARRIER and DISCARD_NOBARRIER writes
 instead of reads

On Sat, 2009-08-29 at 19:03 -0400, Christoph Hellwig wrote:
> 
> The commands are conceptually writes, and in the case of IDE and SCSI
> commands actually are writes.  They were only reads because we thought
> that would interact better with the elevators.  Now the elevators know
> about discard requests, that advantage no longer exists.

Can you drop the final sentence of that? It isn't true, and I never said
it.

s/.  Now.*/, but that isn't necessary, and making them writes makes it
easier for the low-level IDE and SCSI code to cope with the fact that
the command has to be sent with a payload./

The elevators _still_ don't know about discards, and will still let
reads and writes (and discards, which are just a special case of writes)
to the same sector all cross each other on the queue unless there's some
external factor to prevent it.

-- 
David Woodhouse                            Open Source Technology Centre
David.Woodhouse@...el.com                              Intel Corporation

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