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X-Mailer: exmh version 2.5 07/13/2001 with nmh-1.0.4+dev
To: Jens Axboe <axboe@suse.de>
Cc: Nick Piggin <piggin@cyberone.com.au>, Giuliano Pochini <pochini@shiny.it>,
       linux-mm@kvack.org, linux-kernel@vger.kernel.org,
       linux-kernel@alex.org.uk, Alex Tomas <bzzz@tmi.comex.ru>,
       Andrew Morton <akpm@digeo.com>, Oliver Xymoron <oxymoron@waste.org>
Subject: Re: 2.5.59-mm5 
In-Reply-To: Your message of "Fri, 24 Jan 2003 21:04:34 +0100."
             <20030124200434.GD889@suse.de> 
From: Valdis.Kletnieks@vt.edu
References: <XFMail.20030124180942.pochini@shiny.it> <3E31765F.4010900@cyberone.com.au> <200301241934.h0OJYf0V005773@turing-police.cc.vt.edu>
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Date: Fri, 24 Jan 2003 17:02:48 -0500
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On Fri, 24 Jan 2003 21:04:34 +0100, Jens Axboe said:

> Nicks comment refers to the block layer situation, we obviously cannot
> merge reads and writes there. You would basically have to rewrite the
> entire request submission structure and break all drivers. And for zero
> benefit. Face it, it would be stupid to even attempt such a manuever.

As I *said* - "hairy beyond benefit", not "cant".

> Since you bring it up, you must know if a device which can take a single
> command that says "read blocks a to b, and write blocks x to z"? Even
> such thing existed,

They do exist.

IBM mainframe disks (the 3330/50/80 series) are able to do much more than that
in one CCW chain  So it was *quite* possible to even express things like "Go to
this cylinder/track, search for each record that has value XYZ in the 'key'
field, and if found, write value ABC in the data field". (In fact, the DASD I/O
opcodes for CCW chains are Turing-complete).

>                      it would be much better implemented by the driver
> as pulling more requests of the queue and constructing these weirdo

The only operating system I'm aware of that actually uses that stuff is MVS.

> So I quite agree with the "obviously".

My complaint was the confusion of "obviously cant" with "we have decided we
don't want to".

/Valdis

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