linux-kernel - Re: Linux 2.6.29

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Date:	Wed, 25 Mar 2009 11:26:57 -0700 (PDT)
From:	Linus Torvalds <torvalds@...ux-foundation.org>
To:	David Rees <drees76@...il.com>
cc:	Theodore Tso <tytso@....edu>, Jan Kara <jack@...e.cz>,
	Andrew Morton <akpm@...ux-foundation.org>,
	Ingo Molnar <mingo@...e.hu>,
	Alan Cox <alan@...rguk.ukuu.org.uk>,
	Arjan van de Ven <arjan@...radead.org>,
	Peter Zijlstra <a.p.zijlstra@...llo.nl>,
	Nick Piggin <npiggin@...e.de>,
	Jens Axboe <jens.axboe@...cle.com>,
	Jesper Krogh <jesper@...gh.cc>,
	Linux Kernel Mailing List <linux-kernel@...r.kernel.org>
Subject: Re: Linux 2.6.29

On Wed, 25 Mar 2009, Linus Torvalds wrote:
> 
> Even a suck-ass laptop drive can write streaming data fast enough that 
> people don't care. The problem is invariably that writes from different 
> sources (much of it being metadata) interact and cause seeking.

Actually, not just writes.

The IO priority thing is almost certainly that _reads_ (which get higher 
priority by default due to being synchronous) get interspersed with the 
writes, and then even if you _could_ be having streaming writes, what you 
actually end up with is lots of seeking.

Again, good SSD's don't care. Disks do. It doesn't matter if you have a FC 
disk array that can eat 300MB/s when streaming - once you start seeking, 
that 300MB/s goes down like a rock. Battery-protected write caches will 
help - but not a whole lot when streaming more data than they have RAM. 
Basic queuing theory.

			Linus
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