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Date: Wed, 25 Mar 2009 11:26:57 -0700 (PDT)
From: Linus Torvalds <torvalds@linux-foundation.org>
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Subject: Re: Linux 2.6.29
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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