linux-kernel - Re: Linux 2.6.29

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Date:	Wed, 25 Mar 2009 14:55:29 -0400
From:	Ric Wheeler <rwheeler@...hat.com>
To:	Alan Cox <alan@...rguk.ukuu.org.uk>
CC:	Linus Torvalds <torvalds@...ux-foundation.org>,
	David Rees <drees76@...il.com>, Theodore Tso <tytso@....edu>,
	Jan Kara <jack@...e.cz>,
	Andrew Morton <akpm@...ux-foundation.org>,
	Ingo Molnar <mingo@...e.hu>,
	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

Alan Cox wrote:
>> 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.
>>     
>
> Subtly more complex than that. If your mashed up I/O streams fit into the
> 2GB or so of cache (minus one stream to disk) you win. You also win
> because you take a lot of fragmented OS I/O and turn it into bigger
> chunks of writing better scheduled. The latter win arguably shouldn't
> happen but it does occur (I guess in part that says we suck) and it
> occurs big time when you've got multiple accessors to a shared storage
> system (where the host OS's can't help)
>
> Alan
>   

The other thing that can impact random writes on arrays is their 
internal "track" size - if the random write is of a partial track, it 
forces a read-modify-write with a back end disk read.  Some arrays have 
large internal tracks, others have smaller ones.

Again, not unlike what you see with some SSD's and their erase block 
size - give them even multiples of that and they are quite happy.

Ric

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