linux-kernel - Re: [00/17] Large Blocksize Support V3

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Date:	Fri, 27 Apr 2007 01:28:51 +1000
From:	David Chinner <dgc@....com>
To:	Nick Piggin <nickpiggin@...oo.com.au>
Cc:	Andy Whitcroft <apw@...dowen.org>,
	Christoph Lameter <clameter@....com>,
	"Eric W. Biederman" <ebiederm@...ssion.com>,
	linux-kernel@...r.kernel.org, Mel Gorman <mel@...net.ie>,
	William Lee Irwin III <wli@...omorphy.com>,
	David Chinner <dgc@....com>,
	Jens Axboe <jens.axboe@...cle.com>,
	Badari Pulavarty <pbadari@...il.com>,
	Maxim Levitsky <maximlevitsky@...il.com>
Subject: Re: [00/17] Large Blocksize Support V3

On Fri, Apr 27, 2007 at 01:08:17AM +1000, Nick Piggin wrote:
> Andy Whitcroft wrote:
> >Nick Piggin wrote:
> >
> 
> >>I don't understand what you mean at all. A block has always been a
> >>contiguous area of disk.
> >
> >
> >Lets take Nick's definition of block being a disk based unit for the
> >moment.  That does not change the key contention here, that even with
> >hardware specifically designed to handle 4k pages that hardware handles
> >larger contigious areas more efficiently.  David Chinner gives us
> >figures showing major overall throughput improvements from (I assume)
> >shorter scatter gather lists and better tag utilisation.  I am loath to
> >say we can just blame the hardware vendors for poor design.
> 
> So their controllers get double the throughput when going from 512K
> (128x4K pages) to 2MB (128x16K pages) requests. Do you really think
> it is to do with command processing overhead?

No - it has to do with things like the RAID controller caching behaviour, the
number of disks a single request can keep busy, getting I/os large
enough to avoid partial stripe writes, etc. Remember that this
controller is often on the other side of a HBA so large I/Os are
really desirable here....

Cheers,

Dave.
-- 
Dave Chinner
Principal Engineer
SGI Australian Software Group
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