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

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Date:	Fri, 27 Apr 2007 00:33:03 +1000
From:	David Chinner <dgc@....com>
To:	Avi Kivity <avi@...o.co.il>
Cc:	David Chinner <dgc@....com>, Nick Piggin <nickpiggin@...oo.com.au>,
	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>,
	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 Thu, Apr 26, 2007 at 04:53:00PM +0300, Avi Kivity wrote:
> David Chinner wrote:
> >The problem with this approach is that it turns around the whole
> >way we look at bufferheads. Right now we have well defined 1:n
> >mapping of page to bufferheads and so we tpyically lock the
> >page first them iterate all the bufferheads on the page.
> >
> >Going the other way, we need to support m:n which we means
> >the buffer has to become the primary interface for the filesystem
> >to the page cache. i.e. we need to lock the bufferhead first, then
> >iterate all the pages on it. This is messy because the cache indexes
> >via pages, not bufferheads. hence a buffer needs to point to all the
> >pages in it explicitly, and this leads to interesting issues with
> >locking.
> >  
> 
> Why is it necessary to assume that one filesystem block == one buffer?  
> Is it for atomicity, efficiency, or something else?

By definition, really - each filesystem block has it's own state and
it's own disk mapping and so we need something to carry that
information around....

Cheers,

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