linux-kernel - Re: [patch 00/14] Page cache cleanup in anticipation of Large Blocksize support

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Date:	Thu, 14 Jun 2007 19:23:40 -0700
From:	Andrew Morton <akpm@...ux-foundation.org>
To:	Christoph Lameter <clameter@....com>
Cc:	linux-kernel@...r.kernel.org, hch@...radead.org
Subject: Re: [patch 00/14] Page cache cleanup in anticipation of Large
 Blocksize support

On Thu, 14 Jun 2007 19:04:27 -0700 (PDT) Christoph Lameter <clameter@....com> wrote:

> > > Of course there is. The seeks are reduced since there are an factor 
> > > of 16 less metadata blocks. fsck does not read files. It just reads 
> > > metadata structures. And the larger contiguous areas the faster.
> > 
> > Some metadata is contiguous: inode tables, some directories (if they got
> > lucky), bitmap tables.  But fsck surely reads them in a single swoop
> > anyway, so there's no gain there.
> 
> The metadata needs to refer to 1/16th of the earlier pages that need to be 
> tracked. metadata is shrunk significantly.

Only if the filesystems are altered to use larger blocksizes and if the
operator then chooses to use that feature.  Then they suck for small-sized
(and even medium-sized) files.

So you're still talking about corner cases: specialised applications which
require careful setup and administrator intervention.

What can we do to optimise the common case?

> > Other metadata (indirect blocks) are 100% discontiguous, and reading those
> > with a 64k IO into 64k of memory is completely dumb.
> 
> The effect of a larger page size is that the filesystem will 
> place more meta data into a single page instead of spreading it out. 
> Reading a mass of meta data with a 64k read is an intelligent choice to 
> make in particular if there is a large series of such reads.

Again: requires larger blocksize: specialised, uninteresting for what will
remain the common case: 4k blocksize.

The alleged fsck benefit is also unrelated to variable PAGE_CACHE_SIZE. 
It's a feature of larger (unweildy?) blocksize, and xfs already has that
working (doesn't it?)

There may be some benefits to some future version of ext4.
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