Date: Wed, 21 Apr 2010 21:39:37 +0100
From: Jamie Lokier <jamie@shareable.org>
To: Evgeniy Polyakov <zbr@ioremap.net>
Cc: Phillip Susi <psusi@cfl.rr.com>, linux-fsdevel@vger.kernel.org,
       Linux-kernel <linux-kernel@vger.kernel.org>
Subject: Re: readahead on directories
Message-ID: <20100421203937.GX27575@shareable.org>
References: <4BCC7C05.8000803@cfl.rr.com> <20100421004434.GA27420@shareable.org> <4BCF123C.6010400@cfl.rr.com> <20100421161211.GC27575@shareable.org> <20100421183853.GA14897@ioremap.net> <20100421185124.GM27575@shareable.org> <20100421185657.GA21249@ioremap.net> <20100421200243.GU27575@shareable.org> <20100421202134.GB25130@ioremap.net>
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Evgeniy Polyakov wrote:
> On Wed, Apr 21, 2010 at 09:02:43PM +0100, Jamie Lokier (jamie@shareable.org) wrote:
> > FIEMAP might not be the answer, but what part of it requires fs
> > knowledge?  It's supposed to be fs-independent.  I agree it's not
> > always appropriate to use, and I don't know if it would be effective
> > anyway.
> 
> At least we have to know whether given fs supports such interface.
> And more complex is to know how underlying fs is organized. What is
> extent, which types can it have, where exactly information about extent
> metadata is stored, i.e. where can we find what this object is about?

Ummm... Does any of that matter?

> And how to actually populate appropriate blocks into ram to speedup
> readdir()?

Blockdev readahead() :-)

> FIEMAP (which is file mapper btw :) is useful for information gathering
> about how fs is organized, but that's all I'm afraid.

That's all you need to start fetching from the blockdev.  You can't
*use* the blockdev data, but that doesn't matter for this readahead
operation, only that they are approximately the right data blocks.

- Jamie
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