linux-kernel - Re: readahead on directories

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Message-ID: <20100421203937.GX27575@shareable.org>
Date:	Wed, 21 Apr 2010 21:39:37 +0100
From:	Jamie Lokier <jamie@...reable.org>
To:	Evgeniy Polyakov <zbr@...emap.net>
Cc:	Phillip Susi <psusi@....rr.com>, linux-fsdevel@...r.kernel.org,
	Linux-kernel <linux-kernel@...r.kernel.org>
Subject: Re: readahead on directories

Evgeniy Polyakov wrote:
> On Wed, Apr 21, 2010 at 09:02:43PM +0100, Jamie Lokier (jamie@...reable.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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