To: Lars Noschinski <lklml@ml.noschinski.de>
Cc: linux-kernel@vger.kernel.org
Subject: Re: How does ext2 implement sparse files?
From: Andi Kleen <andi@firstfloor.org>
References: <20080131152823.GA29422@lars.home.noschinski.de>
Date: Thu, 31 Jan 2008 19:14:01 +0100
In-Reply-To: <20080131152823.GA29422@lars.home.noschinski.de> (Lars Noschinski's message of "Thu\, 31 Jan 2008 16\:28\:23 +0100")
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Lars Noschinski <lklml@ml.noschinski.de> writes:

> For an university project, we had to write a toy filesystem (ext2-like),
> for which I would like to implement sparse file support. For this, I
> digged through the ext2 source code; but I could not find the point,
> where ext2 detects holes.
>
> As far as I can see from fs/buffer.c, an hole is a buffer_head which is
> not mapped, but uptodate. But I cannot find a relevant source line,
> where ext2 makes usage of this information.

It does not explicitely detect holes; holey data is just never written
so no space for it is allocated.

-Andi
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