linux-kernel - Re: How does ext2 implement sparse files?

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Message-ID: <p73myql50d2.fsf@bingen.suse.de>
Date:	Thu, 31 Jan 2008 19:14:01 +0100
From:	Andi Kleen <andi@...stfloor.org>
To:	Lars Noschinski <lklml@...noschinski.de>
Cc:	linux-kernel@...r.kernel.org
Subject: Re: How does ext2 implement sparse files?

Lars Noschinski <lklml@...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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