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

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Message-ID: <ee101c360802010234m4a7199e7g4cc9131484529b1c@mail.gmail.com>
Date:	Fri, 1 Feb 2008 18:34:40 +0800
From:	"Shuduo Sang" <sangshuduo@...il.com>
To:	"Andi Kleen" <andi@...stfloor.org>
Cc:	"Lars Noschinski" <lklml@...noschinski.de>,
	linux-kernel@...r.kernel.org
Subject: Re: How does ext2 implement sparse files?

On Feb 1, 2008 2:14 AM, Andi Kleen <andi@...stfloor.org> wrote:
> 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.
>

does anybody know how to make a hole in a large file which already has
real content from user space application?
In my project I need this function to delete a piece of content from
an exist large effectively.
thanks.

> -Andi
>
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