linux-ext4 - Re: Files full of zeros with coreutils-8.11 and xfs (FIEMAP related?)

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Message-ID: <4DA87F43.5040807@redhat.com>
Date:	Fri, 15 Apr 2011 11:24:19 -0600
From:	Eric Blake <eblake@...hat.com>
To:	Christoph Hellwig <hch@...radead.org>
CC:	Jim Meyering <jim@...ering.net>,
	Eric Sandeen <sandeen@...deen.net>,
	Dave Chinner <david@...morbit.com>, xfs-oss <xfs@....sgi.com>,
	coreutils@....org, linux-ext4@...r.kernel.org,
	Markus Trippelsdorf <markus@...ppelsdorf.de>
Subject: Re: Files full of zeros with coreutils-8.11 and xfs (FIEMAP related?)

On 04/15/2011 11:16 AM, Christoph Hellwig wrote:
> On Fri, Apr 15, 2011 at 10:53:48AM +0200, Jim Meyering wrote:
>> Can you or anyone else point to authoritative documentation
>> (or even a summary of those "discussions") of FIEMAP semantics?
> 
> A large part of the problem is that there's none and the semantics
> is a hode-podge of random flags that mostly make little sense
> outside of fs developers debugging the layout that we created.  The
> closest to a description is Documentation/filesystems/fiemap.txt
> in the kernel tree, but that doesn't document most of the delicate
> points.

Would it be worth borrowing from Solaris' semantics and adding SEEK_HOLE
and SEEK_DATA to lseek(2), as a higher level (less-detailed, but easier
to define and easier to use) interface for discovering the regions of a
file that only contain NUL bytes?  FIEMAP is a great interface at
exposing everything possible, but coreutils doesn't necessarily need all
that complexity just for determining when it is safe to skip over a
portion of a file that is either unallocated or unchanged from the
initial 0 contents.

-- 
Eric Blake   eblake@...hat.com    +1-801-349-2682
Libvirt virtualization library http://libvirt.org


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