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

To: Christoph Hellwig <hch@xxxxxxxxxxxxx>
Subject: Re: Files full of zeros with coreutils-8.11 and xfs (FIEMAP related?)
From: Eric Blake <eblake@xxxxxxxxxx>
Date: Fri, 15 Apr 2011 11:24:19 -0600
Cc: Jim Meyering <jim@xxxxxxxxxxxx>, Eric Sandeen <sandeen@xxxxxxxxxxx>, Dave Chinner <david@xxxxxxxxxxxxx>, xfs-oss <xfs@xxxxxxxxxxx>, coreutils@xxxxxxx, linux-ext4@xxxxxxxxxxxxxxx, Markus Trippelsdorf <markus@xxxxxxxxxxxxxxx>
In-reply-to: <20110415171629.GA9088@xxxxxxxxxxxxx>
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Organization: Red Hat
References: <20110414102608.GA1678@xxxxxxxxxxxxxx> <20110414120635.GB1678@xxxxxxxxxxxxxx> <20110414140222.GB1679@xxxxxxxxxxxxxx> <4DA70BD3.1070409@xxxxxxxxxxxxxx> <4DA717B2.3020305@xxxxxxxxxxx> <20110414225904.GK21395@dastard> <4DA7836A.5040604@xxxxxxxxxxxxxx> <20110415000940.GL21395@dastard> <87sjtjq4ir.fsf@xxxxxxxxxxxxxxxx> <20110415171629.GA9088@xxxxxxxxxxxxx>
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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@xxxxxxxxxx    +1-801-349-2682
Libvirt virtualization library http://libvirt.org

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