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

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Message-Id: <DD59E27A-FD96-4983-A274-B76CCE99AE7A@dilger.ca>
Date:	Fri, 15 Apr 2011 16:28:37 -0600
From:	Andreas Dilger <adilger@...ger.ca>
To:	Christoph Hellwig <hch@...radead.org>
Cc:	Eric Blake <eblake@...hat.com>, 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 2011-04-15, at 11:26 AM, Christoph Hellwig wrote:
> On Fri, Apr 15, 2011 at 11:24:19AM -0600, Eric Blake wrote:
>> 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?
> 
> Yes, I've already suggested that both in this thread and on IRC.
> 
> For efficient copies it's the only usable interface.

I suspect that these bugs would have still existed whether the interface is SEEK_HOLE/SEEK_DATA, or FIEMAP.  The main problem is that the delalloc pages were not accounted for correctly during layout traversal.

For ext4 I think it is sufficient to add another case to ext4_ext_fiemap_cb() to check the pagecache for unmapped pages when it finds an uninitialized extent on disk.  This will be very similar to the fix for finding holes in the on-disk mapping.

Cheers, Andreas





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