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

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Message-ID: <20110418004040.GS21395@dastard>
Date:	Mon, 18 Apr 2011 10:40:40 +1000
From:	Dave Chinner <david@...morbit.com>
To:	Theodore Tso <tytso@....EDU>
Cc:	Andreas Dilger <adilger@...ger.ca>,
	Pádraig Brady <P@...igBrady.com>,
	Eric Sandeen <sandeen@...deen.net>,
	"linux-ext4@...r.kernel.org" <linux-ext4@...r.kernel.org>,
	"coreutils@....org" <coreutils@....org>,
	Markus Trippelsdorf <markus@...ppelsdorf.de>,
	xfs-oss <xfs@....sgi.com>
Subject: Re: Files full of zeros with coreutils-8.11 and xfs (FIEMAP
 related?)

On Sat, Apr 16, 2011 at 08:21:28AM -0400, Theodore Tso wrote:
> 
> On Apr 16, 2011, at 1:11 AM, Andreas Dilger wrote:
> 
> > In that case, it means cp should just always use FIEMAP_FLAG_SYNC, which is fine. 
> 
> Except that if someone is copying a large delay allocated file, it will cause 
> the file to immediately snapped to disk, which might not be the greatest
> thing in the world. 

Obvious workaround - if the initial fiemap call shows unwritten
extents, redo it with the sync flag set. Though that assumeѕ that
you can trust things like delalloc extents to only cover the range
that valid data exists in. Which, of course, you can't assume,
either. :/

> Christoph is write, SEEK_HOLE and SEEK_DATA are
> a much better API for what cp woulld lke to do.  Unfortunately it hasn't
> been implemented yet in the VFS...

Agreed, SEEK_HOLE/SEEK_DATA is the right way to solve this problem.

Cheers,

Dave.
-- 
Dave Chinner
david@...morbit.com
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