linux-ext4 - Re: sparsify - utility to punch out blocks of 0s in a file

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Message-Id: <9C5B5C07-B0E4-4D13-BCFE-7F35162DF5E8@dilger.ca>
Date:	Sun, 5 Feb 2012 09:55:54 -0700
From:	Andreas Dilger <adilger@...ger.ca>
To:	Eric Sandeen <sandeen@...hat.com>
Cc:	Ron Yorston <rmy@...ress.co.uk>,
	"xfs@....sgi.com" <xfs@....sgi.com>,
	"linux-ext4@...r.kernel.org" <linux-ext4@...r.kernel.org>
Subject: Re: sparsify - utility to punch out blocks of 0s in a file

On 2012-02-05, at 9:36, Eric Sandeen <sandeen@...hat.com> wrote:
> On 2/5/12 3:33 AM, Ron Yorston wrote:
>> Eric Sandeen wrote:
>>> Now that ext4, xfs, & ocfs2 can support punch hole, a tool to
>>> "re-sparsify" a file by punching out ranges of 0s might be in order.
>>> 
>>> I'll see if util-linux wants it after it gets beat into shape.
>>> (or did a tool like this already exist and I missed it?)
> 
> Matthias' suggestion of adding SEEK_HOLE/SEEK_DATA makes very good sense too.

I thought about this, but if SEEK_HOLE/SEEK_DATA (or FIEMAP) worked, then the file would already be sparse, so I don't think that will help in this case...

> I should also untie the read/zero buffer size from the minimum hole size,
> we should do optimal IO sizes regardless of the minimum hole size desired...

Definitely. 4kB IO is a killer for large files.

Cheers, Andreas--
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