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

To: Eric Sandeen <sandeen@xxxxxxxxxx>
Subject: Re: sparsify - utility to punch out blocks of 0s in a file
From: Andreas Dilger <adilger@xxxxxxxxx>
Date: Sun, 5 Feb 2012 09:55:54 -0700
Cc: Ron Yorston <rmy@xxxxxxxxxxxxx>, "xfs@xxxxxxxxxxx" <xfs@xxxxxxxxxxx>, "linux-ext4@xxxxxxxxxxxxxxx" <linux-ext4@xxxxxxxxxxxxxxx>
In-reply-to: <4F2EB017.5090006@xxxxxxxxxx>
References: <4F2D8F30.3090802@xxxxxxxxxx> <201202050933.q159XQpB026380@xxxxxxxxxxxxxxxxxxxxxxxxxxxxx> <4F2EB017.5090006@xxxxxxxxxx>
On 2012-02-05, at 9:36, Eric Sandeen <sandeen@xxxxxxxxxx> 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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