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

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Date:	Sun, 05 Feb 2012 11:23:21 -0600
From:	Eric Sandeen <sandeen@...hat.com>
To:	Andreas Dilger <adilger@...ger.ca>
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 2/5/12 10:55 AM, Andreas Dilger wrote:
> 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...

But only if other tools originally used them, and there will probably be plenty
of cases where they don't, or legacy files, or ....

>> 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.

yeah, it was a quick hack, I'll try to fix that up.

(OTOH for large files you man not want a 4k hole granularity either)

-Eric

> Cheers, Andreas

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