linux-kernel - Re: [ANNOUNCE] Btrfs: a copy on write, snapshotting FS

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Message-ID: <4678E90A.6000707@vlnb.net>
Date:	Wed, 20 Jun 2007 12:44:58 +0400
From:	Vladislav Bolkhovitin <vst@...b.net>
To:	Philipp Matthias Hahn <pmhahn@...an.lahn.de>
Cc:	Chris Mason <chris.mason@...cle.com>,
	Pádraig Brady 
	<P@...igBrady.com>, linux-kernel@...r.kernel.org,
	linux-fsdevel@...r.kernel.org
Subject: Re: [ANNOUNCE] Btrfs: a copy on write, snapshotting FS

Philipp Matthias Hahn wrote:
>>>>I would also suggest one more feature: support for block level
>>>>de-duplication. I mean:
> 
> ...
> 
>>>>That would be very usable feature, which in most cases would allow to
>>>>shrink occupied disk space on 50-90%.
>>>
>>>Have you references for this number?
>>>In my experience one gets a lot of benefit from
>>>the much simpler process of "de-duplication" of files.
>>
>>Yes, I would expect simple hard links to be a better solution for this,
>>but the feature request is not that out of line.  I actually had plans
>>on implementing auto duplicate block reuse earlier in btrfs.
> 
> 
> One problem with hard-links for me is, they also share the meta-data,
> especially file permissions and owners.
> 
> Take a Subversion checkout for example: For each file "$A" Subversion
> saves a backup under ".svm/text-base/$A.svn-base" for file comparison
> and diff generation. The user controls the file permissions of "$A",
> Subversion protects its backup with 0444. You can't hard-link them,
> because than "svn diff" doesn't work anymore if your editor doesn't
> break the hard-link, or worse, your permissions can get wrong.
> 
> If previous versions Subversion also had an extra file for file
> attributes (mime-type, permissions, to-be-ignored, etc.) Since most
> files had no special attributes, each had a file only containing "END".
> Those you could hard-link by hand to save space.
> 
> If somebody want to research this further:
> 
> There is this nice little package called "perforate", which contains
> "finddup" to find duplicate files. Run it two times, once with "-i" to
> ignore permissions while comparing file contents
> 	finddup -i -d /
> and once without "-i" for "content and permissions must match"
> 	finddup -d /
> This will give you a hint on how many files you could hard-link or how
> many files share their content.

So, seems ever for file based de-duplication some support from the FS, 
including some kind of ability for different inodes point to the same 
data blocks to store the meta-data, would be needed anyway.

Vlad
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