linux-kernel - Re: Finding hardlinks

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Message-Id: <E1H01Og-0007TF-00@dorka.pomaz.szeredi.hu>
Date:	Thu, 28 Dec 2006 20:58:38 +0100
From:	Miklos Szeredi <miklos@...redi.hu>
To:	bhalevy@...asas.com
CC:	arjan@...radead.org, mikulas@...ax.karlin.mff.cuni.cz,
	jaharkes@...cmu.edu, linux-kernel@...r.kernel.org,
	linux-fsdevel@...r.kernel.org, nfsv4@...f.org
Subject: Re: Finding hardlinks

> >> It seems like the posix idea of unique <st_dev, st_ino> doesn't
> >> hold water for modern file systems 
> > 
> > are you really sure?
> 
> Well Jan's example was of Coda that uses 128-bit internal file ids.
> 
> > and if so, why don't we fix *THAT* instead
> 
> Hmm, sometimes you can't fix the world, especially if the filesystem
> is exported over NFS and has a problem with fitting its file IDs uniquely
> into a 64-bit identifier.

Note, it's pretty easy to fit _anything_ into a 64-bit identifier with
the use of a good hash function.  The chance of an accidental
collision is infinitesimally small.  For a set of 

         100 files: 0.00000000000003%
   1,000,000 files: 0.000003%

And usually (tar, diff, cp -a, etc.) work with a very limited set of
st_ino's.  An app that would store a million st_ino values and compare
each new to all the existing ones would be having severe performance
problems and yet _almost never_ come across a false positive.

Miklos
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