linux-kernel - Re: Finding hardlinks

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Message-ID: <20070102191504.GA5276@ucw.cz>
Date:	Tue, 2 Jan 2007 19:15:05 +0000
From:	Pavel Machek <pavel@....cz>
To:	Miklos Szeredi <miklos@...redi.hu>
Cc:	bhalevy@...asas.com, 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

Hi!

> > >> 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%

I do not think we want to play with probability like this. I mean...
imagine 4G files, 1KB each. That's 4TB disk space, not _completely_
unreasonable, and collision probability is going to be ~100% due to
birthday paradox.

You'll still want to back up your 4TB server...

							Pavel
-- 
Thanks for all the (sleeping) penguins.
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