linux-kernel - Re: Finding hardlinks

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Message-Id: <E1H1qtW-0001yH-00@dorka.pomaz.szeredi.hu>
Date:	Tue, 02 Jan 2007 22:10:02 +0100
From:	Miklos Szeredi <miklos@...redi.hu>
To:	mikulas@...ax.karlin.mff.cuni.cz
CC:	pavel@....cz, bhalevy@...asas.com, arjan@...radead.org,
	jaharkes@...cmu.edu, linux-kernel@...r.kernel.org,
	linux-fsdevel@...r.kernel.org, nfsv4@...f.org
Subject: Re: Finding hardlinks

> > Certainly, but tar isn't going to remember all the inode numbers.
> > Even if you solve the storage requirements (not impossible) it would
> > have to do (4e9^2)/2=8e18 comparisons, which computers don't have
> > enough CPU power just yet.
> 
> It is remembering all inode numbers with nlink > 1 and many other tools 
> are remembering all directory inode numbers (see my other post on this 
> topic).

Don't you mean they are remembering all the inode numbers of the
directories _above_ the one they are currently working on?  I'm quite
sure they aren't remembering all the directories they have processed.

> It of course doesn't compare each number with all others, it is
> using hashing.

Yes, I didn't think of that.

> > It doesn't matter if there are collisions within the filesystem, as
> > long as there are no collisions between the set of files an
> > application is working on at the same time.
> 
> --- that are all files in case of backup.

No, it's usually working with a _single_ file at a time.  It will
remember inode numbers of files with nlink > 1, but it won't remember
all the other inode numbers.

You could have a filesystem with 4billion files, each one having two
links.  Not a likely scenario though.

Miklos
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