linux-kernel - Re: Finding hardlinks

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Date:	Mon, 08 Jan 2007 13:00:08 +0100
From:	Miklos Szeredi <miklos@...redi.hu>
To:	pavel@....cz
CC:	mikulas@...ax.karlin.mff.cuni.cz, matthew@....cx,
	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

> > There's really no point trying to push for such an inferior interface
> > when the problems which samefile is trying to address are purely
> > theoretical.
> 
> Oh yes, there is. st_ino is powerful, *but impossible to implement*
> on many filesystems.

You mean POSIX compliance is impossible?  So what?  It is possible to
implement an approximation that is _at least_ as good as samefile().
One really dumb way is to set st_ino to the 'struct inode' pointer for
example.  That will sure as hell fit into 64bits and will give a
unique (alas not stable) identifier for each file.  Opening two files,
doing fstat() on them and comparing st_ino will give exactly the same
guarantees as samefile().

> > Currently linux is living with 32bit st_ino because of legacy apps,
> > and people are not constantly agonizing about it.  Fixing the
> > EOVERFLOW problem will enable filesystems to slowly move towards 64bit
> > st_ino, which should be more than enough.
> 
> 50% probability of false positive on 4G files seems like very ugly
> design problem to me.

4 billion files, each with more than one link is pretty far fetched.
And anyway, filesystems can take steps to prevent collisions, as they
do currently for 32bit st_ino, without serious difficulties
apparently.

Miklos
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