linux-kernel - Re: Finding hardlinks

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Message-ID: <4593C524.8070209@poochiereds.net>
Date:	Thu, 28 Dec 2006 08:22:44 -0500
From:	Jeff Layton <jlayton@...chiereds.net>
To:	Benny Halevy <bhalevy@...asas.com>
CC:	Mikulas Patocka <mikulas@...ax.karlin.mff.cuni.cz>,
	Arjan van de Ven <arjan@...radead.org>,
	Jan Harkes <jaharkes@...cmu.edu>,
	Miklos Szeredi <miklos@...redi.hu>,
	linux-kernel@...r.kernel.org, linux-fsdevel@...r.kernel.org,
	nfsv4@...f.org
Subject: Re: Finding hardlinks

Benny Halevy wrote:
> 
> It seems like the posix idea of unique <st_dev, st_ino> doesn't
> hold water for modern file systems and that creates real problems for
> backup apps which rely on that to detect hard links.
> 

Why not? Granted, many of the filesystems in the Linux kernel don't enforce that 
they have unique st_ino values, but I'm working on a set of patches to try and 
fix that.

> Adding a vfs call to check for file equivalence seems like a good idea to me.
> A syscall exposing it to user mode apps can look like what you sketched above,
> and another variant of it can maybe take two paths and possibly a flags field
> (for e.g. don't follow symlinks).
> 
> I'm cross-posting this also to nfsv4@...f. NFS has exactly the same problem
> with <fsid, fileid> as fileid is 64 bit wide. Although the nfs client can
> determine that two filesystem objects are hard linked if they have the same
> filehandle but there are cases where two distinct filehandles can still refer to
> the same filesystem object.  Letting the nfs client determine file equivalency
> based on filehandles will probably satisfy most users but if the exported
> fs supports the new call discussed above, exporting it over NFS makes a
> lot of sense to me... What do you guys think about adding such an operation
> to NFS?
> 

This sounds like a bug to me. It seems like we should have a one to one 
correspondence of filehandle -> inode. In what situations would this not be the 
case?

-- Jeff

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