linux-kernel - Re: Finding hardlinks

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Message-ID: <20070108112916.GB25857@elf.ucw.cz>
Date:	Mon, 8 Jan 2007 12:29:16 +0100
From:	Pavel Machek <pavel@....cz>
To:	Miklos Szeredi <miklos@...redi.hu>
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

Hi!

> > >> No one guarantees you sane result of tar or cp -a while changing the tree.
> > >> I don't see how is_samefile() could make it worse.
> > >
> > > There are several cases where changing the tree doesn't affect the
> > > correctness of the tar or cp -a result.  In some of these cases using
> > > samefile() instead of st_ino _will_ result in a corrupted result.
> > 
> > ... and those are what?
> 
>   - /a/p/x and /a/q/x are links to the same file
> 
>   - /b/y and /a/q/y are links to the same file
> 
>   - tar is running on /a
> 
>   - meanwhile the following commands are executed:
> 
>      mv /a/p/x /b/x
>      mv /b/y /a/p/x
> 
> With st_ino checking you'll get a perfectly consistent archive,
> regardless of the timing.  With samefile() you could get an archive
> where the data in /a/q/y is not stored, instead it will contain the
> data of /a/q/x.
> 
> Note, this is far nastier than the "normal" corruption you usually get
> with changing the tree under tar, the file is not just duplicated or
> missing, it becomes a completely different file, even though it hasn't
> been touched at all during the archiving.
> 
> The basic problem with samefile() is that it can only compare files at
> a single snapshot in time, and cannot take into account any changes in
> the tree (unless keeping files open, which is impractical).

> 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 are of course welcome to combine st_ino with
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.
								Pavel
-- 
(english) http://www.livejournal.com/~pavelmachek
(cesky, pictures) http://atrey.karlin.mff.cuni.cz/~pavel/picture/horses/blog.html
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