linux-kernel - Re: Finding hardlinks

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Message-ID:  <mj+md-20070108.131932.29709.nikam@ucw.cz>
Date:	Mon, 8 Jan 2007 14:26:07 +0100
From:	Martin Mares <mj@....cz>
To:	Miklos Szeredi <miklos@...redi.hu>
Cc:	pavel@....cz, 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

Hello!

> 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().

Good, ... except that it doesn't work. AFAIK, POSIX mandates inodes
to be unique until umount, not until inode cache expires :-)

IOW, if you have such implementation of st_ino, you can emulate samefile()
with it, but you cannot have it without violating POSIX.

> 4 billion files, each with more than one link is pretty far fetched.

Not on terabyte scale disk arrays, which are getting quite common these days.

> And anyway, filesystems can take steps to prevent collisions, as they
> do currently for 32bit st_ino, without serious difficulties
> apparently.

They currently do that usually by not supporting more than 4G files
in a single FS.

				Have a nice fortnight
-- 
Martin `MJ' Mares                          <mj@....cz>   http://mj.ucw.cz/
Faculty of Math and Physics, Charles University, Prague, Czech Rep., Earth
"Oh no, not again!"  -- The bowl of petunias
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