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From: Denis Vlasenko <vda@port.imtp.ilyichevsk.odessa.ua>
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To: Peter Chubb <peter@chubb.wattle.id.au>,
       Andrew McGregor <andrew@indranet.co.nz>
Subject: Re: ISO-9660 Rock Ridge gives different links different inums
Date: Fri, 10 Jan 2003 08:34:44 +0200
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On 10 January 2003 05:34, Peter Chubb wrote:
> >> I don't know enough about the ISO9660 standard to be sure what's
> >> best to do about this.
>
> Andrew> Change it to be the offset to the data area, which should be
> Andrew> the same for all of them?
>
> I thought about that, but I'm unsure if there's any way to get from
> that offset to the directory information.  As far as I can tell,
> there's no concept of an inode separate from directory entry on
> iso9660 --- the directory entry/entries all contain all the
> information that describes a file.  Which means that the inumber has
> to point to some directory node.
>
> Preferably, all the inumbers for the same file would point to the
> same directory entry; but I can see no easy way to do that.  Keeping
> an in-memory table for files with multiple links might be the best
> way, as there aren't that many on a typical filesystem.

And what will happen on a non-typical filesystem with 1 million hardlinks?

The root of the problem is a fundamental layering violation in
traditional Unix filesystems: inode numbers should NOT be visible
to userspace. Userspace just needs a way to tell hardlinks from separate
files, that's all. Exposing inumbers does that, but creates tons
of problems for filesystems which do NOT have such a concept.

There is at least one way to redesign it:
* provide hash number instead of an inumber for each file
  with the following semantics:
  - hardlinks ALWAYS have equal hash numbers
  - different files MAY have equal hash numbers (but rarely)
* provide is_hardlink(file1,file2) system call

But this will cause very long migration period (~10 years?)
and incompatibilities with other Unix variants...
--
vda
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