Re: Our introduction to Reiser-list

["Alexander G. M. Smith" <agmsmith@rogers.com>]

John Gilmore wrote on Wed, 26 Oct 2005 17:02:06 +0000:
> I had understood that a big part of the issue with file-as-directory was the 
> same as the issue with hard links to directories. Which I thought is that if 
> you move one directory into another, you can lose the connection to the root 
> of the filesystem -- I.E. ../../.. etc is no longer guaranteed to get you 
> to / -- And of course this also means that there is no way to get from / to 
> your freshly moved files, and you've just lost a chunk of disk space to 
> complete inaccessibility. fsck would have to be made smarter specifically to 
> be able to figure that one out, too.

The file move operation has to check that it doesn't break the graph into
two graphs, thus disconnecting some files from the root.  Or you can think
of it as being a way of deleting a whole subgraph of files all at once,
kind of like an rmdir that works better than usual :-)

Speaking of connecting to the root, one concept I found useful was to have
a "True Path" to the root.  One of the hard links to a file / directory is
considered to be the main one and the rest are auxiliary (easier done if
each file/dir stores a list of parents).  The main one is guaranteed to
lead to the root (a recursive property) and is used for ".." in directories
and the equivalent operation for files.  Then when you want a canonical
path, you build it by following the main parents up to the root.

The "True Path" comes in useful for faking hard links for POSIX compatibility
by misrepresenting them as symbolic links using the dynamically generated true
path string.  As well, if you remove a hard link and it wasn't the main parent
then you don't have to do any graph traversal to fix up things; all items
will still have a valid link to the root through their unchanged main parent.

- Alex