[TUHS] Early non-Unix filesystems?

John Cowan cowan at mercury.ccil.org
Fri Mar 18 13:59:36 AEST 2016

Warren Toomey scripsit:

> It's a bit off-topic, but what were non-Unix filesystems like around
> 1969-1970?  The PDP-7 filesystem has i-nodes (file metadata) and
> filenames separate from the i-nodes. This allows hard links and thus
> a non-tree structured filesystem.

Not only that, but a non-tree-structured directory system: the directories
themselves were a general directed graph, and it was possible to have any
number of arbitrary hard links to a directory.  This was simplified in
later Unices to the tree-structured directory system we have today, where
the only hard links to a directory are the named link from the parent,
the "." link from self, and the ".." links from the child directories.

There was also no concept of pathnames in PDP-7 Unix, neither relative
nor absolute.  The link syscall was link("foo", "bar", "baz"), meaning
to cause entry "baz" in directory "bar" in the working directory to
be the same file or directory as entry "foo" in the working directory.
So for example, each home directory had a hard link to the main binaries
directory, conventionally named "bin", and the shell could not execute
commands out of a directory that did not contain a "bin" link.  With no
reliable ".." entry, there was also no way to recover from a chdir, and
traversal had to be done as digraph traversal rather than tree traversal.
In particular, incautious use of unlink could cause the directory graph
to become partitioned, requiring low-level disk surgery to recover.

An interesting property of the ODS-1 (RSX) file system was that the
root directory contained the i-node table as a file named INDEXF.SYS.
This allowed ordinary programs to inspect the i-node table without messing
about with raw disks.  It also contained a self-entry named 000000.DIR,
but other directories did not contain self-links.

John Cowan          http://www.ccil.org/~cowan        cowan at ccil.org
The internet is a web of tiny tyrannies giving an illusion of anarchy.
                --David Rush

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