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From: KVC@ENGVAX.SCG.HAC.COM (Kevin Carosso)
Newsgroups: comp.os.vms
Subject: Re: SET FILE/ENTER
Message-ID: <8706301405.AA29636@ucbvax.Berkeley.EDU>
Date: Mon, 29-Jun-87 19:01:00 EDT
Article-I.D.: ucbvax.8706301405.AA29636
Posted: Mon Jun 29 19:01:00 1987
Date-Received: Wed, 1-Jul-87 06:16:08 EDT
Sender: daemon@ucbvax.BERKELEY.EDU
Distribution: world
Organization: The ARPA Internet
Lines: 29

SET FILE/ENTER allows you to make alternate directory entries for a file,
similar to UNIX "hard" links.

Now, it works fine, is used by DEC (for cluster common roots, SYSBOOT, etc)
but is "discouraged" by the documentation.  Why?

The reason is the filesystem doesn't really handle alternate file entries
very well.  If you delete a file, the alternate entry will remain but the
file it points to will be deleted without so much as a warning.  That entry
will then point to a useless "no such file".  This can be an unpleasant
surprise.  A UNIX hard link, on the other hand, maintains a usage count,
and the file is not really deleted until it has no more links.

Also, ANALYZE/DISK doesn't know much about alternate entries and reports that
the file has an invalid directory backlink.

The problem is addressed (well, sort of) in VMS V5.  In V5 a file has
a "primary" entry and 0 or more "secondary" entries.  Deleting a "secondary"
entry only deletes the entry, not the file.  Deleting the primary entry
will delete the file, and can leave "dangling" secondary entries.  The primary
entry is the entry with whom the file's backlink agrees.  Anyway, as you
can see this is an improvement, though they need to take it further.  They
indicated that changes are planned further out for the primary/secondary
mechanism, so there is hope that it'll continue to evolve.

(This info is from the "Backup and File System Futures" talk at DECUS).

        /Kevin Carosso                     kvc@engvax.scg.hac.com
         Hughes Aircraft Co.               kvc%engvax@oberon.usc.edu