damerell@CS.UCL.AC.UK (Dr R M Damerell, RHBNC) (06/26/87)
sorry if this Q has been asked before; I am not on the list, so please could any replies be copied to me? thankyou. The DCL manual says that use of $SET FILE/ENTER is "discouraged" but doesnt say why. Please could somebody kindly tell me why? What precautions should I take if I go ahead & do it anyway? If asked I will explain why I want to do this, but I dont want to inflict a very long explanation on the whole list. mark
ASBED@ACRUX.USC.EDU (Liquid Len) (06/27/87)
One reason you have to be very careful about using this command is
that you end up with many pointers to the same file, and someone
other than you (or even you) may forget their existence, and delete
one by mistake. This causes all the others to point to a nonexistent
file.
If this file is important to booting (like sysboot.exe) and you do
a VMSINSTALL with the purge option, your system may be rendered
unbootable (this has happened to me).
Also, unless you specify the /EXCLUDE switch on pointers to directories,
while backing up files and such, you will end up backing up duplicate
structures.
I'm sure there are other problems... anyone?
_________________________________________________________________________
Asbed Bedrossian, University Computing Services, U of Southern California
ARPA: asbed@oberon.usc.edu BITNET: asbed@uscvaxq
UUCP: {sdcrdcf, cit-vax}!oberon!asbed PHONE: (213) 743-4266
-------KVC@ENGVAX.SCG.HAC.COM (Kevin Carosso) (06/30/87)
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