[HECnet] BRU tapes, extract on UNIX?

[Johnny Billquist]

On 2015-03-16 19:52, Clem Cole wrote:

On Mon, Mar 16, 2015 at 2:28 PM, Johnny Billquist <bqt at softjar.se
<mailto:bqt at softjar.se>> wrote:

      I obviously don't know what Tom might have done. It's definitely
      possible he did. But depending on when this was, it might have been
      another format called DSC (Disk Save and Compress), which is what
      RSX used up until V3.something. Which is, I think, almost mid-80s.


Johnny, 

As I said, neither do it and I don't have the code on-line to look
anymore, but given the dates you mention, DSC sounds more reasonable.
His work would have been 82/83 ish IIRC.   I seem to remember that the
tapes from Mitel came from an RSX system that one of our
officemates/fellow grad students brought with him, telling us the tapes
were an RSX backup format.     When we mounted them and started to poke
around, the tools we had would not work without modification.     Since I
was getting ready to bug out/graduate, I remember Tom dealt with it, I
just don't remember how he did it.

Yeah. And I don't know at all how the DSC format looked like, but I believe it was simpler than BRU. One of the reasons for DEC to create BRU was to improve speed of backups. And one of the tricks were to reduce disk seek times during a backup. And that is done by actually not backing up files in the logical order of the blocks in the file, but in the order they appear on disk, which is why I said the format is slightly tricky.

This is a good example of knowing enough to be dangerous.     I know a
little about RSX, but never dived in deep enough to be sure of any of
some of those details, so if you suggest that it was likely DSC, I
definitely bow to your knowledge.

:-)

That said, a quick look at:
http://www.ibiblio.org/pub/academic/computer-science/history/pdp-11/rsx/decus/rsx86a/301050/bruformat.txt

and, assuming no tape errors .... I suspect that it should not be that
hard to take any of the UNIX ansi tape readers that are available and
hack this format in them.   Interesting to see the RAD50 stuff in there.
That takes me back to painful times.

Yeah, I have already studied that document at a great length.
The format is not hard. But the fact that you need to analyze some meta-information in order to understand where the disk blocks in the backup saveset actually fits into the restored file makes it a bit more interesting.

It also looks like the tape directory is in the front few blocks of the
tape, which was pretty typical of a number of tape formats, particularly
ones for backup. [This makes writing the tapes easier (faster) but it
also make recovery harder if you lose blocks in the directory].

Yes. Like said. It's not rocket science, but it is a bit more complicated than you might first realize.

	Johnny

-- 
Johnny Billquist                                   || "I'm on a bus
                                                                  ||   on a psychedelic trip
email: bqt at softjar.se                         ||   Reading murder books
pdp is alive!                                         ||   tryin' to stay hip" - B. Idol