[Simh] VAX Tape Emulation?

[Johnny Billquist]

On 2018-01-26 00:44, Clem Cole wrote:
> 
> 
> On Thu, Jan 25, 2018 at 4:57 PM, Mark Pizzolato <Mark at infocomm.com 
> <mailto:Mark at infocomm.com>> wrote:
> 
>     I think the documentation comment “cannot write variable-length
>     blocks and do not allow skipping forward over records between read
>     operations” was written when talking about the common cartridge
>     tapes that were available on 80s and 90s Unix workstations.  I don’t
>     recall the name.
> 
> That was not UNIX, that was the QIC standard.   Yes, those were blocked 
> at 512 bytes.   Apollo's domain systems had a b*tch of time with them 
> because their standard disk block was 1056 bytes​

Hmm. Ah. THose QIC tapes. Never liked them much, and never played much 
with them.
Domain OS using 1056 bytes? On what systems? I used a lot of DN3000, 
4000 and 5000 systems, and they used bog standard disk drives with 512 
byte sectors. (Interesting systems in some ways, but their windowing 
system was pretty horrible.)

>        These things only supported fixed block size operations and not
>     variable record lengths (i.e. 80 byte tape labels, then different
>     sized data records, etc.).
> 
> ​Right the 80 byte ANSI label, then different length data records.  UNIX 
> handles that fine, even with RMT.​  FYI: My grad school housemate, Tom 
> Quarles (of SPICE3 fame) wrote the ANSI tape and bunch of 
> other tape support that most UNIX systems used, explicitly so he could 
> read/write VMS tapes for the DEC guys who were doing some of the funding 
> of the USB CAD lab.   Leffler (who wrote rmt) used Tom's tape stuff for 
> the original debug of rmt.
> 
> 
>        Given that the remote tape drive was a drive which could do
>     variable length record activities, I think MultiNet’s rmt support
>     actually worked well.  I don’t remember testing it though.  Whether
>     someone should try to do that now to backup simulated VMS systems is
>     another subject I may write about a little later.
> 
> 
> ​Understood.   I was just suggest​ing trying to keep another emulated 
> system out of the scheme and going directly to the remote device either 
> through DECnet or rmt or maybe even using a NAS as virtual tape files.   
> It just seemed running a Linux with a tape and then running an emulated 
> VAX on top of that seemed like an extra layer of indirection if there 
> was an easier path.

Yes, I think the approaches are trying to do things in too complicated 
ways as well.

   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