[TUHS] Ultrix Tape: Block Size?

[Ethan Dicks]

On Tue, Oct 16, 2018 at 1:49 PM Paul Koning via cctalk
<cctalk at classiccmp.org> wrote:
> > On Oct 16, 2018, at 1:23 PM, William Pechter via cctalk <cctalk at classiccmp.org> wrote:
> >
> > DEC Tape II was the serial driven TU58.
> > The TK50 was CompacTape or something like that.  It was the predecessor of a number of square tapes...
> >
> > See DLT on Wikipedia https://en.m.wikipedia.org/wiki/Digital_Linear_Tape
>
> DLT is something entirely different from "DECtape II"... DLT is fast, 1/2 inch tape, serpentine recording.  It's the direct ancestor of a whole series of cartridge tapes of ever increasing capacity.

Yes.  There's DECtape (TU55/TU56), there's DECtape II (TU58), and
there's "CompacTape" (TK50/TK70) which developed into DLT III and so
on to LTO, etc.

The carts for original CompacTape and DLT are the same size.  The
drives are not of identical reliability.  TK50 drives jammed and had a
variety of physical problems.  Even when they were working, their
performance was affected by the systems they were attached to.  In
particular, they are a streaming technology from an era when low-end
systems couldn't feed them fast enough to keep streaming, so in the
case of the TK50, capacity was always listed as "up to 95MB per tape"
because on writes, if there wasn't data in the buffer, the tape
wouldn't just stop but you'd get a gap until there was enough data, so
your capacity was lower.  For reads, it would have to back up a long
ways, spin back up to speed and begin reading where it left off,
substantially slowing down read performance.

By the time DLT III came out, machines were faster and, AFAIK, the
hardware buffering was better so you didn't have the same problems.
Having cut a few hundred DLT III tapes 15 years ago, I found the tape
drives to be absolutely solid.  Over the next 10 years we had an
every-other-year upgrade/migration because our capacity needs grew
rapidly and the cost per drive stayed about the same.  I think they
were on LTO2 when I left, in a 200 tape/4 drive jukebox.

The short version of this is: TK50s were a PITA then, and they still are.

-ethan