[TUHS] Ultrix Tape: Block Size?

Clem Cole clemc at ccc.com
Tue Oct 16 07:00:27 AEST 2018

Be careful, TK-50 is different than 9-track.  It's a streamer tape like
QIC, 4mm and 8mm.   The blocking is done under the covers by the HW and the
blovk size if just how a DMA is done.  I recommend that you pre-fetch the
read with dd or double-dd setting ibs=64k, obs=20b and conv=sync and pipe
the output to the reader (tar/cpio or the like) [if that fails try
obs=1b].   This should work well as can (TK-50 overall suck - don't set
your hopes high on anything with them -- they were DECtape from a
realiabilty standpoint, they were different from the reset of the world,
the performance was poor and they were expensive).

Anyway, by using dd or the like a front end, it will allow the read
streamer to read as fast as it can.  The problem is that the way it works
under the cover does not shine with traditional UNIX I/O.  BTW: ibs of
anything more than 64K on a VAX (or PDP-11) will not help because of the
dma size on the Unibus caps DMA read/writes at 64K.   On a PMAX or (under
Tru64 on a Alpha), you can try using really large ibs sizes depending on
your physical memory size.

BTW: What will help the most is actually finding a copy of the old
double-dd program (from the UUNET archives) which forks off two child
procees to perform the actual I/O and alternates between the two processes
via pipe between them and controller - so one dd process is reading when
the other dd process is writing.  [It used to be called: ddd before the Gnu
guys grabbed that name for the debugger].   The command line might be
something like:   ddd ibs=64k obs=20b | tar xvpf -

FWIW:  I wrote a version of a fast dd years ago that used pthreads and a
semaphore that I should still have kicking around.   At one point when I
was dealing with streamer tapes for backup, I definitely ran it on Tru64
and FreeBSD, but  I've forgotten where Ultrix fell.

On Mon, Oct 15, 2018 at 4:01 PM Warren Toomey <wkt at tuhs.org> wrote:

> All, I received this request from Matthew who isn't subscribed to either
> the TUHS or cctalk lists. He knows how to read the lists archives. Many
> thanks for any help you can provide.
> Cheers, Warren
> ----- Forwarded message from Matthew Whitehead -----
> Date: Mon, 15 Oct 2018 08:25:39 -0400
> From: Matthew Whitehead
> Subject: Ultrix Tape Blocks
>    Warren,
>      I wonder if you can give me a referral. I want to install Ultrix-32
>    on my MicroVAX II using the ancient TK-50 tape drive. I know the tape
>    files are on your archive, but I need to know the block size for each
>    of the many files; it can vary a lot.
>    Who might be able to help me with this?
>    Matthew Whitehead
> ----- End forwarded message -----
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