[TUHS] Ultrix Tape: Block Size?

[Clem Cole]

#$%^ - they >>weren't<< like DECtape from a reliability standpoint ...
ᐧ

On Mon, Oct 15, 2018 at 5:00 PM Clem Cole <clemc at ccc.com> wrote:

> 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.
> ᐧ
>
> 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 -----
>>
>