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From: mangler@cit-vax.Caltech.Edu (System Mangler)
Newsgroups: comp.unix.questions
Subject: Re: Problems with a tape-reader
Message-ID: <3569@cit-vax.Caltech.Edu>
Date: Tue, 11-Aug-87 17:38:10 EDT
Article-I.D.: cit-vax.3569
Posted: Tue Aug 11 17:38:10 1987
Date-Received: Thu, 13-Aug-87 06:12:27 EDT
References: <2642@lifia.UUCP> <7931@mimsy.UUCP>
Organization: California Institute of Technology
Lines: 33
Keywords: Tape-reader, BSD-4.3, BSD-4.2, UNIX, DLT
Summary: move to head of bus

In article <2642@lifia.UUCP> drabik@lifia.UUCP (Pascal Drabik) writes:
>    We have an UNIBUS on which are connected :
>    - 1 tape-reader M890-II (Cipher) thru TC12/FS (TS11 compatible controller)
     [plus assorted DMA hogs:  RL11, DR11W, UDA50, NP100]

In article <7931@mimsy.UUCP>, chris@mimsy.UUCP (Chris Torek) writes:
> The important bit here is DLT, data late.
>
> The configuration listed confirms the problem, which is that you
> have a 6250 bpi tape drive that cannot tolerate much Unibus access
> latency.  The UDA50/RAxx driver in 4.3BSD was changed to set the
> DMA burst rate to 4; this is too high for some devices.

According to the spec sheet in front of me, the M890-II is a 3200-bpi
drive, with a recording rate of 160 KB/s.  The transfer rate of the
cache is settable, up to 120 KB/s.  That's not really very high.
(It might be useful to check this setting).

If the TC12 is anything like the TC13, it may have a switch to select
larger DMA bursts.

Some brands of DR11W can be even worse DMA hogs than the UDA50.
The DR11W should be the last DMA device on the bus.

Try moving the TC12 to the first (quad) slot of the Unibus - in the CPU
cabinet, not the expansion cabinet - and put the RL11 right behind it.

Another possible reason why it worked with 4.2bsd and not with 4.3bsd
is that tar et al are now so much faster, hence disk and tape I/O are
more likely to overlap and interfere with each other.  (The SunOS 3.0
xt driver had a race condition that was brought out just like this).

Don Speck   speck@vlsi.caltech.edu  {rutgers,amdahl}!cit-vax!speck