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From: neil@zardoz.UUCP (Neil Gorsuch)
Newsgroups: comp.realtime,comp.os.vms
Subject: Re: Streaming tape drives
Message-ID: <13745@zardoz.UUCP>
Date: 8 May 89 07:37:00 GMT
References: <4374@ttidca.TTI.COM>
Reply-To: neil@zardoz.UUCP (Neil Gorsuch)
Organization: Uninet Peripherals, Santa Ana, CA
Lines: 37

In article <4374@ttidca.TTI.COM> kevin@ttidca.TTI.COM (Kevin Carothers) writes:
>Greetings
>I used to work for a company that made a data-acquisition front-end
>to a DEC PDP-11/73 (Cyber systems, Anaheim).  At the time, we had a 
>requirement to write 50Kwords/sec to a Cipher (tm) 9-track streamer tape
>under the RSX O/S.
>Well, we never made it. The best we did was maybe 35Kwords/Sec.
>The tape drive never went into "streaming" mode, and  we all just 
>kind of gave up.
>Does anyone out there know if this is even possible? We tried every
>conceivable buffering size, I/O mode, double-buffering schemes, etc.
>RSX is very (ok, somewhat) similar to VMS in philosophy (ie; DCL), and 
>the PDP and VAX have similar Command/Status/DMA register structure, so it 
>seems like *someone* in the VAX/realtime world would have come across this 
>problem before.

Hi kevin, this is Neil (small world, isn't it?).
I have a 25 inches per second tape drive streaming at 1600 bpi off a stock IBM
AT running Microport Unix.  As long as the input data can keep up, and you
use concurrent forked tasks sharing a common memory pool, and you use IPC
for synchronization, and you write your own tape driver with double chained
DMA channels, and you design your own tape interface board with automatic
DMA channel switching when one DMA channel runs into the segment boundary,
there is no problem.  I am feeding the data from the ethernet (the Solbourne
can spew out 300K per second from disk, nothing else here can keep up
with it, so the tape drive has more than enough data coming in to it).  We
have also used the tape drive in 3200 bpi mode, but that isn't reliable because
of the IBM AT's limited DMA bandwidth.  I believe that if we bothered to put
in a PC clone running at a higher clock speed, we could easily stream at
3200 bpi and 25 ips, but I never bothered, since I don't use 9 track for backup
much any more.  So, if we can do 40 Kwords/second using an AT clone, a
PDP 11/73 should be able to do much higher than that.  It's just a matter
of doing everything yourself all the way through.

Neil Gorsuch
neil@cpd.com
uunet!zardoz!neil