Path: utzoo!utgpu!jarvis.csri.toronto.edu!mailrus!tut.cis.ohio-state.edu!ucbvax!hplabs!hpfcso!rjn
From: rjn@hpfcso.HP.COM (Bob Niland)
Newsgroups: comp.sys.hp
Subject: Re: How to speed performance of HP7980A tape drives (?)
Message-ID: <7370013@hpfcso.HP.COM>
Date: 13 Oct 89 16:41:16 GMT
References: <5785@tank.uchicago.edu>
Organization: Hewlett-Packard, Fort Collins, CO, USA
Lines: 27

re: "I have been benchmarking the HP7980A tape drive (a 125ips streamer) on a
     HP9000 model 835. I have noticed some peculiar results..."

>  The benchmark program opens the tape drive (/dev/rmt/0hn) ...

> For small record sizes, the tape drive writes a bit, stops, backs up to
> the point it would have stopped at if it were a stop/start drive, writes,
> backs up, etc.

Do you have bit 21 (IR Inh) clear in the /dev file minor number?

If this bit is set, each write request cannot complete in the host until
the data is actually on the tape, which usually leds to thrashing on small
record sizes.

IR = Immediate Report, which allows the drive to return a status of "write
complete" as soon as the data is read into the drive's cache, but before
it is written to tape.

With IR enabled, the drive should stream as long as you can send data at an
aggregate rate of 781 Kbytes/sec (at 6250 cpi).  Since the maximum HP-IB bus
rate is 1.2 Mbytes/sec., the overhead of short records could prevent
reaching 781 average.

Regards,                                              Hewlett-Packard
Bob Niland        rjn%hpfcrjn@hplabs.HP.COM           3404 East Harmony Road
                  [hplabs|hpu...!hpfcse]!rjn          Ft Collins CO 80525-9599