Path: utzoo!utgpu!watserv1!watmath!att!occrsh!uokmax!apple!portal!fernwood!hercules!dirt.cisco.com!kph
From: kph@dirt.cisco.com (Kevin Paul Herbert)
Newsgroups: comp.os.rsts
Subject: Re: Additional information - Backu
Message-ID: <19302@hercules.csl.sri.com>
Date: 5 Jul 90 20:36:21 GMT
References: <memo.60129@cix.uucp>
Sender: usenet@csl.sri.com
Reply-To: kph@dirt.cisco.com (Kevin Paul Herbert)
Organization: cisco Systems
Lines: 34

I don't know anything about the specific devices that you are using, but
I can give you the general answer.

BACKUP beats very heavily on MSCP devices. On your PDP-11/84, BACKUP
will probably keep the entire MSCP command ring full for the duration of
the backup. Generally, the only time that this doesn't occur is if you
run out of Unibus Map Registers (the registers used to map from the
18-bit Unibus to the 22-bit memory bus on newer Unibus PDP-11s), and if
this happens, your performance suffers badly. BACKUP has been known to
find microcode bugs in controllers which have not been driven that fast
before. You should be sure that you have good support from whatever
company you buy the controller from, in case they have problems like this.

The BACKUP format written to disk is a standard disk file. BACKUP writes
to it in random order, dependant upon the order of disk I/O completions
from the source device. Some optical disks are designed to be written to
in sequential order; your BACKUP performance may suffer as a result.

When writing to tape, BACKUP works very hard to sustain high data rates,
pausing to "prime the pipe" as an optimization for streaming tape drives
where it is more efficient to write fast bursts than slow steady data. I
don't know precise the start/stop mechanism is on DAT; you should
experiment with the /BUFFER switch to see what works best.

I hope this helps you evaluate the products.

Kevin

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