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From: rod@LION.ISI.EDU (Rod Van Meter)
Newsgroups: mod.computers.vax
Subject: A Sudden Change
Message-ID: <860924202543.005@Lion.ISI.Edu>
Date: Wed, 24-Sep-86 23:26:15 EDT
Article-I.D.: Lion.860924202543.005
Posted: Wed Sep 24 23:26:15 1986
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Problem: Suddenly decreased performance from our system disk.

	We have a cluster of 8 11/750's with an HSC50 controlled
RA81 as our system disk. Currently we have a second HSC50 plugged in
ready to do dual-porting, but it is not yet enabled. This happened
last weekend. At the same time, our apparent access to the system disk
was sharply decreased in speed. Using Monitor and information from the
"Guide to Performance Management" we determined that our (rough) average
current disk access time is from 100 to over 600 milliseconds, depending
on the node and load.

	Does anyone know what kind of access times can normally be
expected in a large cluster, and if a nominally inactive but wired in place
HSC50 could in anyway affect our access? The transition from fast to
slow was sharp, but we have no numbers to substantiate this, just our
own impressions and our users' complaints. Interactive work has become
difficult, and virtually any request (even DIR or a simple delete) takes
6-12 seconds before it leaves the I/O Request Queue, sometimes even more
than a minute.

Any ideas?

Rod Van Meter
USC Information Sciences Institute
(213)822-1511