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More on SCSI disks

1996-12-13 10:23:23
Subject: More on SCSI disks
From: Richard Sims <rbs AT BU DOT EDU>
Date: Fri, 13 Dec 1996 10:23:23 -0500
To add a few more recommendations about dealing with SCSI disks and
potential problems...

If at all possible, NEVER introduce new hardware of any kind to your
shop by immediately using it for important applications, particularly
if it is a brand/model with which you have not had prior experience.
Our procedure with disks is to first put them through a stress test
in terms of repeatedly filling them and driving them at maximum volume
rate.  (In doing this we found conspicuous problems with one non-IBM
vendor's alleged hot-swappable SCSI - which they later admitted was
designed for timing with particular host processors, a bizarre approach
which left it incompatible with RS/6000 access!)  If it passes this
functionality test we then put it to work on our test systems for some
term of daily exercise in real-world use.  Only thereafter do we employ
this external SCSI disk on production systems.

I would particularly like to recommend IBM SSA disk technology.  If you
haven't looked into SSA yet, you should: it is remarkably flexible,
reliable technology with high throughput.  It is basically SCSI disks
driven by full-duplex, serial connections, allowing for multiple-host
connections and probably more disks than you can afford on a single
loop.  It puts standard SCSI to shame, with its cumbersome, extremely
expensive, short cabling and half-duplex communication.  Imagine being
able to have devices 25 METERS APART on a loop.  We currently have a
computer on one side of the computer room using SSA disks located on
the other side of the room.  The only problem we've had so far is
sporadic correctable errors, which IBM Poughkeepsie engineers believe
is due to electrical noise susceptibility in the first generation of
SSA microchannel cards, as when located next to an Ethernet card.
We're happily using SSA for paging, data, and system files.
We and other customers very much want to see IBM start using SSA for
3590 tape drives, to permit more flexible equipment location within
the computer room.

    Richard Sims, Boston University OIT
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