Question: Disk Mirroring preceding RAID?

From: Chuck McManis <cmcmanis_at_mcmanis.com>
Date: Mon Dec 4 11:38:24 2000

At 10:53 AM 12/4/2000 -0800, you wrote:
>Need help in settling argument...does disk mirroring (via hardware)
>predate RAID's mirroring capability? (I'm not interested in
>OS software implemented mirroring here.)

Sounds like a patent suit :-)

>Also, are there any hardware mirror methods that one could point
>to and say: this is not capable of being called any kind of RAID!
>(The argument in question is over the statement that
>"all hardware based mirroring is a form of RAID".)

This is kind of a specious argument. There were volume shadowing (aka
hardware mirroring) products available before people started calling it
"RAID" and in fact the IBM DASD farms could do volume shadowing and it
isn't "RAID" because RAID is "Redundant Array of *Inexpensive* Disks" and
3380's certainly weren't "inexpensive."

The hoopla, if you can call it that, around RAID was that disks were
getting pretty cheap and you could reorganize them in several ways to
"enhance" certain features (like reliability or performance) that normally
you'd have to pay beau coup bucks for. Nearly everything "raid like" had
been done before by people like IBM and DEC but their disk farms didn't
come cheap.

>For example, is there any "stealth hardware mirroring" mechanism,
>where a drive watches the bus, and obeys writes, but nothing else?

Look at some of the CDC dual ported drives. But there is still a controller
on the disk. If you try to say there isn't a controller involved then
you'll be looking at something that is splitting off the write lines to the
platters. (which I wouldn't rule out, just you will have to look for *that*
in the techical manual for so called "high reliability" storage that
perhaps IBM, CDC, Rand, or DEC may have sold to some customers)

>(This would be "drive-based" mirroring, as opposed to "controller-
>based mirroring") (Since RAID is controller-based, a drive-based
>mirroring mechanism would seem to be clearly not any kind of RAID.)

So, you going to tell us the patent number?
--Chuck
Received on Mon Dec 04 2000 - 11:38:24 GMT