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Date:	Sun, 20 Oct 2002 10:00:30 PST7
From:	Peter Skye <pskye@peterskye.com >
Reply-To:	scoug-help@scoug.com
To:	scoug-help@scoug.com
Subject:	SCOUG-Help: RAID, tape backup (was: XCOPY failures (was: Using Infozip to backup))

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Info2SYNass.NET wrote:
>
> I am not having practical experience with RAID.
> Well the mobo (Abit KT7-Raid) of my first selfbuilt system
> is having a RAID feature but I am using it as an activator
> for the 2 more IDE sockets to provide UDMA100.
> I could also activate some RAID but I assume it's not the
> same RAID as you were telling !?

It's the same RAID. There are several different types of RAID
(variously "Redundant Array of Inexpensive Disks" and "Redundant Array
of Independent Drives", depending on vendor) and the types have numbers
like 0, 1, 5, 10. These "types" are actually different ways of doing
things with multiple drives operating in parallel.

For example, you can simply have two drives with the exact same data on
each one ("mirroring"). The controller writes the data to both drives,
and if it can't read a data sector from one drive due to an error it
then reads that sector from the other drive. If one drive fails the
RAID "system" keeps reading and writing using the other, good, drive.

RAID also allows for something called "striping" which is a way of
"making your drives run faster". For a hypothetical example, instead of
writing a 1024-byte 2-sector data record to one drive, the controller
can write the first 512 bytes to drive 1 and *simultaneously* write the
second 512 bytes to drive 2. The entire 1024 bytes is written in "half
the time" because both drives write only half the data.

And RAID allows for something called "parity". Your data is first
"striped" to multiple drives (see the preceding paragraph) and *then*
all of these stripes are XOR'd to create a "parity" stripe which is
written to yet another drive. If one of the data drives fails, the
controller can recreate the missing sector by XOR'ing all of the other
drives and, voila!, the bad sector/track/drive is recreated. (This is
something like the old parity recovery tracks used on mainframe tape
drives.) Many RAID controllers allow you to have more than one parity
drive -- two parity drives, for example, can completely restore the data
even if two drives fail, in fact the RAID array will keep running and
your computer won't even know there was a failure (although when using a
parity drive to restore data the data rate can slow down).

To summarize, RAID is a *group* of technologies which are used on "disk
arrays". These technologies can "mirror" the data so you have a backup,
can speed up the data transfer of the data using "striping", and can
restore bad sectors using "parity". When you set up a RAID array you
pick-and-choose the specific RAID technologies you want to use and
ignore the ones you don't want.

> Your focus ... reminds me of my old times
> with an airline operation center. Within 1 1/2
> minutes we had switched from a misbehaving
> mainframe system to the twin of this system.

You worked in a place that did a good job with their system design.

> Just end of September I had a very bad experience with my backup
> tapes.
> My most important file of Lotus Notes ran into troubles ...
> ... and trying a restore from tapes I realized the unablilty
> to read the tapes ;-(( I do not know what has happened ;-(((

I've never been happy with tape backups. I've tried a number of tape
drives and tape formats and never was comfortable with them.

And even if the tapes are compatible between drives or software vendors,
the compression algorithms may not be -- which is why you shouldn't use
compression when you back up (somebody steals your machine with its tape
drive but you have the backup tape in your desk drawer, and you find you
can't restore it to a new drive because nobody currently sells the tape
drive you need or you can't find your copy of the backup software and
nobody is selling the software any more either).

Svobi, when are you going to come to Southern California for a visit?

- Peter

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