Subject: Re: HD as backup (Re: RAID In general)
To: NetBSD-current Discussion List <current-users@NetBSD.ORG>
From: Greg A. Woods <>
List: current-users
Date: 04/17/2003 16:06:15
[ On Thursday, April 17, 2003 at 14:41:52 (-0400), Chuck Yerkes wrote: ]
> Subject: HD as backup (Re: RAID In general)
> Number of copies?  1.

The cost of more copies is the same as the cost of the media, but those
extra copies (up to some sane number) can be made simultaneously with
the right tools.

Don't forget that off-site archives can usually be recycled too,
especially with very-high-capacity media where you can keep all old data
on the same backup set.

The speed of the hard drive as backup media, as Greywolf said, is still
one of the key features of using low-cost hard drives though.

Having very high-speed high-volume backup media radically changes the
way one thinks about backups.  Think of the "snapshot" feature on NetAPP
file servers, among other things.

I would still use (software) RAID-5 for the backup drives though just so
that I could stand to loose one of them and still not loose my data, and
of course that means having at least three drives in a backup set and at
least a six-drive capacity for the backup server, and that changes the
economics somewhat (depending on the frequency of off-site archive
creation and recycling, etc.).

Of course if I were doing serious backups with tapes I'd definitely want
at least two copies of each too....  (or use RAIT with striped parity)

> I have an DDS 4 that's 4 years old and has made over 300 tapes (in
> not-heavy use).

Even that lifetime sounds like pure luck, though more of my experience
with unreliable DDS drives is with DDS-3 models....  I really don't like
or trust any kind of helical-scanned tape devices, especially not those
with dual-reel tape cartridges.  The narrower the tape the less reliable
the tape and drive too, at least in my experience.

> I have a DAT that I mail to a pal cross country every couple months.
> I'd never do that with an HD.

I suspect the likelihood of physically damaging a DAT tape and a
properly packed hard drive when sending each through the postal mail is
about equal these days.

> I have a DAT sitting on my desk.
> I can't safely do that with an HD.

I don't know what your desk environment is like, but on my desk the hard
drive would stand a hell of a lot better chance of being readable after
a few months than any DAT tape would, even (or especially) if both were
just sitting bare without a box.  In the drawer though they might both
stand an equal chance....

> I have a box of tapes (8mm, QIC150, DAT from DDS-1 to DDS4) sitting
> in a box in a closet.  They all still work (I question the 9-track's
> life after 18 years since backup).

How do you know they all still "work"?  What's your definition of
"work"?  Can you still read exactly the same data, and all of the data,
from them that you recorded on them originally?  How often do you check
them?  How many tests can they survive before the tests alone cause them
to fail?

> I also have a 20MB MFM (or RLL) drive that's got some stuff I might
> want.  Could you run over and get that data off please?

We're not talking about ancient technology near-antique 20MB MFM drives

Obviously you've kept working tape drives that can still read all your
old media, so why didn't you keep a working system with an ST-506 and/or
EIDE interface for your old hard drives?

> - HD's are delicate.

Not in a properly padded box they're not....

> - Tapes survive more external extremes.  Tapes are a consumable,
>   they duplicate for low costs, etc.

It depends on the type of tape.  I wouldn't trust anything but the
hard-cartridge style single-reel tapes, such as DLT or the IBM
equivalent (3480?), but they're expensive and harder to duplicate.

> - Tapes survive TIME far better.

Actually that's one of the things we really don't know yet, especially
when compared to modern high-capacity low-cost drives.

I do know that high-capacity low-cost tape media is FAR less reliable
than high-cost tape media.  I also know that helical-scanned tape media
is usualy a lot less reliable too (mostly because of the way the tape
must be extracted from the cartridge and wrapped around the head, but
also possibly because of the higher head speed too).

(We do know that some CD-ROM and writable CDs often end up having a
shorter lifetime than disks or tapes too.)

								Greg A. Woods

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