[rescue] Tape Backup

[Stephen D. B. Wolthusen]

Hi,

On 17-Sep-2002 Mike Meredith wrote:

> How old is the drive ? And how heavily used has it been ? DDS tape heads 
> do seem to wear out after a while. It could well be the tapes however 
> ... DDS tapes should be replaced regularly and more frequently than 
> managers would like.

DDS tape drives require religious attention to cleaning cycles (just as
Exabytes do). Tapes should be retired as soon as you can afford it since the
wear on the 4mm tapes is quite high. 

> If I was backing up my systems with DDS tapes, I'd take a full backup 
> every night. Also check the backups at random intervals and destroy any 
> tapes that had failed.

Depending on the volume of the data to be backed up, that may not be an option
with a DDS-2; these are rather slow.
 
>> Would some other drive be better?
> 
> We use DLTs at work, and the tapes seem to fail less frequently than 
> DDS.

Well yes, definitely. For home/small business use the pickings are rather slim,
though. DDS tape systems have a number of problems inherent in their design
that make them undesirable for critical backups. They're helical scan, so you
get quite a bit of gunk from abrasion (which is why you want to use high
quality, well-treated tapes in the first place and use cleaning tapes
regularly), and the tape path is a contortionist's nightmare, which places
significant strain on a rather narrow tape. That inevitably leads to stretching
and at some point the data becomes unreadable or - at worst - the tape snaps if
the situation is just right (flaky mechanics, moisture levels, old tapes...).

Another basic problem of DDS tape drives is that their calibration is rather
finicky and depends on the drive being `just right'. If you have a drive with
unknown maintenance history or one that's just worn out you stand a good chance
that there will be no other DDS drive on this planet that will read your tapes.
Which is a Bad Thing since drives conk out occasionally.

If all you have to back up is in fact 2GB or less you might want to look into
the newish MO drives - somewhere around $400 for a 3.5" drive that does 2GB
media. No contact, no mechanical wear, limited worries about tape storage
conditions.

DLTs (or SDLTs) are fine for critical backups, but both drives and tapes are
hideously expensive - you get what you pay for. They're self-calibrating based
on the tape cartridge with simple and robust mechanics (basically the
mechanical construction hasn't changed since the old TK50/TK70 days, but
capacity has gone up from 250 or so MB on the TK70 to 160 GB on SDLTs).

If it wasn't for the fact that its a single-vendor solution, OnStream's ADR
would look good. It's also a linear tape with embedded servos (just like DLT),
and it is comparable in price and capacity to DDS drives (between 15 and 60 GB
capacity native). Trouble is you're paying twice as much for the same capacity
as in DDS drives - so its price per GB capacity is even worse than DLT,
although the latter tapes can be used many times over.

-- 

        later,
        Stephen

Stephen Wolthusen (stephen at wolthusen.com)