Archiving Data Permanently

[Joel Jaeggli]

On Thu, 18 Aug 2005, Ezra Nugroho wrote:

>
> I kinda doubt it.
>
> HDs are now built with precisions and standards that are equal or exceed
> those of jet engine making. Consider the engineering that people use to
> make micro HDs for IPods, etc.

Micro-hd's have fairly high tolerances to operating and non-operating 
shock. A lot of that can be attributed to extremely low mass. the inertia 
of the head when the thing plumments towards the floor then stops 
suddently is not enough to overcome the airgap, and thus no headcrash. the 
cornice 1.5GB hd in my mp3 player weighs a whole 14 grams. The duty cycle 
of microhd's is orders of magnitude lower than that of modern 
high-performance disks sold into the enterprise market however, so 
asserting that they are higher precision than 133GB per plater 400GB 
drives, or 80GB per platter 15krpm disks is a little far from the mark.

> Tape cassettes only use a lot of moving objects. Not only that, the tape
> touches it self in the roll, the head of the reader touches the tape.
> This means wear and tear. Furthermore, tape is magnetized plastic. They
> are less heat resistant.

For long term archiving (eg indefinetly) the expected media life can be a 
lot more important than how it holds up under continious use. Different 
tape media/transports/technologies have vastly different characteristics 
from each other, so it's really not fair to generalize without treating 
the difference's between say 4mm and 8mm cassettes and 1/2" carts (carts 
have a less torturous tape path and get retensioned every time they're 
mounted) or the difference between dlt and and lto (data written to both 
sides of the tape, vs one which has long term implications for electron 
migration), etc.

> HD heads now float above the platters, they do not touch the platters at
> all.

When unloaded intentionally heads are lifted away from the platter by a 
ramp, that probably doesn't apply to all hard-disks that you have in your 
possesion, since it's fairly recent. The head is still designed to take of 
and land on the disk.

>HDs are air tight.

No actually they aren't. hence the blow hole, and the issues revolving 
around condensing moisture, very high humidity has a measurable effect on 
the service life of disks...

> The only advantage that tape has over HD is in shock resistance.

Well that's not the only advantange and again, over-generalizes on the 
nature of tape. the upfront costs on big tape backup are pretty high, (my 
latest tape robot was about $12,000) but in the end the costs on my spread 
sheet work about to around $500 per 1TB with an expected service life if 
this particular setup in the 5-7 year range. That's much cheaper than disk 
is likely to be on the scale that I need it for the long term backups.

These days lots of people are moving towards or already have a mixture of 
near-line disk-based backup with long term tape palying a lesser role. a 
lot of this depends or your requirements, if you use backups to recover 
from mistakes in a multi-user enviromnet, then spooling them of huge tapes 
on a daily basis is a pain in the ass and makes short-term 
near-line recovery options much more attractive. your disaster recovery 
policy, or statutory legal data retention requirements may also make tape 
make economic or practical sense.

> The cassette may break from a drop, but you may still (expensively)
> recover the data from the tape, if it is not damaged.

A 1/2 cart is not something to treat like an mp3 player. we don't drop 
them subject them to excess heat etc, any more then we do a data-center 
full of hard-disks

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> On Fri, 2005-08-19 at 02:00 +0930, Tim wrote:
>> On Thu, 2005-08-18 at 08:14 -0600, Robin Laing wrote:
>>> This is one option that is used around here but there are reports that
>>> I have heard about HD's that don't like being left alone on the shelf
>>> and need to be part of a computer or they commit suicide. :)
>>
>> I wouldn't be surprised.  They're mechanical devices, and bearings do
>> stuff up.
>>
>> In around 20+ years of working in audio and video production, I can say
>> that things like tape decks left sitting in storage often need some
>> maintenance to free up stuck bearings.  Sometimes quite a lot of effort
>> is required to get them working again.
>>
>> Of course, some of that may be due to the bearings not being sealed.
>> But tight bearing fittings and temperature changes might just contribute
>> to something that wasn't designed as well as it ought to be, getting
>> stuck.
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Joel Jaeggli  	       Unix Consulting 	       joelja at darkwing.uoregon.edu
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