On 12/4/2014 6:52 PM, berg...@merctech.com wrote:
In the message dated: Thu, 04 Dec 2014 14:36:07 -0800,
The pithy ruminations from "Michael D. Parker" on
<Re: [lopsa-tech] Data on longevity of powered off disk drives?> were:
=> I think that a bigger problem might be the quickly changing disk technology
=> and having to maintain possibly obsolete hardware that could read the disks
=> in the future if need be.
Actually, I'd claim that's just the opposite situation, particularly
when comparing disk to tape.
The 'quickly changing disk technology' won't affect the drives you've
got locked away in a fireproof safe, as long as the disk _interface_
(SAS, SATA, FC, etc.) is still available in the future.
One advantage cited for disks over tape for off-line storage is that a
disk drive has everything needed to read data within the device--only the
interface & power must be supplied by the server, while a tape cartridge
relies on the mechanism within a tape drive or library to physically
move the tape, as well as logically read the media.
My biggest concern about the longevity of disk drives for off-line storage
would be mechanical. A disk drive has a fair number of moving parts,
which have been designed to move, not sit still for extremely long
periods. Many years ago, disk drives were well known for having problems
spinning up if they were shut down for too long -- typically the problem
was ascribed to stuck bearings, congealed grease, etc. This may not be
[as much of] an issue with modern drives.
There are very many modern drives that are designed for many power
cycles, both for desktop and for big data. (Companies like Facebook,
which have tremendous buying power and market influence, use a lot of
drives like this to save power for 'old stuff'. They even have designed
'cold storage' servers where the power envelope is such that only one
drive be accessed at a time in the entire server.
However, I haven't seen any mention of data integrity. An average low
cost sata drive has an expected, published bit error rate of about
1E10-14. Modern tape has a published, expected bit error rate of about
1E10-19 or 1E10-20. That's 5-6 orders of magnitude better data integrity
for shelf storage! Plus tape is still cheaper by quite a bit, no matter
what many of the storage manufacturers and integrators have tried to
claim, it's easier to ship (lighter, more shock resistant), higher
density, and higher data rate per unit. The big advantage for disk is in
access latency, but you completely lose that as soon as you are talking
about putting it on a shelf.
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