Re: backup archive format saved to disk

On Tue, Dec 05, 2006 at 06:58:35PM -0600, Mike McCarty wrote:
Andrew Sackville-West wrote:
On Tue, Dec 05, 2006 at 07:08:54PM -0500, Douglas Tutty wrote:

Yes. But I don't want to loose any data at all.


there is no way to guarantee this. you could improve your odds by
having multiple storage locations with multiple copies and a rigorous
method for routinely testing the backup media for corruption and
making new replacement copies of the backups to prevent future loss.

For example, make multiple identical backups. sprinkle them in various
locations. on a periodic, routine basis, test those backups for
possible corruption. If their clean, make a new copy anyway to put in
rotation, throwing away the old ones after so many periods. If you

Respectfully, I disagree with this last recommendation. You are
suggesting that he continually keep his backup media on the
infant mortality portion of the Weibull distribution. The usual
way for devices which are not subjected to periodic high stress
to fail is to have an infant mortality rate which is high, but falls
down to a low level, then begins to rise again with wearout. In this
case, wearout would be eventual degradation of the metallization
layer in the disc.

find a corrupt one, get one of your clean ones to reproduce it and
start over.

Be sure to use an odd number of copies. Don't want no tied votes
on whether a given bit is a 0 or a 1 :-)

there is now way, using only one physical storage medium, to guarantee
no loss of data.

There is no way, using any number of physical storage media, to
gurantee no loss of data.

On any storage medium, if the probability of error in a data bit is less
than 50%, then given any e > 0 there exists an FEC method which reduces
the probability of data loss to be less than e.

If the probability of error on any given bit is greater than 50%,
then there is no way, by adding additional information, to make
the eventual error rate be less than a single copy. The additional
bits are more likely to be in error than the original.

Speaking pedantically, if the probability of error is greater than 50%,
you can complement every bit and gte a probability less than 50%.

-- hendrik


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