Re: backup archive format saved to disk

[hendrik@topoi.pooq.com]

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