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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.

good point and one I hadn't thought of, though its obvious now that
you mention it. I'm thinking of moldy floppies and aging cd's that
definitely fall apart over time with probably less of the typical
bathtub shape and more of a slippery slope... 

> >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.

absolutely right.

> 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.
> >maybe I;'m reading this wrong , but it seems to be what you're asking for.
> There is no way to guarantee that every bit of computer information
> on the Earth is not destroyed by a comet strike :-)

That's why I'm hoping the universe is circular. I can catch the
broadcast on the way back ;-)


ps. Mike, I got one of those bounces the other day and the whois
pointed to US DOD servers. heh.

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