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 -- To UNSUBSCRIBE, email to [EMAIL PROTECTED] with a subject of "unsubscribe". Trouble? Contact [EMAIL PROTECTED]
