UMG Developer (3/25/2011)
Steve - whilst you may not have heard of checksum (Vs parity) in relation to RAID 5 , try searching google for "RAID 5 checksum". I do agree that you can view this as a "parity" but I am one of them dinosaurs who have been around since the days of punch cards, paper tape and magnetic tape. Parity back then was more to do with detecting error.
Isn't that still what the parity/checksum in RAID5 is for to detect and correct errors?
No, it's never used for detection, only for correction.
Incidentally, I suspect that I'm a much older dinosaur that happycat58 and for me "parity" has always meant "an extra bit that tells you whether the rest of the bits have an even number or odd number of 1s" which is exactly what parity on a RAID5 array is; you could use that for detecting error, yes, and on paper tape and cards longitudinal parity was often used for that (and was close to useless, since common hardware problems tended to produce error patterns that that wouldn't detect); but of course you couldn't use horizontal parity (which would have been rather less useless) if the number of bits on your horizontal line was the number of data bits you needed, which it almost always was for binary data.
On a single unit medium like cards or tape horizontal parity (which I'm sure is what happycat59 was thinking of, there was a reference to byte parity somewhere) was useless for error correction, because if you detected an error it could be that the parity bit or any of the data bits was wrong; in RAID5 horizontal parity is fine for error correction, because the error detected is that a drive has packed up - so we know which bits are OK and which or wrong. And if you have 9 drives, you can think of RAID5 parity as being just like byte parity on a 9 channel tape - exceptthat (a) which channel is the parity swaps at regular intervals and (b) when you do recovery you know which of the bits in each row is the failed one.
BTW, don't take my explaining RAID5 as a sensible use of parity means I approve of the thing - if data reliability and integrity mean more to you than a couple of per cent on your hardware budget eschew RAID 5 and use RAID 10.