Thiemo Nagel wrote:
For errors occurring on the level of hard disk blocks (signature: most
bytes of the block have D errors, all with same z), the probability for
multidisc corruption to go undetected is ((n-1)/256)**512. This might
pose a problem in the limiting case of n=255, however for practical
applications, this probability is negligible as it drops off
exponentially with decreasing n:
That assumes fully random data distribution, which is almost certainly a
false assumption.
Agreed. This means, that the formula only serves to specify a lower limit
to the probability. However, is there an argumentation, why a pathologic
case would be probable, i.e. why the probability would be likely to
*vastly* deviate from the theoretical limit? And if there is, would that
argumentation not apply to other raid 6 operations (like "check") also?
And would it help to use different Galois field generators at different
positions in a sector instead of using a uniform generator?
What you call "pathologic" cases when it comes to real-world data are
very common. It is not at all unusual to find sectors filled with only
a constant (usually zero, but not always), in which case your **512
becomes **1.
It doesn't mean it's not worthwhile, but don't try to claim it is
anything other than opportunistic.
-hpa
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