Re: On the subject of RAID-6 corruption recovery

Thiemo Nagel wrote:

quoted

quoted

quoted

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.

That's why I was asking about the generator.  Theoretically, this
situation might be countered by using a (pseudo-)random pattern of
generators for the different bytes of a sector, though I'm not sure
whether it is worth the effort.

Kind regards,

Thiemo Nagel