On 2012-12-11 16:44, Jim Klimov wrote:
For single-break-per-row tests based on hypotheses from P parities, D data disks and R broken rows, we need to checksum P*(D^R) userdata recombinations in order to determine that we can't recover the block.
A small maths correction: the formula above reflects that we change some one item from on-disk value to reconstrycted hypothesis on some one data disk(column) in all rows, or on P disks if we try to recover from more than one failed item in a row. Reality is worse :) Our original info (parity errors and checksum mismatch) warranted only that we have at least one error in userdata. It is possible that other (R-1) errors are on the parity disk, so the recombination should also check all variants with (0..R-1) unchanged rows with their on-disk contents intact. This gives us something like P*(D + D^2 + ... + D^R) variants to test, which is roughly a 25% increase in recombinations in the range of computationally feasible amounts of error-matching.
Heck, just counting from 1 to 2^64 in a "i++" loop takes a lot of CPU time
By my estimate, even that would take until the next Big Bang, at least on my one computer ;) Just for fun: a count to 2^32 took 42 seconds, so my computer can do 10^8 trivial loops per second - but that's just a data point. What really matters is that 4^64 == (2^32)^33, which is a lot. Roughly, 2^3 = 8 ~= 10, so the plain count from 1 to 4^64 would take about 42*10^30 seconds, or roughly 10^24 years. If the astronomers' estimates are correct, this amounts to 10^13 lifetimes of our universe, or so ;) //Jim _______________________________________________ zfs-discuss mailing list zfs-discuss@opensolaris.org http://mail.opensolaris.org/mailman/listinfo/zfs-discuss
