Garrett D'Amore wrote: > Scott Rotondo wrote: >> In any case, I think it's safe to conclude that SHA-256 is more than >> adequate for filesystem block equality comparisons. > > That's true today. At what point will Moore's law catch up though? > (In other words, how long will it take for storage densities to reach > the point where where the risk of a collision becomes significant?) > Start from a petabyte (probably about the largest practical filesystem > size in use today), and double every 12 months. (I think storage has > been outpacing Moore somewhat.) >
To answer that question, consult the table Krishna provided: http://en.wikipedia.org/wiki/Birthday_paradox#Probability_table First, select an acceptable collision probability. Let's choose 10^-18, which is the smallest probability found in the table, and (according to the same article) at the low end of the uncorrectable bit error rate for a typical hard disk. According to the table, SHA-256 can handle 4.8 x 10^29 (approx 2^98) blocks given our acceptable collision probability. That exceeds the ZFS limit of 2^64 *bytes* per filesystem. If we ignore the ZFS limit on filesystem size, and assume a disk block is 2K bytes, that's 2^59 petabytes. Your assumed rate of filesystem growth means we'll need a new plan in 60 years. Scott -- Scott Rotondo Principal Engineer, Solaris Security Technologies President, Trusted Computing Group Phone/FAX: +1 408 850 3655 (Internal x68278)
