Scott Rotondo wrote:
> 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.
The fact that it exceeds the 2^64 limit is good enough for me. :-)
-- Garrett
>
> Scott
>
