> -----Original Message----- > From: Philipp Marek [mailto:philipp.ma...@emerion.com] > Sent: Wednesday, February 25, 2009 12:31 PM > That's the question ... if it's "cryptographically secure", > it means (AFAIU) that it's "hard" to get collisions ... but > it's not impossible. > Really, it's *guaranteed* that on a large-enough filesystem > (some TB, anyone?) you'll get two blocks with the same hash value. With a 512bit hash value from SHA-2(which is considdered collision-resistant), you'll probably get a collision roughly after 2**256 blocks you hash(Birthday paradox). This equates to an extremely large filesystem(2**218 PiB). By using SHA-1(which has Some problems, besides is limited size), you'll get a collision After about 2**80 blocks. 2**80 blocks is still a very large filesystem(2**42 PiB).
> Therefore I asked whether the risk is acceptable ... there > has been some filesystem (I think that was more than 10 years > ago, didn't find a link) that tried deduplication by some > hash - but got shot down, because without > *verification* that the data is identical you might > *silently* shoot yourself (and all others) in the foot. By using a large enough hash-value there shouldn't be a problem. But it might be a filesystem-option. > Ok. > But if verification is needed anyway, then something *much* > simpler (and > *much* faster) would be ok, too. Any hash-function that you'll use have a much shorter calculation time than any access to rotating media. Even SSD's are slower than what a mainstream CPU can calculate secure hashes from. -- +-----------------------------------------------------+ + Stefan Christensen, Dept. Eng, | Ph: +47 7764 6406 + + IT-department | Fax: +47 7764 4100 + +-----------------------------------------------------+ _______________________________________________ Tux3 mailing list Tux3@tux3.org http://mailman.tux3.org/cgi-bin/mailman/listinfo/tux3
