On Thu, Jan 06, 2011 at 06:07:47PM -0500, David Magda wrote:
> On Jan 6, 2011, at 15:57, Nicolas Williams wrote:
>
> > Fletcher is faster than SHA-256, so I think that must be what you're
> > asking about: "can Fletcher+Verification be faster than
> > Sha256+NoVerification?" Or do you have some other goal?
>
> Would running on recent T-series servers, which have have on-die
> crypto units, help any in this regard?
Yes, particularly for larger blocks.
Hash collisions don't matter as long as ZFS verifies dups, so the real
question is: what is the false positive dup rate (i.e., the accidental
collision rate). But that's going to vary a lot by {hash function,
working data set}, thus it's not possible to make exact determinations,
just estimates.
For me the biggest issue is that as good as Fletcher is for a CRC, I'd
rather have a cryptographic hash function because I've seen incredibly
odd CRC failures before. There's a famous case from within SWAN a few
years ago where a switch flipped pairs of bits such that all too often
the various CRCs that applied to the moving packets failed to detect the
bit flips, and we discovered this when an SCCS file in a clone of the ON
gate got corrupted. Such failures (collisions) wouldn't affect dedup,
but they would mask corruption of non-deduped blocks.
Nico
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