Re: Enabling Checksums

On Sat, Apr 13, 2013 at 06:14:28PM +0300, Ants Aasma wrote:
> > CRCs are known to be good for that sort of thing; it's what they were
> > designed for.  I'd like to see some evidence that any substitute
> > algorithm has similar properties.  Without that, I'm going to vote
> > against this idea.
> 
> Sorry for creating confusion here by playing fast and loose with the
> terminology. We are not talking about that hash function at all. What
> we are talking about here is Fowler-Noll-Vo-ish
> (http://en.wikipedia.org/wiki/Fowler%E2%80%93Noll%E2%80%93Vo_hash_function)
> hash function that is restructured to be parallelisable with SIMD
> instructions with the explicit goal of being as fast as possible. The
> resulting hash function is roughly two orders of magnitude faster than
> 1-byte-at-a-time CRC32 currently in use. Performance is about
> comparable with optimized fixed size memcpy running in cache.
> 
> Based on current analysis, it is particularly good at detecting single
> bit errors, as good at detecting burst errors as can be expected from
> 16 bits and not horrible at detecting burst writes of zeroes. It is
> quite bad at detecting multiple uncorrelated single bit errors and
> extremely bad at detecting repeating patterns of errors in low order
> bits.
> 
> All in all I would say that the performance is worth the loss in
> detection capability as we are not talking about using the checksum to
> prove correctness.

Agreed. It would be good to get these details into the patch so others
are not confused in the future.

--  Bruce Momjian  <bruce@momjian.us>        http://momjian.us EnterpriseDB
http://enterprisedb.com
 + It's impossible for everything to be true. +