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Re: Uh, this is *not* a 64-bit CRC ...

From: Bruce Momjian <pgman(at)candle(dot)pha(dot)pa(dot)us>
To: pgsql-hackers(at)postgresql(dot)org
Subject: Re: Uh, this is *not* a 64-bit CRC ...
Date: 2001-03-01 02:17:19
Message-ID: 200103010217.VAA26763@candle.pha.pa.us (view raw or flat)
Thread:
Lists: pgsql-hackers
Added to TODO:

	* Correct CRC WAL code to be normal CRC32 algorithm 

> On Wed, Feb 28, 2001 at 04:53:09PM -0500, Tom Lane wrote:
> > I just took a close look at the COMP_CRC64 macro in xlog.c.
> > 
> > This isn't a 64-bit CRC.  It's two independent 32-bit CRCs, one done
> > on just the odd-numbered bytes and one on just the even-numbered bytes
> > of the datastream.  That's hardly any stronger than a single 32-bit CRC;
> > it's certainly not what I thought we had agreed to implement.
> > 
> > We can't change this algorithm without forcing an initdb, which would be
> > a rather unpleasant thing to do at this late stage of the release cycle.
> > But I'm not happy with it.  Comments?
> 
> This might be a good time to update:
> 
>   The CRC-64 code used in the SWISS-PROT genetic database is (now) at:
> 
>     ftp://ftp.ebi.ac.uk/pub/software/swissprot/Swissknife/old/SPcrc.tar.gz
> 
>   From the README:
> 
>   The code in this package has been derived from the BTLib package
>   obtained from Christian Iseli <chris(at)ludwig-alpha(dot)unil(dot)ch>.
>   From his mail:
> 
>   The reference is: W. H. Press, S. A. Teukolsky, W. T. Vetterling, and
>   B. P.  Flannery, "Numerical recipes in C", 2nd ed., Cambridge University
>   Press.  Pages 896ff.
> 
>   The generator polynomial is x64 + x4 + x3 + x1 + 1.
> 
> I would suggest that if you don't change the algorithm, at least change
> the name in the sources.  Were you to #ifdef in a real crc-64, and make 
> a compile-time option to select the old one, you could allow users who 
> wish to avoid the initdb a way to continue with the existing pair of 
> CRC-32s.
> 
> Nathan Myers
> ncm(at)zembu(dot)com
> 


-- 
  Bruce Momjian                        |  http://candle.pha.pa.us
  pgman(at)candle(dot)pha(dot)pa(dot)us               |  (610) 853-3000
  +  If your life is a hard drive,     |  830 Blythe Avenue
  +  Christ can be your backup.        |  Drexel Hill, Pennsylvania 19026

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