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Re: Enabling Checksums

Ants Aasma <ants@cybertec.at>

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From: Ants Aasma <ants@cybertec.at>
To: Florian Pflug <fgp@phlo.org>
Cc: Jeff Davis <pgsql@j-davis.com>, Greg Smith <greg@2ndquadrant.com>, Tom Lane <tgl@sss.pgh.pa.us>, Andres Freund <andres@2ndquadrant.com>, Bruce Momjian <bruce@momjian.us>, Heikki Linnakangas <hlinnakangas@vmware.com>, Simon Riggs <simon@2ndquadrant.com>, PostgreSQL-development <pgsql-hackers@postgresql.org>
Date: 2013-04-18T17:13:15Z
Lists: pgsql-hackers 
On Thu, Apr 18, 2013 at 8:05 PM, Florian Pflug <fgp@phlo.org> wrote:
> On Apr18, 2013, at 19:04 , Jeff Davis <pgsql@j-davis.com> wrote:
>> On Wed, 2013-04-17 at 20:21 -0400, Greg Smith wrote:
>>> -Original checksum feature used Fletcher checksums.  Its main problems,
>>> to quote wikipedia, include that it "cannot distinguish between blocks
>>> of all 0 bits and blocks of all 1 bits".
>>
>> That is fairly easy to fix by using a different modulus: 251 vs 255.
>
> At the expense of a drastic performance hit though, no? Modulus operations
> aren't exactly cheap.

The modulus can be done in the end. By using a modulus of 65521 the
resulting checksum is called Adler-32. [1] However the quality of
Fletcher-32/Adler-32 is strictly worse than even the first iteration
of multiply-add based checksums proposed.

[1] http://en.wikipedia.org/wiki/Adler-32

Regards,
Ants Aasma
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