| From: | Ants Aasma <ants(at)cybertec(dot)at> |
|---|---|
| To: | Florian Pflug <fgp(at)phlo(dot)org> |
| Cc: | Jeff Davis <pgsql(at)j-davis(dot)com>, Greg Smith <greg(at)2ndquadrant(dot)com>, Tom Lane <tgl(at)sss(dot)pgh(dot)pa(dot)us>, Andres Freund <andres(at)2ndquadrant(dot)com>, Bruce Momjian <bruce(at)momjian(dot)us>, Heikki Linnakangas <hlinnakangas(at)vmware(dot)com>, Simon Riggs <simon(at)2ndquadrant(dot)com>, PostgreSQL-development <pgsql-hackers(at)postgresql(dot)org> |
| Subject: | Re: Enabling Checksums |
| Date: | 2013-04-18 17:13:15 |
| Message-ID: | CA+CSw_tQZw2oZPQ5TUv8kVweKPuHhZaDYka-8KJg30y_Kr-Lbg@mail.gmail.com |
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| Lists: | pgsql-hackers |
On Thu, Apr 18, 2013 at 8:05 PM, Florian Pflug <fgp(at)phlo(dot)org> wrote:
> On Apr18, 2013, at 19:04 , Jeff Davis <pgsql(at)j-davis(dot)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
--
Cybertec Schönig & Schönig GmbH
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Web: http://www.postgresql-support.de
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