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Re: Enabling Checksums

Ants Aasma <ants@cybertec.at>

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From: Ants Aasma <ants@cybertec.at>
To: Ants Aasma <ants@cybertec.at>
Cc: Florian Pflug <fgp@phlo.org>, Andres Freund <andres@2ndquadrant.com>, Tom Lane <tgl@sss.pgh.pa.us>, Bruce Momjian <bruce@momjian.us>, Jeff Davis <pgsql@j-davis.com>, Heikki Linnakangas <hlinnakangas@vmware.com>, Simon Riggs <simon@2ndquadrant.com>, Greg Smith <greg@2ndquadrant.com>, PostgreSQL-development <pgsql-hackers@postgresql.org>
Date: 2013-04-18T18:02:49Z
Lists: pgsql-hackers 
On Thu, Apr 18, 2013 at 8:24 PM, Ants Aasma <ants@cybertec.at> wrote:
> On Thu, Apr 18, 2013 at 8:15 PM, Florian Pflug <fgp@phlo.org> wrote:
>> So either the CRC32-C polynomial isn't irreducible, or there something
>> fishy going on. Could there be a bug in your CRC implementation? Maybe
>> a mixup between big and little endian, or something like that?
>
> I'm suspecting an implementation bug myself. I already checked the
> test harness and that was all sane, compiler hadn't taken any
> unforgivable liberties there. I will crosscheck the output with other
> implementations to verify that the checksum is implemented correctly.

Looks like the implementation is correct. I cross-referenced it
against a bitwise algorithm for crc32 with the castagnoli polynomial.
This also rules out any endianness issues as the bitwise variant
consumes input byte at a time.

What ever it is, it is something specific to PostgreSQL page layout.
If I use /dev/urandom as the source the issue disappears. So much for
CRC32 being proven good.

Regards,
Ants Aasma
--
Cybertec Schönig & Schönig GmbH
Gröhrmühlgasse 26
A-2700 Wiener Neustadt
Web: http://www.postgresql-support.de



-- 
Cybertec Schönig & Schönig GmbH
Gröhrmühlgasse 26
A-2700 Wiener Neustadt
Web: http://www.postgresql-support.de