Re: Enabling Checksums

From: Ants Aasma <ants(at)cybertec(dot)at>
To: Ants Aasma <ants(at)cybertec(dot)at>
Cc: Florian Pflug <fgp(at)phlo(dot)org>, Andres Freund <andres(at)2ndquadrant(dot)com>, Tom Lane <tgl(at)sss(dot)pgh(dot)pa(dot)us>, Bruce Momjian <bruce(at)momjian(dot)us>, Jeff Davis <pgsql(at)j-davis(dot)com>, Heikki Linnakangas <hlinnakangas(at)vmware(dot)com>, Simon Riggs <simon(at)2ndquadrant(dot)com>, Greg Smith <greg(at)2ndquadrant(dot)com>, PostgreSQL-development <pgsql-hackers(at)postgresql(dot)org>
Subject: Re: Enabling Checksums
Date: 2013-04-18 18:02:49
Message-ID: CA+CSw_tfmXkGz_AE+f+3y99b06XZwZ-N1+4a3DFB389OoL-bsQ@mail.gmail.com
Views: Whole Thread | Raw Message | Download mbox | Resend email
Thread:
Lists: pgsql-hackers

On Thu, Apr 18, 2013 at 8:24 PM, Ants Aasma <ants(at)cybertec(dot)at> wrote:
> On Thu, Apr 18, 2013 at 8:15 PM, Florian Pflug <fgp(at)phlo(dot)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

In response to

Responses

Browse pgsql-hackers by date

  From Date Subject
Next Message Florian Pflug 2013-04-18 18:11:28 Re: Enabling Checksums
Previous Message Jeff Davis 2013-04-18 17:40:00 Re: Enabling Checksums