Re: 16-bit page checksums for 9.2

From: Bruce Momjian <bruce(at)momjian(dot)us>
To: Heikki Linnakangas <heikki(dot)linnakangas(at)enterprisedb(dot)com>
Cc: Simon Riggs <simon(at)2ndQuadrant(dot)com>, Tom Lane <tgl(at)sss(dot)pgh(dot)pa(dot)us>, PostgreSQL-development <pgsql-hackers(at)postgresql(dot)org>
Subject: Re: 16-bit page checksums for 9.2
Date: 2012-02-06 18:05:01
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On Mon, Feb 06, 2012 at 08:51:34AM +0100, Heikki Linnakangas wrote:
> >I wonder if we should just dedicate 3 page header bits, call that the
> >page version number, and set this new version number to 1, and assume
> >all previous versions were zero, and have them look in the old page
> >version location if the new version number is zero. I am basically
> >thinking of how we can plan ahead to move the version number to a new
> >location and have a defined way of finding the page version number using
> >old and new schemes.
> Three bits seems short-sighted, but yeah, something like 6-8 bits
> should be enough. On the whole, though. I think we should bite the
> bullet and invent a way to extend the page header at upgrade.

I just emailed a possible layout that allows for future page version
number expansion.

I don't think there is any magic bullet that will allow for page header
extension by pg_upgrade. If it is going to be done, it would have to
happen in the backend while the system is running. Anything that
requires pg_upgrade to do lots of reads or writes basically makes
pg_upgrade useless.

Bruce Momjian <bruce(at)momjian(dot)us>

+ It's impossible for everything to be true. +

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