Re: 16-bit page checksums for 9.2

On 06.02.2012 05:48, Bruce Momjian wrote:
> On Sun, Feb 05, 2012 at 08:40:09PM +0000, Simon Riggs wrote:
>> In the proposed scheme there are two flag bits set on the page to
>> indicate whether the field is used as a checksum rather than a version
>> number. So its possible the checksum could look like a valid page
>> version number, but we'd still be able to tell the difference.
>
> Thanks. Clearly we don't need 16 bits to represent our page version
> number because we rarely change it. The other good thing is I don't
> remember anyone wanting additional per-page storage in the past few
> years except for a checksum.

There's this idea that I'd like to see implemented:
http://archives.postgresql.org/pgsql-hackers/2010-05/msg01176.php

In any case, I think it's a very bad idea to remove the page version
field. How are we supposed to ever be able to change the page format
again if we don't even have a version number on the page? I strongly
oppose removing it.

I'm also not very comfortable with the idea of having flags on the page
indicating whether it has a checksum or not. It's not hard to imagine a
software of firmware bug or hardware failure that would cause pd_flags
field to be zeroed out altogether. It would be more robust if the
expected bit-pattern was not 0-0, but 1-0 or 0-1, but then you need to
deal with that at upgrade somehow. And it still feels a bit whacky anyway.

> 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.

--
Heikki Linnakangas
EnterpriseDB http://www.enterprisedb.com