Re: Enabling Checksums

Jeff Davis <pgsql@j-davis.com>

From: Jeff Davis <pgsql@j-davis.com>

To: Greg Stark <stark@mit.edu>

Cc: Florian Pflug <fgp@phlo.org>, Bruce Momjian <bruce@momjian.us>, Ants Aasma <ants@cybertec.at>, Tom Lane <tgl@sss.pgh.pa.us>, Andres Freund <andres@2ndquadrant.com>, Heikki Linnakangas <hlinnakangas@vmware.com>, Simon Riggs <simon@2ndquadrant.com>, Greg Smith <greg@2ndquadrant.com>, PostgreSQL-development <pgsql-hackers@postgresql.org>

Date: 2013-04-17T21:00:36Z

Lists: pgsql-hackers

On Wed, 2013-04-17 at 16:58 +0100, Greg Stark wrote:
> On Wed, Apr 17, 2013 at 4:28 PM, Florian Pflug <fgp@phlo.org> wrote:
> > Is there any way we can change the checksum algorithm in 9.4
> > *without* breaking pg_upgrade?
> 
> Personally I think we're going to need a solution for page format
> changes someday eventually....
> 
> What advantages are we postponing now to avoid it?
> 
> * 32-bit checksums?
> * Being able to enable/disable checksums?
> 
> Anything else?

I'm not sure that changing the page format is the most difficult part of
enabling/disabling checksums. It's easy enough to have page header bits
if the current information is not enough (and those bits were there, but
Heikki requested their removal and I couldn't think of a concrete reason
to keep them).

Eventually, it would be nice to be able to break the page format and
have more space for things like checksums (and probably a few other
things, maybe some visibility-related optimizations). But that's a few
years off and we don't have any real plan for that.

What I wanted to accomplish with this patch is the simplest checksum
mechanism that we could get that would be fast enough that many people
would be able to use it. I expect it to be useful until we do decide to
break the page format.

Regards,
	Jeff Davis