Re: 16-bit page checksums for 9.2

Excerpts from Heikki Linnakangas's message of mié feb 29 16:09:02 -0300 2012:
> On 29.02.2012 19:54, Simon Riggs wrote:
> > I'm beginning to lose faith that objections are being raised at a
> > rational level. It's not a panel game with points for clever answers,
> > its an engineering debate about how to add features real users want.
> > And they do want, so let me solve the problems by agreeing something
> > early enough to allow it to be implemented, rather than just
> > discussing it until we run out of time.
> 
> I thought my view on how this should be done was already clear, but just 
> in case it isn't, let me restate: Enlarge the page header to make room 
> for the checksum. To handle upgrades, put code in the backend to change 
> the page format from old version to new one on-the-fly, as pages are 
> read in. Because we're making the header larger, we need to ensure that 
> there's room on every page. To do that, write a utility that you run on 
> the cluster before running pg_upgrade, which moves tuples to ensure 
> that. To ensure that the space doesn't get used again before upgrading, 
> change the old version so that it reserves those N bytes in all new 
> insertions and updates (I believe that approach has been discussed 
> before and everyone is comfortable with backpatching such a change). All 
> of this in 9.3.

Note that if we want such an utility to walk and transform pages, we
probably need a marker in the catalogs somewhere so that pg_upgrade can
make sure that it was done in all candidate tables -- which is something
that we should get in 9.2 so that it can be used in 9.3.  Such a marker
would also allow us get rid of HEAP_MOVED_IN and HEAP_MOVED_OUT.

-- 
Álvaro Herrera <alvherre@commandprompt.com>
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