On Wed, Nov 8, 2023 at 8:04 AM Stephen Frost <sfrost@snowman.net> wrote:
Greetings,
* Andres Freund (andres@anarazel.de) wrote: > On 2023-05-09 17:08:26 -0500, David Christensen wrote: > > From 965309ea3517fa734c4bc89c144e2031cdf6c0c3 Mon Sep 17 00:00:00 2001 > > From: David Christensen <david@pgguru.net> > > Date: Tue, 9 May 2023 16:56:15 -0500 > > Subject: [PATCH v4 1/3] Add reserved_page_space to Page structure > > > > This space is reserved for extended data on the Page structure which will be ultimately used for > > encrypted data, extended checksums, and potentially other things. This data appears at the end of > > the Page, after any `pd_special` area, and will be calculated at runtime based on specific > > ControlFile features. > > > > No effort is made to ensure this is backwards-compatible with existing clusters for `pg_upgrade`, as > > we will require logical replication to move data into a cluster with > > different settings here. > > The first part of the last paragraph makes it sound like pg_upgrade won't be > supported across this commit, rather than just between different settings...
Yeah, that's vague, but you picked up on what I meant.
> I think as a whole this is not an insane idea. A few comments:
Thanks for all the feedback!
> - Why is it worth sacrificing space on every page to indicate which features > were enabled? I think there'd need to be some convincing reasons for > introducing such overhead.
In conversations with folks (my memory specifically is a discussion with Peter G, added to CC, and my apologies to Peter if I'm misremembering) there was a pretty strong push that a page should be able to 'stand alone' and not depend on something else (eg: pg_control, or whatever) to provide info needed be able to interpret the page. For my part, I don't have a particularly strong feeling on that, but that's what lead to this design.
Unsurprisingly, I agree that it's useful to keep these features on the page itself; from a forensic standpoint this seems much easier to interpret what is happening here, as well it would allow you to have different features on a given page or type of page depending on need. The initial patch utilizes pg_control to store the cluster page features, but there's no reason it couldn't be dependent on fork/page type or stored in pg_tablespace to utilize different features.
