Re: [PATCHES] Post-special page storage TDE support

Greetings,

* Andres Freund (andres(at)anarazel(dot)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(at)pgguru(dot)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...
>
> 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.

Getting a consensus on if that's a requirement or not would definitely
be really helpful.

Thanks,

Stephen