Date: Wed, 8 Nov 2023 09:04:13 -0500
From: Stephen Frost <sfrost@snowman.net>
To: Andres Freund <andres@anarazel.de>
Cc: David Christensen <david.christensen@crunchydata.com>,
	Matthias van de Meent <boekewurm+postgres@gmail.com>,
	Peter Geoghegan <pg@bowt.ie>,
	PostgreSQL-development <pgsql-hackers@postgresql.org>
Subject: Re: [PATCHES] Post-special page storage TDE support
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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 wi=
ll 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 runtim=
e 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.
>=20
> 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=
=2E..
>=20
> 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 featu=
res
>   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

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