Re: better page-level checksums

On Mon, Jun 13, 2022 at 3:06 PM Bruce Momjian  wrote:
> That is encryption done in a virtual file system independent of
> Postgres.  So, I guess the answer to your question is that this is not
> how EDB Advanced Server does it.

Okay, thanks for clearing that up. The term "block based" does appear
in the article I linked to, so you can see why I didn't understand it
that way initially.

Anyway, I can see how it would be useful to be able to know the offset
of a nonce or of a hash digest on any given page, without access to a
running server. But why shouldn't that be possible with other designs,
including designs closer to what I've outlined?

A known fixed offset in the special area already assumes that all
pages must have a value in the first place, even though that won't be
true for the majority of individual Postgres servers. There is
implicit information involved in a design like the one Robert has
proposed; your backup tool (or whatever) already has to understand to
expect something other than no encryption at all, or no checksum at
all. Tools like pg_filedump already rely on implicit information about
the special area.

I'm not against the idea of picking a handful of checksum/encryption
schemes, with the understanding that we'll be committing to those
particular schemes indefinitely -- it's not reasonable to expect
infinite flexibility here (and so I don't). But why should we accept
something that seems to me to be totally inflexible, and doesn't
compose with other things?

-- 
Peter Geoghegan