Re: better page-level checksums

On Tue, Jun 14, 2022 at 12:13 PM Robert Haas <robertmhaas(at)gmail(dot)com> wrote:
> Peter, unless I have missed something, this email is the very first
> one where you or anyone else have said anything at all about a PD_*
> bit. Even here, it's not very clear exactly what you are proposing.
> Therefore I have neither said anything bad about it in the past, nor
> can I now answer the question as to what is "so bad about it." If you
> want to make a concrete proposal, I will be happy to tell you what I
> think about it.

I am proposing that we not commit ourselves to relying on implicit
information about what must be true for every page in the cluster.
Just having a little additional page-header metadata (in pd_flags)
would accomplish that much, and wouldn't in itself impose any real
overhead on TDE.

It's not like the PageHeaderData.pd_flags bits are already a precious
commodity, in the same way as the heap tuple infomask status bits are.
We can afford to use some of them for this purpose, and then some.

Why wouldn't we do it that way, just on general principle?

You may still find it useful to rely on high level context at the
level of code that runs on the server, perhaps for performance reasons
(though it's unclear how much it matters). In which case the status
bit is technically redundant information as far as the code is
concerned. That may well be fine.

--
Peter Geoghegan