Re: Add 64-bit XIDs into PostgreSQL 15

On Fri, Jan 7, 2022 at 03:53:51PM +0000, Finnerty, Jim wrote:
> Re:  The "prepare" approach was the first tried.
> https://github.com/postgrespro/pg_pageprep But it appears to be
> very difficult and unreliable.  After investing many months into
> pg_pageprep, "double xmax" approach appears to be very fast to
> implement and reliable.
>
> I'd still like a plan to retire the "double xmax" representation
> eventually.  Previously I suggested that this could be done as a
> post-process, before upgrade is complete, but that could potentially
> make upgrade very slow.
>
> Another way to retire the "double xmax" representation eventually
> could be to disallow "double xmax" pages in subsequent major version
> upgrades (e.g. to PG16, if "double xmax" pages are introduced in
> PG15).  This gives the luxury of time after a fast upgrade to convert
> all pages to contain the epochs, while still providing a path to more
> maintainable code in the future.

This gets into the entire issue we have discussed in the past but never
resolved --- how do we manage state changes in the Postgres file format
while the server is running?  pg_upgrade and pg_checksums avoid the
problem by doing such changes while the server is down, and other file
formats have avoided it by allowing perpetual reading of the old format.

Any such non-perpetual changes while the server is running must deal
with recording the start of the state change, the completion of it,
communicating such state changes to all running backends in a
synchronous manner, and possible restarting of the state change.

-- 
  Bruce Momjian  <bruce@momjian.us>        https://momjian.us
  EDB                                      https://enterprisedb.com

  If only the physical world exists, free will is an illusion.