On Thu, Jul 11, 2019 at 06:45:36AM -0600, Ryan Lambert wrote:
> > >>Uh, what if a transaction modifies page 0 and page 1 of the same table
>
> > >>--- don't those pages have the same LSN.
> > >
> > >No, because WAL being a physical change log, each page gets its own
> > >WAL record with its own LSN.
> > >
> >
> > What if you have wal_log_hints=off? AFAIK that won't change the page LSN.
>
> >
>
> > Alvaro suggested elsewhere that we require checksums for these, which
> > would also force wal_log_hints to be on, and therefore the LSN would
> > change.
>
> Yes, it sounds like the agreement was LSN is unique when wal_log_hints is on.
> I don't know enough about the internals to know if pg_class.oid is also needed
> or not.
Well, so, as far as we know now, every change to a heap/index page
advances the LSN, except for hint bits, which we can force to advance
LSN via wal_log_hints. We automatically enable wal_log_hints for
checksums, so we can easily enable it automatically for encrypted
clusters.
I assume the LSN used for 8k pages and the segment numbers (for WAL) do
not overlap in numbering, for our nonce.
I think we will eventually have to have this setup verified by security
experts but I think we need to work through what is possible using
existing Postgres facilities before we present a possible solution.
--
Bruce Momjian <bruce@momjian.us> http://momjian.us
EnterpriseDB http://enterprisedb.com
+ As you are, so once was I. As I am, so you will be. +
+ Ancient Roman grave inscription +
