On 08/09/2026 11:17, Daniel Gustafsson wrote:
On 7 Sep 2026, at 17:57, Heikki Linnakangas <[email protected]> wrote:
3: Primary on, replica off
You end up in this situation, if you turn on run pg_checksums to turn on
checksums in primary. You stay that you really really shouldn't stay for long
in this state, but why? What's the harm?
One harm is that it's confusing, but if we have to deal with it anyway, why is
it so bad?
Tools like pg_rewind etc rely on the fact that nodes are equal in checksum
state. What if the replica is promoted? I'm personally unconvinced that there
is a good usecase for per-node settings, but I've spent X years thinking about
checksums being replicated so I am clearly biased.
Considering how complicated it was to get replicated states right, if we want
to make it per-node I think we need to go back to the drawing board and
re-think properly rather than settle for that it seems to work. (Not that I
think that's what you're advocating, I just expect getting it work will be
complicated.)
Sure, I don't see any point in this setup either. It's just something
that you can end up with. But as long as you can end up with it, we need
to deal with it gracefully.
If the replica is promoted, that seems fine. Checksums will be off.
In principle, I think pg_rewind would still work as long as
wal_log_hints=on. But I don't think we need to cater for that, erroring
out is fine.
4: Primary off, replica on
Is this possible? Does it make sense?
This is possible using pg_checksums, but like the inverse case I don't think it
makes sense to use different settings across replication.
How about we forbid this completely? Forbid running pg_checksums on
replica, unless the primary already has checksums on.
I guess that would make it impossible to run pg_checksums concurrently
in the primary and the standby. You'd have to run pg_checksums on
primary first, wait for it to finish, and only then launch it in the
standby.
Maybe print a warning if you run pg_checksums in a replica:
WARNING: unless you run pg_checksums on the primary at the same time,
the checksums will be immediately disabled again after startup
- Heikki
