Re: Two issues leading to discrepancies in FSM data on the standby server

On Sat, Sep 12, 2026 at 11:05 PM Noah Misch <noah(at)leadboat(dot)com> wrote:
> On Mon, Sep 07, 2026 at 06:22:22PM +0300, Alexander Korotkov wrote:
> > 0002 is patch for FSM readme to highlight this aspect
> >
> > I'm going to push (and backpatch) 0001. 0002 needs review.
>
> > Subject: [PATCH v1 2/2] Say in the FSM README why its hint writes still need
> > full page images
>
> > +That RBM_ZERO_ON_ERROR is a property of this directory, not of the FSM fork
> > +itself. The files themselves are ordinary relation files, and plenty of code
> > +outside freespace.c reads them and does verify checksums:
> > +RelationCopyStorage(), which ALTER TABLE ... SET TABLESPACE runs over every
> > +fork; the read stream in RelationCopyStorageUsingBuffer(), used by
> > +CREATE DATABASE ... STRATEGY = wal_log; and the checksum verification in base
> > +backups and in pg_checksums. The last two are the reason this cannot simply be
> > +relaxed: an external verification tool has no way to tell a tear that we
> > +consider harmless from one caused by failing storage, so making it accept
> > +the former blinds it to the latter.
>
> One could relax the "last two" by having those tools skip checksum
> verification for *_fsm files, so let's not claim it's infeasible. It's true
> that this could miss failing-storage evidence that coincidentally landed in
> FSM, but accepting that could be the right thing someday. Other than that, I
> agree with this README change. Thanks.

I've corrected that paragraph. I'm going to push this to master if no
objections.

------
Regards,
Alexander Korotkov
Supabase