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

On Sun, Sep 13, 2026 at 03:10:58PM +0300, Alexander Korotkov wrote:
> 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.

> +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 could instead be taught to skip
> +*_fsm files, and that may be the right call someday. The cost would be that
> +a tool told to ignore a tear in the FSM can no longer report one caused by
> +failing storage. Today they do verify, so what we write here has to be
> +valid.

I'd handle the last sentences differently, yielding a paragraph like this:

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 alternative would be all such code skipping checksum
validation of *_fsm files, and that may be the right call someday. Today they
do verify, so what we write here has to be valid.