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

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.