On Sun, Sep 13, 2026 at 03:10:58PM +0300, Alexander Korotkov wrote: > On Sat, Sep 12, 2026 at 11:05 PM Noah Misch <[email protected]> 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.
