Re: standby recovery fails (tablespace related) (tentative patch and discussion)

At Mon, 28 Mar 2022 12:17:50 -0400, Robert Haas <robertmhaas(at)gmail(dot)com> wrote in
> On Mon, Mar 21, 2022 at 3:02 PM Alvaro Herrera <alvherre(at)alvh(dot)no-ip(dot)org> wrote:
> > > 2. Why not instead change the code so that the operation can succeed,
> > > by creating the prerequisite parent directories? Do we not have enough
> > > information for that? I'm not saying that we definitely should do it
> > > that way rather than this way, but I think we do take that approach in
> > > some cases.
> >
> > It seems we can choose freely between these two implementations -- I
> > mean I don't see any upsides or downsides to either one.
>
> What got committed here feels inconsistent to me. Suppose we have a
> checkpoint, and then a series of operations that touch a tablespace,
> and then a drop database and drop tablespace. If the first operation
> happens to be CREATE DATABASE, then this patch is going to fix it by
> skipping the operation. However, if the first operation happens to be
> almost anything else, the way it's going to reference the dropped
> tablespace is via a block reference in a WAL record of a wide variety
> of types. That's going to result in a call to
> XLogReadBufferForRedoExtended() which will call
> XLogReadBufferExtended() which will do smgrcreate(smgr, forknum, true)
> which will in turn call TablespaceCreateDbspace() to fill in all the
> missing directories.

Right. I thought that recovery avoids that but that's wrong. This
behavior creates a bare (non-linked) directly within pg_tblspc. The
directory would dissapear soon if recovery proceeds to the consistency
point, though.

> I don't think that's very good. It would be reasonable to decide that
> we're never going to create the missing directories and instead just
> remember that they were not found so we can do a cross check. It's
> also reasonable to just create the directories on the fly. But doing a
> mix of those systems doesn't really seem like the right idea -
> particularly because it means that the cross-check system is probably
> not very effective at finding actual problems in the code.
>
> Am I missing something here?

No. I agree that mixing them is not good. On the other hand we
already doing that by heapam. AFAICS sometimes it avoid creating a
new page but sometimes creates it. But I don't mean to use the fact
for justifying this patch to do that, or denying to do that.

regards.

--
Kyotaro Horiguchi
NTT Open Source Software Center