Re: [PoC] pg_upgrade: allow to upgrade publisher node

On Thu, Aug 10, 2023 at 7:07 PM Masahiko Sawada <sawada.mshk@gmail.com> wrote:
>
> On Thu, Aug 10, 2023 at 12:52 PM Amit Kapila <amit.kapila16@gmail.com> wrote:
> >
> >
> > Are you suggesting doing this before we start the old cluster or after
> > we stop the old cluster? I was thinking about the pros and cons of
> > doing this check when the server is 'on' (along with other upgrade
> > checks something like the patch is doing now) versus when the server
> > is 'off'. I think the advantage of doing it when the server is 'off'
> > (after check_and_dump_old_cluster()) is that it will be ensured that
> > there is no extra WAL that could be generated during the upgrade and
> > has not been verified against confirmed_flush_lsn location. But OTOH,
> > to retrieve slot information when the server is 'off', we need a
> > separate utility or probably a functionality for the same in
> > pg_upgrade and also some WAL reading stuff which sounds to me like a
> > larger change that may not be warranted here. I think anyway the extra
> > WAL (if any got generated during the upgrade) won't be required after
> > the upgrade so not convinced to make such a check while the server is
> > 'off'. Are there reasons which make it better to do this while the old
> > cluster is 'off'?
>
> What I imagined is that we do this check before
> check_and_dump_old_cluster() while the server is 'off'. Reading the
> slot state file would be simple and I guess we would not need a tool
> or cli program for that. We need to expose RepliactionSlotOnDisk,
> though.
>

Won't that require a lot of version-specific checks as across versions
the file format could be different? For the case of the control file,
we use version-specific pg_controldata (for the old cluster, the
corresponding version's pg_controldata) utility to read the old
version control file. I thought we need something similar here if we
want to do what you are suggesting.

>
> After reading the control file and the slots' state files we
> check if slot's confirmed_flush_lsn matches the latest checkpoint LSN
> in the control file (BTW maybe we can get slot name and plugin name
> here instead of using pg_dump?).

But isn't the advantage of doing via pg_dump (in binary_mode) that we
allow some outside core in-place upgrade tool to also use it if
required? If we don't think that would be required then we can
probably use the info we retrieve it in pg_upgrade.

>
> the first commit. Or another idea would be to allow users to mark
> replication slots "upgradable" so that pg_upgrade skips the
> confirmed_flush_lsn check.
>

I guess for that we need to ask users to ensure that confirm_flush_lsn
is up-to-date and then provide some slot-level API to mark the slots
with the required status. If so, that sounds a bit complicated for
users.

-- 
With Regards,
Amit Kapila.