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

On Thu, Oct 5, 2023 at 1:48 AM Amit Kapila <amit.kapila16@gmail.com> wrote:
>
> Then, we should also try to create slots before invoking pg_resetwal.
> The idea is that we can write a new binary mode function that will do
> exactly what pg_resetwal does to compute the next segment and use that
> location as a new location (restart_lsn) to create the slots in a new
> node. Then, pass it pg_resetwal by using the existing option '-l
> walfile'. As we don't have any API that takes restart_lsn as input, we
> can write a new API probably for binary mode to create slots that do
> take restart_lsn as input. This will ensure that there is no new WAL
> inserted by background processes between resetwal and the creation of
> slots.

+1. I think this approach makes it foolproof. pg_resetwal uses
FindEndOfXLOG and we need that to be in a binary mode SQL callable
function. FindEndOfXLOG ignores TLI to compute the new WAL file name,
but that seems to be okay for the new binary mode function because
pg_upgrade uses TLI 1 anyways and doesn't copy WAL files from old
cluster.

FWIW, pg_upgrades does use -l in copy_xact_xlog_xid, I'm not sure if
it has anything to do with the above proposed change.

> The other potential problem Andres pointed out is that during shutdown
> if due to some reason, the walreceiver goes down, we won't be able to
> send the required WAL and users won't be able to ensure that because
> even after restart the same situation can happen. The ideal way is to
> have something that puts the system in READ ONLY state during shutdown
> and then we can probably allow walreceivers to reconnect and receive
> the required WALs. As we don't have such functionality available and
> it won't be easy to achieve the same, we can leave this for now.
>
> Thoughts?

You mean walreceiver for streaming replication? Or the apply workers
going down for logical replication? If there's yet-to-be-sent-out WAL,
pg_upgrade will fail no? How does the above scenario a problem for
pg_upgrade of a cluster with just logical replication slots?

--
Bharath Rupireddy
PostgreSQL Contributors Team
RDS Open Source Databases
Amazon Web Services: https://aws.amazon.com