Re: replication_slots usability issue

On 2018-10-29 16:02:18 -0300, Alvaro Herrera wrote:
> On 2018-Oct-29, Joshua D. Drake wrote:
>
> > -Hackers,
> >
> >
> > Working on 9.6 today (unsure if fixed in newer versions). Had an issue where
> > the wal was 280G despite max_wal_size being 8G. Found out there were stale
> > replication slots from a recent base backup. I went to drop the replication
> > slots and found that since the wal_level was set to minimal vs replica or
> > higher, I couldn't drop the replication slot. Clearly that makes sense for
> > creating a replication slot but it seems like an artificial limitation for
> > dropping them.
>
> This sounds closely related to
> https://www.postgresql.org/message-id/20180508143725.mn3ivlyvgpul6ovr%40alvherre.pgsql
> (commit a1f680d962ff) wherein we made it possible to drop a slot in
> single-user mode.
>
> Seems worth fixing. Send a patch?

I don't think this quite is the problem. ISTM the issue is rather that
StartupReplicationSlots() *needs* to check whether wal_level > minimal,
and doesn't. So you can create a slot, shutdown, change wal_level,
startup. A slot exists but won't work correctly.

Greetings,

Andres Freund