Re: speed up a logical replica setup

Hi,

I have reviewed the v21 patch. And found an issue.

Initially I started the standby server with a new postgresql.conf file
(not the default postgresql.conf that is present in the instance).
pg_ctl -D ../standby start -o "-c config_file=/new_path/postgresql.conf"

And I have made 'max_replication_slots = 1' in new postgresql.conf and
made 'max_replication_slots = 0' in the default postgresql.conf file.
Now when we run pg_createsubscriber on standby we get error:
pg_createsubscriber: error: could not set replication progress for the
subscription "pg_createsubscriber_5_242843": ERROR: cannot query or
manipulate replication origin when max_replication_slots = 0
NOTICE: dropped replication slot "pg_createsubscriber_5_242843" on publisher
pg_createsubscriber: error: could not drop publication
"pg_createsubscriber_5" on database "postgres": ERROR: publication
"pg_createsubscriber_5" does not exist
pg_createsubscriber: error: could not drop replication slot
"pg_createsubscriber_5_242843" on database "postgres": ERROR:
replication slot "pg_createsubscriber_5_242843" does not exist

I observed that when we run the pg_createsubscriber command, it will
stop the standby instance (the non-default postgres configuration) and
restart the standby instance which will now be started with default
postgresql.conf, where the 'max_replication_slot = 0' and
pg_createsubscriber will now fail with the error given above.
I have added the script file with which we can reproduce this issue.
Also similar issues can happen with other configurations such as port, etc.

The possible solution would be
1) allow to run pg_createsubscriber if standby is initially stopped .
I observed that pg_logical_createsubscriber also uses this approach.
2) read GUCs via SHOW command and restore them when server restarts

I would prefer the 1st solution.

Thanks and Regards,
Shlok Kyal