pg_receivelwal vs synchronous

On Wed, Jun 26, 2019 at 5:06 PM Jesper Pedersen <jesper.pedersen@redhat.com>
wrote:

Hi,

PostgreSQL 11.4

Given,

postgresql.conf:
----------------
wal_level = replica
synchronous_commit = remote_apply
synchronous_standby_names = '*'

CREATE ROLE repluser WITH LOGIN REPLICATION PASSWORD 'mypwd';
SELECT pg_create_physical_replication_slot('replica1');

Execute

pg_receivewal -D /tmp/wal/ -S replica1 --synchronous -p 5432 -h
localhost -U repluser -W --dbname="application_name=replica1"

gives

LOG: standby "replica1" is now a synchronous standby with priority 1

Then

psql -c 'CREATE DATABASE test' postgres

which hangs (committed locally).

postgres=# SELECT * FROM pg_replication_slots ;
slot_name | plugin | slot_type | datoid | database | temporary |
active | active_pid | xmin | catalog_xmin | restart_lsn |
confirmed_flush_lsn

-----------+--------+-----------+--------+----------+-----------+--------+------------+------+--------------+-------------+---------------------
replica1 | | physical | | | f | t
| 12502 | | | 0/1655508 |
(1 row)

postgres=# SELECT * FROM pg_stat_replication;
pid | usesysid | usename | application_name | client_addr |
client_hostname | client_port | backend_start |
backend_xmin | state | sen
t_lsn | write_lsn | flush_lsn | replay_lsn | write_lag |
flush_lag | replay_lag | sync_priority | sync_state

-------+----------+----------+------------------+-------------+-----------------+-------------+-------------------------------+--------------+-----------+----

-------+-----------+-----------+------------+-----------------+-----------------+-----------------+---------------+------------
12502 | 16384 | repluser | replica1 | ::1 |
| 45816 | 2019-06-26 11:00:45.098276-04 | |
streaming | 0/1
655508 | 0/1655508 | 0/1655508 | | 00:00:00.000267 |
00:00:00.000267 | 00:02:14.938836 | 1 | sync
(1 row)

So, replica1 is active and in sync mode, but replay_lsn is never
updated, and replay_lag keeps increasing.

What am I missing here ?

I believe your problem is remote_apply.

pg_receivewal never *applies* any WAL, so it just updates the write and
flush locations. Notice how the replay_lsn remains NULL.

So you need synchronous_commit to be 'on' or 'remote_write', not
'remote_apply'.

--
Magnus Hagander
Me: https://www.hagander.net/ <http://www.hagander.net/&gt;
Work: https://www.redpill-linpro.com/ <http://www.redpill-linpro.com/&gt;

Hi Magnus,

On 6/27/19 8:37 AM, Magnus Hagander wrote:

What am I missing here ?

I believe your problem is remote_apply.

pg_receivewal never *applies* any WAL, so it just updates the write and
flush locations. Notice how the replay_lsn remains NULL.

So you need synchronous_commit to be 'on' or 'remote_write', not
'remote_apply'.

Yeah, remote_write works :)

I'll a patch for the pg_receivewal documentation to make this more clear.

Thanks !

Best regards,
Jesper