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Date: Wed, 26 Feb 2025 13:38:22 +0100
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From: =?UTF-8?Q?Fran=C3=A7ois_Lafont?= <francois.lafont.1978@gmail.com>
Subject: PG16 and replication, ensure a clean switchover after a stop of the
 primary server
To: pgsql-general@postgresql.org
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Hi,

First, I have put all details of this post in this gist
https://gist.github.com/flaf/ccedf78d0290d231e79581077fd92dbc
(with a little video/demo to see with your eyes ;)).

I have 2 PostgreSQL servers version 16.8 on RedHat 8.10, with
physical streaming async replication:

* postgres-1 the primary server,
* postgres-2 the warm standby server.

It's a detail but on these servers, PGDATA=/pg_data/pginc and
the UniX account and superuser is "pginc". There is continuous WAL
archiving via the /pg_archives/pginc/ NFS share mounted on postgres-1
and postgres-2.

My goal is to stop nicely the PostgreSQL daemon on the primary
server and then ensure that the standby server is well synchronized
(ie it contains *exactly* the same data as the stopped primary server).

To check that, I use this command on the primary and on the standby:

~$ pg_controldata /pg_data/pginc/ | grep REDO -C 6

Here an example of output on the primary when PostgreSQL has been
stopped:

-------------------------------------------------------------------
Catalog version number:               202307071
Database system identifier:           7470764453209630447
Database cluster state:               shut down
pg_control last modified:             Tue 25 Feb 2025 03:51:34 PM CET
Latest checkpoint location:           0/6F000028
Latest checkpoint's REDO location:    0/6F000028
Latest checkpoint's REDO WAL file:    00000001000000000000006F
Latest checkpoint's TimeLineID:       1
Latest checkpoint's PrevTimeLineID:   1
Latest checkpoint's full_page_writes: on
Latest checkpoint's NextXID:          0:4349
Latest checkpoint's NextOID:          18418
-------------------------------------------------------------------

To check if the standby server is well synchronized I compare the value
of "Latest checkpoint's REDO location" on primary and on standby (but is
it a reliable check? <= it's my question 4).

Here are my questions.

1. When I stop nicely PostgreSQL on the primary server (postgres-1) with
`systemctl stop postgresql.service` and then I run the `CHECKPOINT;` query
on the secondary server (postgres-2), the latest checkpoint's REDO location
are not the same on postgres-1 and postgres-2, except from time to time
(rather rarely) in a rather random way. Why? I thought that walsender on the
primary server would delay the PostgreSQL shutdown request until confirming
all sent data are flushed on remote side? Is it false?

This page https://postgrespro.com/list/thread-id/2630031 seems to confirm
it's well true.

Note: servers are just VM for test on my desktop. There is no trafic
(read/write) on the primary except some INSERTs I do manually to
simulate a little traffic (see my video on my gist).


2. In fact, concerning this point (walsenders delay the shutdown request until
confirming all sent data are flushed on remote side), is there official
documentation? I have not found.


3. Now, I add this line in the postgresql.conf of primary server:

     wal_keep_size = '512MB'

The value seems to be without any importance, I just set the parameter
to a non zero value. I reload the configuration. Now, I'm doing exactly the
same test as 1. and, at the end, I have **systematically** the same the
latest checkpoint's REDO location on postgres-1 and postgres-2. Why? Is it
normal? Or is the case 1. abnormal?


4. When the primary server is stopped, if have exactly the same latest
checkpoint location and the same latest checkpoint's REDO location, on the
primary server and the secondary server, can I be sure that the data on the
secondary server is exactly the same as the data on the primary server,
with the last changes made to the primary server before its PostgreSQL
daemon was shutdown?

Thanks for your help.

Note: here is the common postgresql.conf of the two servers
(but all is in my gist https://gist.github.com/flaf/ccedf78d0290d231e79581077fd92dbc):

-------------------------------------------------------------------
listen_addresses               = 'localhost,<IP-address-of-postgres-X>'
port                           = '4900'
unix_socket_directories        = '/var/run/postgresql'
unix_socket_permissions        = 0700
max_connections                = 810
superuser_reserved_connections = 10
timezone                       = 'Europe/Paris'
lc_messages                    = 'en_US.UTF8'
lc_monetary                    = 'fr_FR.UTF8'
lc_numeric                     = 'fr_FR.UTF8'
lc_time                        = 'fr_FR.UTF8'
default_text_search_config     = 'pg_catalog.french'
datestyle                      = 'iso,dmy'
log_directory                  = '/pg_logs/pginc'
logging_collector              = on
log_filename                   = 'postgresql-%a.log'
log_rotation_age               = 1d
log_rotation_size              = 0
log_truncate_on_rotation       = on
log_timezone                   = 'Europe/Paris'
log_min_duration_statement     = 5000 # 5 seconds
log_autovacuum_min_duration    = 0
password_encryption            = scram-sha-256
wal_level                      = replica
archive_mode                   = on
archive_command                = 'cp %p /pg_archives/pginc/%f'
restore_command                = 'cp /pg_archives/pginc/%f %p'
#wal_keep_size                  = '512MB'
dynamic_shared_memory_type     = posix
max_worker_processes           = 2
max_parallel_workers           = 2
shared_buffers                 = 1420MB # ~0.25 x RAM
effective_cache_size           = 4260MB # ~0.75 x RAM
work_mem                       = 10MB
maintenance_work_mem           = 512MB # 512 or 1024 MiB
wal_buffers                    = 16MB
min_wal_size                   = 1GB
max_wal_size                   = 4GB
max_standby_streaming_delay    = 20min # To allow long readonly queries on the standby server.
effective_io_concurrency       = 200
random_page_cost               = 1.1
track_activity_query_size      = 8096
shared_preload_libraries       = 'pg_stat_statements,auto_explain'
pg_stat_statements.max         = 10000
auto_explain.log_min_duration  = -1 # disable auto_explain extension currently
-------------------------------------------------------------------

-- 
François Lafont