Thread

could sent_lsn be lower than write/flush/replay_lsn?

Jaime Casanova <jcasanov@systemguards.com.ec> — 2025-12-26T17:49:33Z

Hi,

We have a customer that for the second time have most of its logical
replicas (13 of 16) in a catchcup state, they have been working fine
for some time now and suddenly the pg_stat_replication view shows
something like this for all of the replicas in catchup state:

"""
pid              | 2667517
state            | catchup
sent_lsn         | 38B4/67C403A8
write_lsn        | 38B7/D2C9C038
flush_lsn        | 38B7/D2C9C038
replay_lsn       | 38B7/D2C9C038
"""

This doesn't make sense for me. This is 16.9 btw.

The pg_stat_activity says:

"""
wait_event_type  | IO
wait_event       | ReorderBufferWrite
state            | active
backend_xid      |
backend_xmin     |
query_id         |
query            | START_REPLICATION SLOT "sub_down_tables" LOGICAL
38B7/CEBC9330 (proto_version '4', origin 'any', publication_names
'"pub_down_tables"')
backend_type     | walsender
"""

And the logs keeps showing this:

"""
2025-12-26 12:17:41.861 -05 [pid=2667517;l=1;tx=0] LOG:  38B7/CEBC9330
has been already streamed, forwarding to 38B7/D2C9C038
2025-12-26 12:17:41.861 -05 [pid=2667517;l=2;tx=0] STATEMENT:
START_REPLICATION SLOT "sub_down_tables_central_trx001" LOGICAL
38B7/CEBC9330 (proto_version '4', origin 'any', publication_names
'"pub_elipsys_cresio_down_tables"')
2025-12-26 12:17:41.867 -05 [pid=2667517;l=3;tx=0] LOG:  starting
logical decoding for slot "sub_down_tables_central_trx001"
2025-12-26 12:17:41.867 -05 [pid=2667517;l=4;tx=0] DETAIL:  Streaming
transactions committing after 38B7/D2C9C038, reading WAL from
38B0/2261B890.
2025-12-26 12:17:41.867 -05 [pid=2667517;l=5;tx=0] STATEMENT:
START_REPLICATION SLOT "sub_down_tables_central_trx001" LOGICAL
38B7/CEBC9330 (proto_version '4', origin 'any', publication_names
'"pub_elipsys_cresio_down_tables"')
2025-12-26 12:17:41.868 -05 [pid=2667517;l=6;tx=0] LOG:  logical
decoding found consistent point at 38B0/2261B890
2025-12-26 12:17:41.868 -05 [pid=2667517;l=7;tx=0] DETAIL:  Logical
decoding will begin using saved snapshot.
2025-12-26 12:17:41.868 -05 [pid=2667517;l=8;tx=0] STATEMENT:
START_REPLICATION SLOT "sub_down_tables_central_trx001" LOGICAL
38B7/CEBC9330 (proto_version '4', origin 'any', publication_names
'"pub_elipsys_cresio_down_tables"')
2025-12-26 12:30:35.953 -05 [pid=2678504;l=1;tx=0] ERROR:  replication
slot "sub_down_tables_central_trx001" is active for PID 2667517
2025-12-26 12:30:40.959 -05 [pid=2678564;l=1;tx=0] ERROR:  replication
slot "sub_down_tables_central_trx001" is active for PID 2667517
"""

any idea what to check?

--
Jaime Casanova
SYSTEMGUARDS S.A.