xmin of slot is not moving | hot standby feedback sending old xmin.

Hi Team,

*Issues :-* xmin of physical replication slot is not moving/hung, slave is
refering/stuck on old xmin. which causes it to send old xmin to primary
when hot_standby_feedback is on.

*PG Version* :- 10.12

*Slot Details : - *

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
---------------+--------+-----------+--------+----------+-----------+--------+------------+-----------+--------------+--------------+---------------------
repmgr_slot_3 | | physical | | | f |
t | 4841 | *193762035* | | 560/96081368 |
repmgr_slot_1 | | physical | | | f |
t | 18787 | 200856612 | | 560/96081368 |
repmgr_slot_4 | | physical | | | f |
t | 22825 | 200856607 | | 560/96081368 |
(3 rows)

*Observation :- *

1. message in DB logs.

2022-07-28 08:51:43 UTC [32119]: [2-1] user=,db=,app=,client=DEBUG:
sending hot standby feedback xmin 193762035 epoch 0 catalog_xmin 0
catalog_xmin_epoch 0
2022-07-28 08:51:46 UTC [32119]: [61-1] user=,db=,app=,client=DEBUG:
sending hot standby feedback xmin 193762035 epoch 0 catalog_xmin 0
catalog_xmin_epoch 0
2022-07-28 08:51:56 UTC [32119]: [292-1] user=,db=,app=,client=DEBUG:
sending hot standby feedback xmin 193762035 epoch 0 catalog_xmin 0
catalog_xmin_epoch 0
2022-07-28 08:52:06 UTC [32119]: [583-1] user=,db=,app=,client=DEBUG:
sending hot standby feedback xmin 193762035 epoch 0 catalog_xmin 0
catalog_xmin_epoch 0
2022-07-28 08:52:16 UTC [32119]: [824-1] user=,db=,app=,client=DEBUG:
sending hot standby feedback xmin 193762035 epoch 0 catalog_xmin 0
catalog_xmin_epoch 0
2022-07-28 08:52:26 UTC [32119]: [1056-1] user=,db=,app=,client=DEBUG:
sending hot standby feedback xmin 193762035 epoch 0 catalog_xmin 0
catalog_xmin_epoch 0
2022-07-28 08:52:36 UTC [32119]: [1347-1] user=,db=,app=,client=DEBUG:
sending hot standby feedback xmin 193762035 epoch 0 catalog_xmin 0
catalog_xmin_epoch 0
2022-07-28 08:52:46 UTC [32119]: [1621-1] user=,db=,app=,client=DEBUG:
sending hot standby feedback xmin 193762035 epoch 0 catalog_xmin 0
catalog_xmin_epoch 0
2022-07-28 08:52:56 UTC [32119]: [1848-1] user=,db=,app=,client=DEBUG:
sending hot standby feedback xmin 193762035 epoch 0 catalog_xmin 0
catalog_xmin_epoch 0
2022-07-28 08:53:06 UTC [32119]: [2161-1] user=,db=,app=,client=DEBUG:
sending hot standby feedback xmin 193762035 epoch 0 catalog_xmin 0
catalog_xmin_epoch 0

*Tried troubleshooting :-*

1. Checked for a long running query on primary and slave and found no SQL
present.
2. Turn off hot_standby_feedback for sometime.Post turning on
hot_standby_feedback
the same xmin is assigned to the slot.
3. Killed wal receiver process.
4. Restarted slave/standby databases still hot standby feedback
sending/refering old xmin.

Can you please help us to understand why this is happening, Is this a bug ?

Also is there any solution other than recreating the slot or turning off
the hot_standby_feedback to fix this?

-
Thanks & Regards,

Tushar K Takate .
Mob-No : +91-860-030-2404
LinkedIn : Tushar Takate <https://in.linkedin.com/in/tushar-takate-93660867>