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I have PostgreSQL streaming replication setup using replication slot. Node1 is primary and Node2 is slave. Now, node1 is down and promoted node2. Node2 is running and in read-write mode. There is heavy load on node2 now. How i can make sure enough WALs getting preserved on node2, so that when node1 joins, it could do diff sync using pg_rewind. This is PostgreSQL 14. There is no WAL archiving setup.

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For that, you'd have to create a replication slot on Node2 during failover:

SELECT pg_create_physical_replication_slot(
          slot_name => 'slot',
          immediately_reserve => TRUE
       );
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  • If lets say i do not create slot, then how much WALs it will preserve? Is it wal_keep_size or min_wal_size? Dec 15, 2022 at 9:04
  • wal_keep_size. min_wal_size is about WAL for the future, not old WAL. Dec 15, 2022 at 9:13
  • Don't we need to preserve the newer WAL after the promotion on new primary so that pg_rewind works correctly? Dec 15, 2022 at 9:26
  • Here, default value for min_wal_size is just 80MB and i have heavy workload on promoted primary. I want to preserve at least 1GB of newer workload or WAL on newer primary without creating slot. What should be values for wal_keep_size and min_wal_size in this case? Dec 15, 2022 at 9:29
  • I'm not talking about recent WAL in the context of min_wal_size, I am talking about future WAL. Please read the documentation on wal_keep_size. That is the parameter that makes the primary retain old WAL. Replication slots are better. Dec 15, 2022 at 10:00

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