Write path
WAL buffer → WAL segment file (16MB default). Flushed on commit (fsync). Then data pages later. Recovery replays WAL after crash.
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wal_level
replica: standard replication. logical: logical decoding for CDC + logical replication. minimal: no replication.
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Synchronous replication
synchronous_commit + synchronous_standby_names. Wait for standby WAL fsync before commit ack. Zero data loss + latency cost.