From 9cb5ef40afe2ac7f290cb77ee3f43af97069c253 Mon Sep 17 00:00:00 2001 From: Hayato Kuroda Date: Sun, 29 Jun 2025 15:19:29 +0900 Subject: [PATCH v2] Update comment for last_saved_restart_lsn --- src/include/replication/slot.h | 19 +++++++++++++++++++ 1 file changed, 19 insertions(+) diff --git a/src/include/replication/slot.h b/src/include/replication/slot.h index 76aeeb92242..19b4e8b6a03 100644 --- a/src/include/replication/slot.h +++ b/src/include/replication/slot.h @@ -220,6 +220,25 @@ typedef struct ReplicationSlot * Latest restart_lsn that has been flushed to disk. For persistent slots * the flushed LSN should be taken into account when calculating the * oldest LSN for WAL segments removal. + * + * Do not assume that restart_lsn will always move forward, i.e., that the + * previously flushed restart_lsn is always behind data.restart_lsn. In + * streaming replication using a physical slot, the restart_lsn is updated + * based on the flushed WAL position reported by the walreceiver. + * + * This replication mode allows duplicate WAL records to be received and + * overwritten. If the walreceiver receives older WAL records and then + * reports them as flushed to the walsender, the restart_lsn may appear to + * move backward. + * + * This typically occurs at the beginning of replication. One reason is + * that streaming replication starts at the beginning of a segment, so, if + * restart_lsn is in the middle of a segment, it will be updated to an + * earlier LSN, see RequestXLogStreaming. Another reason is that the + * walreceiver chooses its startpoint based on the replayed LSN, so, if + * some records have been received but not yet applied, they will be + * received again and leads to updating the restart_lsn to an earlier + * position. */ XLogRecPtr last_saved_restart_lsn; -- 2.50.1.windows.1