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Update comment for ReplicationSlot.last_saved_restart_lsn

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Document that restart_lsn can go backwards and explain why this could happen.

Discussion: https://postgr.es/m/1d12d2-67235980-35-19a406a0%4063439497
Discussion: https://postgr.es/m/CAPpHfdvuyMrUg0Vs5jPfwLOo1M9B-GP5j_My9URnBX0B%3DnrHKw%40mail.gmail.com
Author: Hayato Kuroda <kuroda.hayato@fujitsu.com>
Co-authored-by: Amit Kapila <amit.kapila16@gmail.com>
Reviewed-by: Vignesh C <vignesh21@gmail.com>
Reviewed-by: Amit Kapila <amit.kapila16@gmail.com>
Reviewed-by: Alexander Korotkov <aekorotkov@gmail.com>
This commit is contained in:
Alexander Korotkov
2025-07-21 15:07:34 +03:00
parent da71717f0a
commit 0810fbb02d
1 changed files with 19 additions and 0 deletions
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19
src/include/replication/slot.h
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@ -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;