Re: Synchronizing slots from primary to standby

On Fri, Feb 9, 2024 at 10:04 AM Amit Kapila <amit(dot)kapila16(at)gmail(dot)com> wrote:
>
> +reserve_wal_for_local_slot(XLogRecPtr restart_lsn)
> {
> ...
> + /*
> + * Find the oldest existing WAL segment file.
> + *
> + * Normally, we can determine it by using the last removed segment
> + * number. However, if no WAL segment files have been removed by a
> + * checkpoint since startup, we need to search for the oldest segment
> + * file currently existing in XLOGDIR.
> + */
> + oldest_segno = XLogGetLastRemovedSegno() + 1;
> +
> + if (oldest_segno == 1)
> + {
> + TimeLineID cur_timeline;
> +
> + GetWalRcvFlushRecPtr(NULL, &cur_timeline);
> + oldest_segno = XLogGetOldestSegno(cur_timeline);
> ...
> ...
>
> This means that if the restart_lsn of the slot is from the prior
> timeline then the standby needs to wait for longer times to sync the
> slot. Ideally, it should be okay because I don't think even if
> restart_lsn of the slot may be from some prior timeline than the
> current flush timeline on standby, how often that case can happen?

I tested this behaviour on v85 patch, it is working as expected i.e.
if remot_slot's lsn belongs to a prior timeline then on executing
pg_sync_replication_slots() function, it creates a temporary slot and
waits for primary to catch up. And once primary catches up, the next
execution of SQL function persistes the slot and syncs it.

Setup: primary-->standby1-->standby2

Steps:
1) Insert data on primary. It gets replicated to both standbys.
2) Create logical slot on primary and execute
pg_sync_replication_slots() on standby1. The slot gets synced and
persisted on standby1.
3) Shutdown standby2.
4) Insert data on primary. It gets replicated to standby1.
5) Shutdown primary and promote standby1.
6) Insert some data on standby1/new primary directly.
7) Start standby2: It now needs to catch up old data of timeline1
(from step 4) + new data of timeline2 (from step 6) . It does that. On
reaching the end of the old timeline, walreceiver gets restarted and
starts streaming using the new timeline.
8) Execute pg_sync_replication_slots() on standby2 to sync the slot.
Now remote_slot's lsn belongs to a prior timeline on standby2. In my
test-run, remote_slot's lsn belonged to segno=4 on standby2, while the
oldest segno of current_timline(2) was 6. Thus it created the slot
locally with lsn belonging to the oldest segno 6 of cur_timeline(2)
but did not persist it as remote_slot's lsn was behind.
9) Now on standby1/new-primary, advance the logical slot by calling
pg_replication_slot_advance().
10) Execute pg_sync_replication_slots() again on standby2, now the
local temporary slot gets persisted as the restart_lsn of primary has
caught up.

thanks
Shveta