Horton

Re: How can end users know the cause of LR slot sync delays?

Ashutosh Sharma <ashu.coek88@gmail.com>

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From: Ashutosh Sharma <ashu.coek88@gmail.com>
To: Shlok Kyal <shlok.kyal.oss@gmail.com>
Cc: Amit Kapila <amit.kapila16@gmail.com>, PostgreSQL Hackers <pgsql-hackers@lists.postgresql.org>
Date: 2025-09-22T05:28:54Z
Lists: pgsql-hackers

Commits

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  1. Rename column slotsync_skip_at to slotsync_last_skip.

  2. Add slotsync_skip_reason column to pg_replication_slots view.

  3. Fix test failure caused by commit 76b78721ca.

  4. Fix a BF failure caused by commit 76b78721ca.

  5. Add slotsync skip statistics.

Hi,

@@ -646,7 +670,11 @@ synchronize_one_slot(RemoteSlot *remote_slot, Oid
remote_dbid)
     remote_slot->name,
     LSN_FORMAT_ARGS(latestFlushPtr)));

- return false;
+ /* If slot is present on the local, update the slot sync skip stats */
+ if ((slot = SearchNamedReplicationSlot(remote_slot->name, true)))
+ skip_reason = SLOT_SYNC_SKIP_STANDBY_BEHIND;
+ else
+ return false;

With this change, you’re likely enforcing sync slot creation, whereas
earlier that might not have been the case. This introduces a
behavioral change, which may not be well received.

--

I think we can avoid passing skip_reason as a new argument to
update_local_synced_slot(). It only needs to be passed to
update_and_persist_local_synced_slot(). When
update_local_synced_slot() is invoked from within
update_and_persist_local_synced_slot(), we can simply rely on the two
flags, remote_slot_precedes and found_consistent_snapshot and set the
skip_reason accordingly, thoughts?

If update_local_synced_slot is being called from any other place that
means the slot is already persisted.

--

+typedef enum SlotSyncSkipReason
+{
+ SLOT_SYNC_SKIP_NONE, /* No skip */
+ SLOT_SYNC_SKIP_STANDBY_BEHIND, /* Standby is behind the remote slot */
+ SLOT_SYNC_SKIP_REMOTE_BEHIND, /* Remote slot is behind the local slot */
+ SLOT_SYNC_SKIP_NO_CONSISTENT_SNAPSHOT /* Standby could not reach a
+ * consistent snapshot */
+} SlotSyncSkipReason;
+

I would suggest shortening the enum names like maybe SS_SKIP_NONE
instead of SLOT_SYNC_SKIP_NONE.

--
With Regards,
Ashutosh Sharma.