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

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.