Log size is different between primary and secondary node with log_since_last_log_backup_mb

Question

We get log backups from the secondary node. But now I see that log_since_last_log_backup_mb in sys.dm_db_log_stats returns null for databases which are in Availability groups, so Ola Hallengren's log backup procedure with

@LogSizeSinceLastLogBackup = 25,@TimeSinceLastLogBackup = 21000 

limitations works on those databases. The backup process is currently working on the secondary node.

But I could not understand why it returns null? Have you encountered such a situation?

The following pictures show result of log size since last log backup on the primary and secondary nodes.

Primary Node

Secondary Node

Answer 1

I'd expect it to be NULL on the primary node if a backup has never happened on this node. You can confirm this by checking when the last log backup happened for those databases on the primary node:

SELECT   d.name,
         d.dbid,
         MAX(b.backup_finish_date) AS backup_finish_date
FROM     master.sys.sysdatabases d
         LEFT OUTER JOIN msdb..backupset b
         ON       b.database_name = d.name
         AND      b.type          = 'L'
WHERE d.dbid in (19,34,21) --from your primary node
GROUP BY d.name, d.dbid
ORDER BY backup_finish_date DESC

And then on the secondary:

SELECT   d.name,
         d.dbid,
         MAX(b.backup_finish_date) AS backup_finish_date
FROM     master.sys.sysdatabases d
         LEFT OUTER JOIN msdb..backupset b
         ON       b.database_name = d.name
         AND      b.type          = 'L'
WHERE d.dbid in (19,34,33) --from your secondary node
GROUP BY d.name, d.dbid
ORDER BY backup_finish_date DESC