The Mirroring Unrestored Log (KB) alert indicates that the amount of unrestored logs (in KB) queued on the mirrored database reached or exceeded an alert threshold.
This warning helps measure failover time. Failover time consists mainly of the time that the mirror server requires to roll forward any log remaining in its redo queue, plus a short additional time.
This alert is raised based on the diagnostic data collection interval specified in Server Properties. For more information, see Set general server options.
For more information about unrestored logs, see the Microsoft document, Use Warning Thresholds and Alerts on Mirroring Performance Metrics (SQL Server).