The Mirroring Unsent Log (KB) alert indicates that the amount of unsent logs (in KB) queued on the mirrored database reached or exceeded an alert threshold.
This alert specifies how many kilobytes (KB) of unsent logs generate a warning on the principal server instance. This warning helps measure the potential for data loss in terms of KB and is especially relevant for high-performance mode. However, the warning is also relevant for high-safety mode when mirroring is paused or suspended because the partners become disconnected.
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 unsent logs, see the Microsoft document, Use Warning Thresholds and Alerts on Mirroring Performance Metrics (SQL Server).