The Mirroring Oldest Unsent Transactions (Minutes) alert provides the age of unsent transactions queued on the processor and reached an alert threshold.
This alert specifies the number of minutes worth of transactions that can accumulate in the send queue before a warning is generated on the principal server instance. This warning helps measure the potential for data loss in terms of time 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 oldest unsent transactions, see the Microsoft document, Use Warning Thresholds and Alerts on Mirroring Performance Metrics (SQL Server).