This database mirroring event is issued on the principal server instance to indicate that the amount of unsent log has reached a user-specified threshold value. Typically, this event occurs because the performance of the system has changed. Either the bandwidth between the two systems has decreased, or the load has increased.
The amount of unsent log is a performance metric that can help you evaluate the potential for data loss in terms of the number of kilobytes (KB) of unsent log. This metric is particularly relevant for high-performance mode sessions. However, this metric is also a relevant for high-safety mode session when mirroring is paused or suspended because the partners become disconnected.