When you use the AlwaysOn Availability Group feature in Microsoft SQL Server 2012 or in Microsoft SQL Server 2014, the Log_Send_Rate column in the sys.dm_hadr_database_replica_states view does not work correctly. It cannot reflect the rate that log records are sent to the secondary databases at a given point in time, in kilobytes (KB) per second. Instead, it reflects a high water mark. This makes the Log_Send_Rate column fairly useless when it is paired with the Log_send_queue_size column in the same view to determine the drain rate.
Additionally, when Receive Segment Coalescing (RSC) network optimization is disabled, the behavior of this column changes, and the values are updated more frequently.
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Microsoft has confirmed that this is a problem in the Microsoft products that are listed in the "Applies to" section.