Pending task metrics
Pending task metrics track requests that have been received by a node, but are waiting to be processed. An accumulation of pending tasks on a node can indicate a potential bottleneck in performance and should be investigated.
Cassandra maintains distinct thread pools for different stages of execution. Each of these thread pools provide granular statistics on the number of pending tasks for that particular process. If you see pending tasks accumulating, it is indicative of a cluster that is not keeping up with the workload. Essentially, pending tasks mean that things are backing up, which is usually caused by a lack of (or failure of) cluster resources such as disk bandwidth, network bandwidth or memory.