The flush process flushes memtables to disk as SSTables. This metric shows the number of memtables queued for the flush process. The optimal number of pending flushes is 0 (or at most a very small number). A value greater than 0 indicates either I/O contention or degrading disk performance (see disk metrics such as disk latency, disk throughput, and disk utilization for indications of disk health).

When an insert or update to a row is written, the affected row is replicated to all other nodes that manage a replica for that row. This is called the ReplicateOnWriteStage. This metric tracks the pending tasks related to this stage of the write process. During low or moderate write load, you should see 0 pending replicate on write tasks (or at most a very low number). A continuous high number signals a need to investigate disk I/O or network contention problems.