开启了增量checkpoint，同时checkpoint的个数设置为只保留2个

增量checkpoint与web界面的信息其实没有直接联系，增量checkpoint的信息记录由CheckpointCoordinator中的SharedStateRegistry[1] 进行计数管理，而保留多少checkpoint则由 CheckpointStore管理 [2]. 保留2个checkpoint的执行过程如下： chk-1 completed --> register chk-1 in state registry --> add to checkpoint store chk-2 completed --> register chk-2 in state registry --> add to checkpoint store chk-3 completed --> register chk-3 in state registry --> add to checkpoint store --> chk-1 subsumed --> unregister chk-1 in state registry --> discard state with reference=0 chk-4 completed --> register chk-4 in state registry --> add to checkpoint store --> chk-2 subsumed --> unregister chk-2 in state registry --> discard state with reference=0

从上面可以看懂整个执行流程，所以当chk-3 仍然有部分数据依赖chk-1时，那些state数据在unregister时，其计数统计并不会降为0，也就不会删掉，也不需要copy到本次中。

[1] https://github.com/apache/flink/blob/f8ce30a50b8dd803d4476ea5d83e7d48708d54fa/flink-runtime/src/main/java/org/apache/flink/runtime/checkpoint/CheckpointCoordinator.java#L192 [2] https://github.com/apache/flink/blob/f8ce30a50b8dd803d4476ea5d83e7d48708d54fa/flink-runtime/src/main/java/org/apache/flink/runtime/checkpoint/StandaloneCompletedCheckpointStore.java#L41*来自志愿者整理的flink