Currently NameNode include excessive replicas in blockMap and count them as live replicas. Although excessive replicas have marked as invalid, scheduling deletion may be delayed and also datanode does not send deletion confirmation until the next block report. As a result, excessive replicas may stay in blocksMap for quite a while. This may cause underReplicated blocks undetected in NameNode.
For example, assume that block b is at datanode d1, d2, and d3. We have the following scenario
1. d1 loses heartbeat, NN will replicate b to another datanode, assuming it is d4.
2. d1 comes back. NN finds out b is over-replicated therefore choosing one replica, assuming d4, as a excessive replica and marking it as invalid. Now b has 3 valid replicas d1, d2, d3 and 1 excessive (invalid) replica d4, all in blocksMap.
3. d2 loses heartbeat. d2 gets removed from blocksMap. Block b has 2 valid replicas d1 and d3, and 1 excessive invalid replica d4. Block b is under-replicated; But NN still counts block b has 3 live replicas so does not take any action to replicate block b.