Affects Version/s: None
Fix Version/s: None
After a node fails 4 mappers (tasks), it is added to blacklist thus it will no longer accept tasks.
But, it will continue serve the map outputs of any mappers that ran successfully there.
However, the node may not be able serve the map outputs either.
This will cause the reducers to mark the corresponding map outputs as from slow hosts,
but continue to try to get the map outputs from that node.
This may lead to waiting forever.