Support for Multiple Standby NameNodes
HDFS NameNode high availability with QuorumJournalManager uses a Paxos quorum to store the NameNode edit log. With a three-node quorum, this change means we can tolerate the loss of any one node and still continue operation.
However, business-critical deployments may wish to run with higher levels of fault-tolerance, e.g. a five-node quorum to be able to tolerate the loss of any two nodes.
QuorumJournalManager already supports an arbitrary number of nodes, but fault tolerance was limited since HDFS was only able to run a single active and single standby NameNode. Hadoop 3 eliminates this restriction by supporting running multiple standby NameNodes. This improves the fault tolerance of HDFS.
This one is huge to me. It was a sad day when I learned that the “secondary” NameNode was no such thing.