2. Hardware Resources

Ensure that you prepare the following hardware resources:

NameNode machines: The machines where you run Active and Standby NameNodes, should have exactly the same hardware. For recommended hardware for NameNodes, see "Hardware for Master Nodes" in the Cluster Planning Guide.

JournalNode machines: The machines where you run the JournalNodes. The JournalNode daemon is relatively lightweight, so these daemons may reasonably be co- located on machines with other Hadoop daemons, for example the NameNodes or the YARN ResourceManager.

	Note
There must be at least three JournalNode daemons, because edit log modifications must be written to a majority of JournalNodes. This lets the system tolerate failure of a single machine. You may also run more than three JournalNodes, but in order to increase the number of failures that the system can tolerate, you must run an odd number of JournalNodes. For example, 3, 5, 7, and so on. When you run <N> number of JournalNodes, the system can tolerate at most (<N> - 1) / 2 failures and continue to function normally.

Note

There must be at least three JournalNode daemons, because edit log modifications must be written to a majority of JournalNodes. This lets the system tolerate failure of a single machine. You may also run more than three JournalNodes, but in order to increase the number of failures that the system can tolerate, you must run an odd number of JournalNodes. For example, 3, 5, 7, and so on.

When you run <N> number of JournalNodes, the system can tolerate at most (<N> - 1) / 2 failures and continue to function normally.

Zookeeper machines: For automated failover functionality, there must be an existing Zookeeper cluster available. The Zookeeper service nodes can be co-located with other Hadoop daemons.

In an HA cluster, the Standby NameNode also performs checkpoints of the namespace state. Therefore, do not deploy a Secondary NameNode, CheckpointNode, or BackupNode in an HA cluster.

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