Configuring Hardware for HDFS HA
To deploy an HA cluster using Quorum-based Storage, you should prepare the following:
- NameNode hosts - These are the hosts on which you run the active and standby NameNodes. They should have equivalent hardware to each other, and equivalent hardware to what would be used in a non-HA cluster.
- JournalNode hosts - These are the hosts on which you run the JournalNodes. Cloudera recommends that you deploy the JournalNode daemons on the "master" host or hosts (NameNode, Standby NameNode, JobTracker, and so on) so the JournalNodes' local directories can use the reliable local storage on those machines.
- If co-located on the same host, each JournalNode process and each NameNode process should have its own dedicated disk. You should not use SAN or NAS storage for these directories.
- There must be at least three JournalNode daemons, since edit log modifications must be written to a majority of JournalNodes. This will allow the system to tolerate the failure of a single host. You can also run more than three JournalNodes, but to actually increase the number of failures the system can tolerate, you should run an odd number of JournalNodes, (three, five, seven, and so on). Note that when running with N JournalNodes, the system can tolerate at most (N - 1) / 2 failures and continue to function normally. If the requisite quorum is not available, the NameNode will not format or start, and you will see an error similar to this:
12/10/01 17:34:18 WARN namenode.FSEditLog: Unable to determine input streams from QJM to [10.0.1.10:8485, 10.0.1.10:8486, 10.0.1.10:8487]. Skipping. java.io.IOException: Timed out waiting 20000ms for a quorum of nodes to respond.