During the first iteration of the TeraGen job, the goal is to obtain a performance baseline on the disk I/O subsystem. The HDFS replication factor should be overridden from the default value 3 and set to 1 so that the data generated by the TeraGen job is not replicated to additional data nodes. Replicating the data over the network obscures the raw disk performance with potential network bandwidth constraints. Once the first TeraGen job has been run, a second iteration should be run with the HDFS replication factor set to the default value. This applies a high load on the network, and deltas between the first run and second run can provide an indication of network bottlenecks in the cluster. Please check below documentation for set of commands and details - https://docs.cloudera.com/cdp-reference-architectures/latest/cdp-pvc-base-ra/topics/ra-cdpdc-teragen-and-terasort-performance-baseline.html ... View more

When the local NameNode is healthy, the ZKFC holds a session open in ZooKeeper. If the local NameNode is active, it also holds a special “lock” znode. This lock uses ZooKeeper’s support for “ephemeral” nodes; if the session expires, the lock node will be automatically deleted. Ephemeral znodes - These znodes exists as long as the session that created the znode is active. When the session ends the znode is deleted. Zookeeper has a concept of Watches which can be set on znodes to track them for any changes. Check the link how Watches work in zookeeper - https://zookeeper.apache.org/doc/current/zookeeperProgrammers.html#ch_zkWatches ... View more