21. Suppose Hadoop spawned 100 tasks for a job and one of the task failed. What will Hadoop do?
It will restart the task again on some other TaskTracker and only if the task fails more than four ( the default setting and can be changed) times will it kill the job.
22. What are Problems with small files and HDFS?
HDFS is not good at handling large number of small files. Because every file, directory and block in HDFS is represented as an object in the namenode’s memory, each of which occupies approx 150 bytes So 10 million files, each using a block, would use about 3 gigabytes of memory. when we go for a billion files the memory requirement in namenode cannot be met.
23. What does ‘jps’ command do?
It gives the status of the deamons which run Hadoop cluster. It gives the output mentioning the status of namenode, datanode , secondary namenode, Jobtracker and Task tracker.
24. How to restart Namenode?
Step-1. Click on stop-all.sh and then click on start-all.sh OR
Step-2. Write sudo hdfs (press enter), su-hdfs (press enter), /etc/init.d/ha (press enter) and then /etc/init.d/hadoop-0.20-namenode start (press enter).
25. What does /etc /init.d do?
/etc /init.d specifies where daemons (services) are placed or to see the status of these daemons. It is very LINUX specific, and nothing to do with Hadoop.
26. Mention what is the use of Context Object?
The Context Object enables the mapper to interact with the rest of the Hadoop system. It includes configuration data for the job, as well as interfaces which allow it to emit output.
27. Mention what is the number of default partitioner in Hadoop?
In Hadoop, the default partitioner is a “Hash” Partitioner.
28. Explain what is the purpose of RecordReader in Hadoop?
In Hadoop, the RecordReader loads the data from its source and converts it into (key, value) pairs suitable for reading by the Mapper.
29. Mention what is the best way to copy files between HDFS clusters?
The best way to copy files between HDFS clusters is by using multiple nodes and the distcp command, so the workload is shared.
30. What is “speculative execution” in Hadoop?
If a node appears to be executing a task slower, the master node can redundantly execute another instance of the same task on another node. Then, the task which finishes first will be accepted and the other one is killed. This process is called “speculative execution”.
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