What is concept of storing data in HDFS

Hadoop cluster is made up of how many classes of servers

Can anyone tell me about Hadoop Toolbox

What is Concept of Data compression in Hadoop

What is the characteristic of streaming API that makes it flexible run MapReduce jobs in languages like Perl, Ruby, Awk etc.?

What is Hadoop Streaming?

How does speculative execution work in Hadoop?

Suppose Hadoop spawned 100 tasks for a job and one of the task failed. What will Hadoop do?

What is the relationship between Jobs and Tasks in Hadoop?

What is JobTracker?