地址： http://www.cnblogs.com/jarlean/archive/2013/04/10/3011667.html
Q11. Give an example scenario where a cobiner can be used and where it cannot be used
There can be several examples following are the most common ones
- Scenario where you can use combiner
Getting list of distinct words in a file (进行distinct操作时用combiner)
- Scenario where you cannot use a combiner
Calculating mean of a list of numbers (计算平均值时用)
Q12. What is job tracker
Job Tracker is the service within Hadoop that runs Map Reduce jobs on the cluster (jt是运行MR作业的集群)
Q13. What are some typical functions of Job Tracker(jobtracker 的作用)
The following are some typical tasks of Job Tracker
- Accepts jobs from clients（接收客户端的任务）
- It talks to the NameNode to determine the location of the data（获取namenode返回的数据位置信息）
- It locates TaskTracker nodes with available slots at or near the data（定位于数据近的可用的tasktracker节点）
- It submits the work to the chosen Task Tracker nodes and monitors progress of each task by receiving heartbeat signals from Task tracker（将作业提交给tasktracker，接收tasktracker的heartbeat信息）
Q14. What is task tracker
Task Tracker is a node in the cluster that accepts tasks like Map, Reduce and Shuffle operations - from a JobTracker（从jobtracker接收Map，Reduce任务和进行Shuffle操作）
Q15. Whats the relationship between Jobs and Tasks in Hadoop
One job is broken down into one or many tasks in Hadoop.（job分解为1或多个task）