notes for official-website(hadoop always to be continue..)

1　　Other services (such as Web App Proxy Server and MapReduce Job History server) are usually run either on dedicated hardware or on shared infrastrucutre, depending upon the load.

2　　control  scripts: bin/ directory of the distribution

setting site-specific values via the etc/hadoop/hadoop-env.sh and etc/hadoop/yarn-env.sh.

3　　HDFS daemons are NameNode, SecondaryNameNode, and DataNode.

YARN damones are ResourceManager, NodeManager, and WebAppProxy.

4　　Hadoop daemons’ process environment:  _OPTS env vars or  _DIR or _PID

5　　configuring the daemons

1. io.file.buffer.size:　　Size of read/write buffer used in SequenceFiles. 
2. namenode.{name.dir, handler.count}:　　namespace and transactions logs persistently.replicated if comma-delimited　　More NameNode server threads to handle RPCs
3. log-aggregation
4. acl:　　set admins on the yarn cluster.ACLs are of for comma-separated-usersspacecomma-separated-groups.