HDFS-6620. Snapshot docs should specify about preserve options with cp command. Contributed by Stephen Chu.

git-svn-id: https://svn.apache.org/repos/asf/hadoop/common/trunk@1607685 13f79535-47bb-0310-9956-ffa450edef68
HDFS-6613. Improve logging in caching classes. (wang)

git-svn-id: https://svn.apache.org/repos/asf/hadoop/common/trunk@1607697 13f79535-47bb-0310-9956-ffa450edef68
HDFS-6493. Change dfs.namenode.startup.delay.block.deletion to second instead of millisecond. Contributed by Juan Yu.

git-svn-id: https://svn.apache.org/repos/asf/hadoop/common/trunk@1607732 13f79535-47bb-0310-9956-ffa450edef68
HDFS-6511. BlockManager#computeInvalidateWork() could do nothing. Contributed by Juan Yu.

git-svn-id: https://svn.apache.org/repos/asf/hadoop/common/trunk@1607735 13f79535-47bb-0310-9956-ffa450edef68