svn merge -c 1483150 from trunk for HDFS-4826. TestNestedSnapshots times out due to repeated slow edit log flushes when running on virtualized disk.


git-svn-id: https://svn.apache.org/repos/asf/hadoop/common/branches/branch-2@1488090 13f79535-47bb-0310-9956-ffa450edef68