From test result of a recent jenkins nightly https://builds.apache.org/job/Hadoop-Hdfs-trunk/2663/testReport/junit/org.apache.hadoop.hdfs.server.namenode/TestDecommissioningStatus/testDecommissionStatus/
The test failed because the number of under replicated blocks is 4, instead of 3.
Looking at the log, there is a strayed block, which might have caused the faillure:
The block size 16384 suggests this is left over from the sibling test case testDecommissionStatusAfterDNRestart. This can happen, because the same minidfs cluster is reused between tests.
The test implementation should do a better job isolating tests.
Another case of failure is when the load factor comes into play, and a block can not find sufficient data nodes to place replica. In this test, the runtime should not consider load factor: