First of all, DirectoryCollection#verifyDirUsingMkdir doesn't do what it claims to do, it creates a file but its Java doc says it create a dir. Because of that, it doesn't compatible with ReadWriteDiskValidator because the validator will throw an exception if it isn't dir. What's more, it is no longer needed after
HADOOP-13738 (DiskChecker should perform some disk IO). It duplicates what HADOOP-13738 does. We should remove it.