In the current code, the administrator can run savenamespace only after putting the namenode in safemode. This means that applications that are writing to HDFS encounters errors because the NN is in safemode. We would like to allow saveNamespace even when the namenode is not in safemode.
The savenamespace command already acquires the FSNamesystem writelock. There is no need to require that the namenode is in safemode too.