public Set<PersistentID> getOfflineMembers() {
    return offlineMembers;
  }
Retrieve the set of disk directories which are known to hold data for the missing buckets, but
are not online.

@return the set of disk directories which are known to hold data for the missing buckets, but
        are not online
geode-core/src/main/java/org/apache/geode/cache/persistence/PartitionOfflineException.java