void removeDrf(Oplog oplog) {
    synchronized (getOplogIdToOplog()) {
      drfOnlyOplogs.remove(oplog.getOplogId());
    }
  }
Returns the next available DirectoryHolder which has space. If no dir has space then it will
return one anyway if compaction is enabled.

@param minAvailableSpace the minimum amount of space we need in this directory.
geode-core/src/main/java/org/apache/geode/internal/cache/PersistentOplogSet.java