public List<String> getAllFilesFolders() {
    List<String> folders = new ArrayList<>(seqDir2TierLevel.keySet());
    folders.addAll(unSeqDir2TierLevel.keySet());
    return folders;
  }
total space of each tier, Long.MAX_VALUE when one tier contains remote storage
iotdb-core/datanode/src/main/java/org/apache/iotdb/db/storageengine/rescon/disk/TierManager.java