public List<Long> getFolderUsableSpaceList() {
    List<Long> spaceList = new ArrayList<>();
    for (int i = 0; i < folders.size(); i++) {
      String folder = folders.get(i);
      spaceList.add(JVMCommonUtils.getUsableSpace(folder));
    }
    return spaceList;
  }
get space list of all folders.
iotdb-core/datanode/src/main/java/org/apache/iotdb/db/storageengine/rescon/disk/strategy/RandomOnDiskUsableSpaceStrategy.java