public Set<Long> getMemTablesId() {
    return memTablesId;
  }
Metadata exists at the end of each wal file, including each entry's size, search index of first
entry and the number of entries.
iotdb-core/datanode/src/main/java/org/apache/iotdb/db/storageengine/dataregion/wal/io/WALMetaData.java