@Override
  public int serializedSize() {
    return FIXED_SERIALIZED_SIZE
        + buffersSize.size() * Integer.BYTES
        + (memTablesId.isEmpty() ? 0 : Integer.BYTES + memTablesId.size() * Long.BYTES);
  }
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