public void serialize(ByteBuffer buffer) {
    buffer.put(type.getCode());
    buffer.putInt(memTableInfos.size());
    for (MemTableInfo memTableInfo : memTableInfos) {
      memTableInfo.serialize(buffer);
    }
  }
Checkpoint is the basic element of .checkpoint file, including type, number of memTables, and
brief information of each memTable.
iotdb-core/datanode/src/main/java/org/apache/iotdb/db/storageengine/dataregion/wal/checkpoint/Checkpoint.java