@Override
  public IMemMNode addChild(final IMemMNode parent, final String childName, final IMemMNode child) {
    final IMemMNode result = parent.addChild(childName, child);
    if (result == child) {
      requestMemory(child.estimateSize());
    }
    return result;
  }
This is a memory-based implementation of IMTreeStore. All MNodes are stored in memory.
iotdb-core/datanode/src/main/java/org/apache/iotdb/db/schemaengine/schemaregion/mtree/impl/mem/MemMTreeStore.java