@Override
  public IDeviceMNode<IMemMNode> setToEntity(final IMemMNode node) {
    int rawSize = node.estimateSize();
    if (MNodeUtils.setToEntity(node)) {
      regionStatistics.addDevice();
      requestMemory(node.estimateSize() - rawSize);
    }

    return node.getAsDeviceMNode();
  }
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