static ICachedMNode getFlatTree(int flatSize, String id) {
    ICachedMNode root = nodeFactory.createInternalMNode(null, "root");
    ICachedMNode test = nodeFactory.createInternalMNode(root, "test");
    ICachedMNode internalNode = nodeFactory.createDatabaseDeviceMNode(null, "vRoot1");

    for (int idx = 0; idx < flatSize; idx++) {
      String measurementId = id + idx;
      IMeasurementSchema schema = new MeasurementSchema(measurementId, TSDataType.FLOAT);
      IMeasurementMNode<ICachedMNode> mNode =
          nodeFactory.createMeasurementMNode(
              internalNode.getAsDeviceMNode(), measurementId, schema, measurementId + "als");
      internalNode.addChild(mNode.getAsMNode());
    }

    test.addChild(internalNode);
    return internalNode;
  }
related methods shall be merged further: {@linkplain SchemaFile#reEstimateSegSize}
,{@linkplain PageManager#reEstimateSegSize}
iotdb-core/datanode/src/test/java/org/apache/iotdb/db/metadata/mtree/schemafile/SchemaFileTest.java