add get_average_osd_load method and test

tests/test_dataDistribution.py

@@ -158,6 +158,22 @@ class TestDataDistribution(unittest.TestCase):
         total_folder_sizes = list(map(lambda x: distribution.OSDs[x].total_folder_size, osds))
         self.assertTrue(2 * min(total_folder_sizes) == max(total_folder_sizes))
 
+    def test_average_osd_load(self):
+        folder_sizes = [49, 123, 1, 7]
+        num_folders = 2
+        num_osds = 4
+        osd_capacities = [100, 150]
+
+        distribution = dataDistribution.DataDistribution()
+        distribution.add_osd_list(create_test_osd_list(num_osds, osd_capacities))
+        distribution.add_folders(create_test_folder_list(num_folders, folder_sizes),
+                                 create_osd_information(num_osds, osd_capacities),
+                                 osd_capacity_key)
+
+        average = (sum(folder_sizes) * num_folders) / (sum(osd_capacities * num_osds))
+        self.assertEqual(average, distribution.get_average_osd_load(create_osd_information(num_osds, osd_capacities),
+                                                                    osd_capacity_key))
+
 
 def create_test_osd_list(num_osds, osd_capacities):
     test_osds = []

xtreemfs_client/dataDistribution.py

@@ -86,6 +86,18 @@ class DataDistribution(object):
             return 0
         return total_size / total_number_of_folders
 
+    def get_average_osd_load(self, osd_information, capacity):
+        """
+        calculate the average OSD load, that is,
+        the ratio between the sum of all folder sizes and the total OSD capacity.
+        """
+        total_folder_size = 0
+        total_osd_capacity = 0
+        for osd_uuid in self.OSDs.keys():
+            total_folder_size += self.OSDs[osd_uuid].total_folder_size
+            total_osd_capacity += osd_information[osd_uuid][capacity]
+        return total_folder_size / total_osd_capacity
+
     def add_folders(self, folders,
                     osd_information=None, ratio_parameter='', capacity='',
                     ignore_osd_capacities=True,