use more meaningful per-OSD reassignment limit

xtreemfs_client/dataDistribution.py

@@ -104,6 +104,18 @@ class DataDistribution(object):
             self.OSDs[new_osd].add_folder(folder_id, self.OSDs[old_osd.uuid].folders[folder_id])
             self.OSDs[old_osd.uuid].remove_folder(folder_id)
 
+    def get_total_folder_size(self):
+        total_size = 0
+        for one_osd in self.OSDs.values():
+            total_size += one_osd.total_folder_size
+        return total_size
+
+    def get_total_bandwidth(self):
+        total_bandwidth = 0
+        for one_osd in self.OSDs.values():
+            total_bandwidth += one_osd.bandwidth
+        return total_bandwidth
+
     def get_average_folder_size(self):
         """
         get the average folder size of all folders of all OSDs.
@@ -258,17 +270,13 @@ class DataDistribution(object):
         """
         movements = {}
         folders_to_be_reassigned = []
-        # TODO reassignment factor based on load or based on processing time?
-        # as long as we use OSDs with the same bandwidth, there is no difference.
-        reassignment_limit = self.get_average_processing_time() * rebalance_factor
 
         # for each OSD, remove the smallest folder until its total_folder_size does not exceed the reassignment_limit
         # unrolling
         for osd in self.OSDs.values():
-            # TODO how to calculate the 'unrolling limit' for each individual OSD?
-            # again, as long as all OSDs have the same bandwidth, there is no difference.
-            # BUT it should definitely not depend on the load or pt of the OSD.
-            # so for now we use a static limit, the same for all OSDs.
+            # self.get_total_folder_size / self.get_total_bandwidth() is the optimal processing time for each OSD:
+            # this value is a lower bound for the makespan
+            reassignment_limit = osd.bandwidth * (self.get_total_folder_size() / self.get_total_bandwidth())
             while osd.get_processing_time() > reassignment_limit:
                 folder_id, folder_size = osd.get_smallest_folder()
                 folders_to_be_reassigned.append(folder.Folder(folder_id, folder_size, None))
@@ -281,7 +289,7 @@ class DataDistribution(object):
         for folder_id, target in new_assignments:
             movements[folder_id] = (movements[folder_id], target)
 
-        return movements, reassignment_limit
+        return movements
 
     def rebalance_one_folder(self):
         """