Use multiple connections when syncing.

example.cfg

@@ -5,6 +5,12 @@
 # A folder where vdirsyncer can store some metadata about each pair.
 status_path = ~/.vdirsyncer/status/
 
+# The amount of processes to use when synchronizing. If the CPU of your server
+# is the bottleneck (which is plausible for small homeservers), set this to the
+# number of cores * 2. Defaults to one process for each collection. The value
+# 0 will be ignored.
+#processes =
+
 # CONTACTS
 [pair bob_contacts]
 # Similar to accounts in OfflineIMAP.

vdirsyncer/cli.py

@@ -190,10 +190,32 @@ def _main(env, file_cfg):
             config_a.update(all_storages[a_name])
             config_b = dict(storage_defaults)
             config_b.update(all_storages[b_name])
+
+
+            actions.append({
+                'config_a': config_a,
+                'config_b': config_b,
+                'pair_name': pair_name,
+                'collection': collection,
+                'pair_options': pair_options,
+                'general': general
+            })
+
+        from multiprocessing import Pool
+        p = Pool(processes=general.get('processes', 0) or len(actions))
+        p.map(_sync_collection, actions)
+
+    app.register_command('sync', sync_command)
+    app()
+
+def _sync_collection(x):
+    return sync_collection(**x)
+
+def sync_collection(config_a, config_b, pair_name, collection, pair_options,
+                    general):
     a = storage_instance_from_config(config_a)
     b = storage_instance_from_config(config_b)
 
-            def x(a=a, b=b, pair_name=pair_name, collection=collection):
     status_name = \
             '_'.join(filter(bool, (pair_name, collection)))
     pair_description = \
@@ -203,10 +225,3 @@ def _main(env, file_cfg):
     sync(a, b, status,
          pair_options.get('conflict_resolution', None))
     save_status(general['status_path'], status_name, status)
-            actions.append(x)
-
-        for action in actions:
-            action()
-
-    app.register_command('sync', sync_command)
-    app()