diff --git a/planb/default_settings.py b/planb/default_settings.py
index 3da44a8..ff62f6f 100644
--- a/planb/default_settings.py
+++ b/planb/default_settings.py
@@ -109,10 +109,10 @@
 Q_MAIN_QUEUE = 'main'
 Q_CLUSTER_QUEUE = os.environ.get('Q_CLUSTER_QUEUE', Q_MAIN_QUEUE)
 
-# The worker queue for dutree tasks, limited to 1 worker. See how this is set
+# The worker queue for dutree tasks, limited to 3 workers. See how this is set
 # in the bqcluster management command.
 Q_DUTREE_QUEUE = 'dutree'
-Q_DUTREE_WORKERS = 1
+Q_DUTREE_WORKERS = 3
 
 Q_CLUSTER = {
     'name': 'planb',    # redis prefix AND default broker (yuck!)
diff --git a/planb/storage/zfs.py b/planb/storage/zfs.py
index 7d74678..b2f8add 100644
--- a/planb/storage/zfs.py
+++ b/planb/storage/zfs.py
@@ -26,6 +26,21 @@
 logger = logging.getLogger(__name__)
 
 
+class WorkonFailure(Exception):
+    pass
+
+
+class LocalFilesetMissing(WorkonFailure):
+    """Expected storage "fileset" was not found"""
+    pass
+
+
+class LocalMountFailure(WorkonFailure):
+    """Expected storage "fileset+snapshot" mount failed"""
+    # (maybe the snapshot doesn't exist)
+    pass
+
+
 class PerformCommands:
     @classmethod
     def ensure_defaults(cls, config):
@@ -490,8 +505,8 @@ def begin_work(self, data_path=None):
                 break
         else:
             # No luck after the Nth attempt. Fail.
-            raise ValueError('Failed to work on {!r} ({})'.format(
-                path, self.name))  # FIXME: better exception
+            raise LocalMountFailure('Failed to work on {!r} ({})'.format(
+                path, self.name))
 
     def end_work(self):
         # Leave directory, so it can be unmounted.
@@ -519,7 +534,7 @@ def get_data_path(self):
         if not hasattr(self, '_get_data_path'):
             local_path = self.get_mount_path()
             if not local_path:
-                raise ValueError(
+                raise LocalFilesetMissing(
                     'path {!r} for {!r} does not exist'.format(
                         local_path, self.name))
             self._get_data_path = os.path.join(local_path, 'data')