Report zero capacity if GPFS is unmounted

_get_available_capacity() calls df command to gather the capacity of GPFS
filesystem. If GPFS is unmounted, df command will return the capacity of
root file system. This change will let get_volume_stats() to report zero
capacity, so that Cinder can schedule the volume request to another volume
service After GPFS is mounted again, the correct capacity will be reported.

Change-Id: I4c229d78e7d659604a23a50e6ab46060642b4088
Closes-Bug: #1246779

cinder/volume/drivers/gpfs.py

@@ -589,6 +589,18 @@ def _mkfs(self, volume, fs, label=None):
 
     def _get_available_capacity(self, path):
         """Calculate available space on path."""
+        # Check if GPFS is mounted
+        try:
+            self._verify_gpfs_path_state(path)
+            mounted = True
+        except exception.VolumeBackendAPIException:
+            mounted = False
+
+        # If GPFS is not mounted, return zero capacity. So that the volume
+        # request can be scheduled to another volume service.
+        if not mounted:
+            return 0, 0
+
         out, _ = self._execute('df', '-P', '-B', '1', path,
                                run_as_root=True)
         out = out.splitlines()[1]