perf(mtda): reduce costly calls to storage.status

The calls to storage.status are costly, as every time the device state
is queried via an external process invocation. This patch optimizes this
by:

1. not calling the GUI callback on each chunk, but only every 0.5s
2. in case we are in writing mode, the storage is always connected to
   the host. Hence, do not query the status from the driver.

These improvements speedup writes and seeks by 2-3 times.

Signed-off-by: Felix Moessbauer <felix.moessbauer@siemens.com>

mtda/client.py

@@ -174,13 +174,15 @@ def _storage_write(self, image, imgname,
         data = image.read(self._agent.blksz)
         dataread = len(data)
         totalread = 0
+        lastreport = time.time()
         while totalread < inputsize:
             totalread += dataread
 
             # Report progress via callback
-            if callback is not None:
+            if callback is not None and time.time() - lastreport > 0.5:
                 _, _, written = self._impl.storage_status(self._session)
                 callback(imgname, totalread, inputsize, written, imagesize)
+                lastreport = time.time()
 
             # Write block to shared storage device
             bytes_wanted = self._impl.storage_write(data, self._session)

mtda/main.py

@@ -702,7 +702,9 @@ def storage_status(self, session=None):
             self.mtda.debug(4, "storage_status(): no shared storage device")
             result = CONSTS.STORAGE.UNKNOWN, False, 0
         else:
-            status = self.storage_controller.status()
+            # avoid costly query of storage state when we know it anyways
+            status = CONSTS.STORAGE.ON_HOST \
+                if self._writer.writing else self.storage_controller.status()
             result = status, self._writer.writing, self._writer.written
 
         self.mtda.debug(3, "main.storage_status(): %s" % str(result))