Patch for the Drupal Varnish module (7.x-1.0-beta2) to log successful command runs.

varnish_watchdog_success.patch

diff --git a/varnish.module b/varnish.module
index 80e7af2..21dfd8e 100644
--- a/varnish.module
+++ b/varnish.module
@@ -288,6 +288,7 @@ function _varnish_execute_command($client, $command) {
   }
   else {
     // successful connection
+    watchdog('varnish', 'Successfully ran command: %command', array('%command' => $command), WATCHDOG_INFO);
     return $status;
   }
 }