I'm staring into description of Change Deployment Configuration Azure Management Service operation. There's Mode element which can be set to Simultaneous.
Suppose I only want to change instance count for a specific role. AFAIK the only way to do that is to obtain the current deployment configuration (using Get Deployment operation), locate the role in the obtained XML, locate the attribute responsible for instance count, change that attribute and then use the altered configuration in Change Deployment Configuration operation.
Suppose I also set Mode to Simultaneous. What will be the difference compared to default?
When a configuration change happens (either a normal config change of a config setting, or a scale change) the Azure fabric will walk the upgrade domains one at a time and apply the configuration setting. With simultaneous upgrade the Azure fabric will apply the configuration change to all instances in all upgrade domains at the same time.
Services which cancel the RoleEnvironment.Changing event in order to recycle the instance during a config change will experience an entire service outage if the mode is set to simultaneous.
See http://blogs.msdn.com/b/kwill/archive/2013/12/07/topology-blast-send-topology-change-updates-to-all-instances-at-once.aspx for more information.
