Active Discovery and instance deletion

I ran into this again, and I think that the current implementation is lacking.

The problem is that there are many types of instances that can be 'down' in normal operation, but you want to alarm if they were up and now are down.  However, if they are down when you add the device, you don't want to add the instance and have to manually delete them.

Several examples:

Unused redundant power supply

Unused stack port

Unused interface

The current configuration leads to errors; I had support tell me to switch to 'save for 30 days' to get alarms for deleted instances, and today found that the 'Cisco Switch Stack Ports-' datasource from LM has the same problem (it filters on status and alarms on status).  These are also the worst type of errors because they are generally only visible when bad things happen and you don't get notified.

You could have an option for a filter to only apply on the first device discovery (I don't know what other issues that might create).  You could also had instances alarm if they are removed by active discovery (maybe only if the 'save for 30 days' option is enabled).  I am in favor of having instances with the '30 day' option still be visible in the tree so that you can reference historical data, so it wouldn't be too hard to extend that concept to an alarm.