Prior to Stein, Ceilometer issued a metric named /cpu_util/, which I could use to trigger alarms and autoscaling when CPU utilization was too high.
cpu_util doesn't exist anymore. Instead, we are asked to use Gnocchi's /rate/ feature. However, when using rates, alarms on a group of resources require more parameters than just one metric: Both an aggregation and a reaggregation method are needed.
For example, a group of instances that implement "myapp":
gnocchi measures aggregation -m cpu --reaggregation mean --aggregation rate:mean --query server_group=myapp --resource-type instance
Actually, this command uses a deprecated API (but from what I can see, Aodh still uses it). The new way is like this:
gnocchi aggregates --resource-type instance '(aggregate rate:mean (metric cpu mean))' server_group=myapp
If rate:mean is in the archive policy, it also works the other way around:
gnocchi aggregates --resource-type instance '(aggregate mean (metric cpu rate:mean))' server_group=myapp
Without reaggregation, I get quite unexpected numbers, including negative CPU rates. If you want to understand why, see this discussion with one of the Gnocchi maintainers [1].
*My problem*: Aodh allows me to set an aggregation method, but not a reaggregation method. How can I create alarms based on rates? The problem extends to Heat and autoscaling.
Thanks much,
Bernd.