[aodh] [heat] Stein: How to create alarms based on rate metrics like CPU utilization?
Prior to Stein, Ceilometer issued a metric named /cpu_util/, which I
could use to trigger alarms and autoscaling when CPU utilization was too
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
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 .
*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.
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