Open Stack

Hi devs,

I'm thinking to add new type of alarm for time based auto scaling, but not
sure if there is a better way to achieve it outside ceilometer scope

Currently we can auto scaling based on vm load, but it will take several
minutes to do it. For the worst case, when the vm load is heavy, ceilometer
needs 10 minutes (by default) to collect the performance data, and alarm
need 1 minutes to evaluate it, maybe there is evaluation_periods which set
to higher that 1 to avoid performance peak.

So even though we can collect data by 1 minutes, but evaluation periods may
be set to 3, so the worst case is after vm load perfomance in high level
for 4 minutes, then the alarm is triggered, then heat will expand vm count,
nova will take dozens seconds or more to create, finally the service on the
in the heat server group will performance bad or unavailable for several
minutes, which is not acceptable for some critical applications.

But if we can know some high load which related with time, for example,
08:00am will be a peak, and after 22:00pm will definitely drop to idel
level, then heat can increase some vms at 07:30am, then decrease some vms
at 22:00 (or decrease by load as normal routine)

However, current ceilometer only provide time constraint alarm, which will
only evaluate but not guarantee it will be triggered. And heat cannot
provide something like timer, but just can wait for the signal.

So I propose to add a new type of alarm, which will definitely send a
signal to action when it is evaluated (with or without repeat, it will work
well with time constraint)

Any suggestion?
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