[ClusterLabs] Q: Resource balancing opration

[Ken Gaillot]

On 04/20/2016 01:17 AM, Ulrich Windl wrote:
> Hi!
> 
> I'm wondering: If you boot a node on a cluster, most resources will go to another node (if possible). Due to stickiness configured, those resources will stay there.
> So I'm wondering whether or how I could cause a rebalance of resources on the cluster. I must admit that I don't understand the details of stickiness related to other parameters. In my understanding stickiness should be related to a percentage of utilization dynamically, so that a resource running on a node that is "almost full" should dynamically lower its stickiness to allow resource migration.
> 
> So if you are going to implement a manual resource rebalance operation, could you dynamically lower the stickiness for each resource (by some amount or some factor), wait if something happens, and then repeat the process until resources look balanced. "Looking balanced" should be no worse as if all resources are started when all cluster nodes are up.
> 
> Spontaneous pros and cons for "resource rebalancing"?
> 
> Regards,
> Ulrich

Pacemaker gives you a few levers to pull. Stickiness and utilization
attributes (with a placement strategy) are the main ones.

Normally, pacemaker *will* continually rebalance according to what nodes
are available. Stickiness tells the cluster not to do that.

Whether you should use stickiness (and how much) depends mainly on how
significant is the interruption that occurs when a service is moved. For
a large database supporting a high-traffic website, stopping and
starting can take a long time and cost a lot of business -- so maybe you
want an infinite stickiness in that case, and only rebalance manually
during a scheduled window. For a small VM that can live-migrate quickly
and doesn't affect any of your customer-facing services, maybe you don't
mind setting a small or zero stickiness.

You can also use rules to make the process intelligent. For example, for
a server that provides office services, you could set a rule that sets
infinite stickiness during business hours, and small or zero stickiness
otherwise. That way, you'd get no disruptions when people are actually
using the service during the day, and at night, it would automatically
rebalance.

Normally, pacemaker's idea of "balancing" is to simply distribute the
number of resources on each node as equally as possible. Utilization
attributes and placement strategies let you add more intelligence. For
example, you can define the number of cores per node or the amount of
RAM per node, along with how much each resource is expected to use, and
let pacemaker balance by that instead of just counting the number of
resources.