If the IBM® PowerVC Dynamic Resource Optimizer (DRO) determines that a host is overburdened, a DRO Capacity on Demand (CoD) operation is not performed, and migrations are enabled, it migrates virtual machines to remedy the situation.
How the virtual machine is chosen
If there are several virtual machines on the host, DRO tries to move the fewest number of virtual machines by predicting what each host's utilization would be after the migration. It will not migrate a virtual machine if it causes the utilization on the destination host to go above its threshold. The virtual machine chosen for migration depends on which resource is being optimized.
- PowerVC categorizes each virtual machine
as one of two types:
- 1: Moving only this virtual machine would resolve the issue.
- 2: Moving only this virtual machine would not resolve the issue.
- The virtual machine that reduces the utilization the most is selected.
PowerVC cannot monitor memory usage on HMC managed systems. Therefore, if you have a host group that contains both NovaLink managed systems and HMC managed systems and you choose to monitor available memory, the HMC managed systems are ignored. That is, when monitoring memory, DRO will not migrate a virtual machine onto or off of an HMC managed host.
If DRO cannot find a suitable virtual machine during an optimization cycle, nothing is migrated and DRO reevaluates possible migrations during the next cycle.
How the target host is chosen
The DRO selects the target host in a two-step process. First, it determines the candidate hosts.
Candidate hosts include any host that would be a candidate for live migration. For details, see
Migration requirements. From the set of candidate hosts, it chooses the host that has the
lowest anticipated post-migration utilization. This avoids causing a new threshold violation when
remedying the current violation.
If the DRO cannot find a suitable target host during an optimization cycle, nothing is migrated and DRO reevaluates possible migrations during the next cycle.