DRO migrations

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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.

When a host becomes over-utilized and is not a member of an Enterprise Pool, virtual machines from the violated host are migrated to less-burdened hosts. The following animation shows three hosts in a host group that have DRO enabled. The hosts are not members of an Enterprise Pool. When Host 1 becomes overburdened, the virtual machines are migrated to other hosts in the host group:
This figure shows how DRO can migrate an over-utilized virtual machine

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.

CPU
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.
If there are any virtual machines in category 1, the virtual machine with the least memory is migrated because it will take less time. If there are no virtual machines in category 1, the virtual machine from category 2 that will reduce the CPU utilization the most is migrated.
Memory
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.

Note: If soft pinned virtual machines is moved to a different host and if the host to which it is initially pinned is operating, then you cannot migrate the virtual machine to a new host.