DRO with Dynamic Capacity on Demand

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Capacity on Demand (CoD) from IBM® Power Systems is a powerful tool that activates unused cores or mobile memory for a host, giving it more computing power or memory instantly. On eligible systems, the PowerVC Dynamic Resource Optimizer (DRO) works with CoD to relieve overburdened systems much more quickly than when it has to migrate virtual machines to balance system usage. For more information about CoD, see Capacity on Demand.

Overview

When DRO and CoD work together, DRO has the following additional capabilities:
  • DRO detects changes in your Enterprise Pool definitions and periodically updates them in PowerVC, even if the Enterprise Pool license is purchased after PowerVC was set up.
  • DRO uses free CoD mobile cores to automatically ease the workload on busy hosts.
  • DRO activates mobile memory to make more space available to overburdened hosts.

Example: Mobile core activation

When a host's CPU utilization is imbalanced in relation to a user-defined threshold and is a member of an Enterprise Pool, DRO tries to use CoD to optimize host usage. This is much faster than migrating virtual machines and does not burden the network. The following animation illustrates how the DRO can activate mobile cores as the CPU utilization for a host becomes overburdened:
This figure shows how DRO can activate mobile cores as workload demands change

Activating mobile cores

The DRO tries to activate cores from the Enterprise Pool before reclaiming cores from other hosts. If it must reclaim cores from a host, it tries to ensure that reclaiming a core will not violate the threshold for the source host. Additionally, it tries to minimize the number of cores that get reclaimed from any single host. For example, if the DRO needs to assign three cores to a host and there are no available cores in the Enterprise Pool, then it would try to reclaim one core each from three hosts instead of reclaiming three cores from one host. If the DRO cannot assign the number of cores that are needed to satisfy the threshold but there is at least one available core, it continues with CoD-based optimization. If the DRO cannot assign any cores, it tries to migrate a virtual machine instead.
Notes:
  • You cannot activate more mobile cores than the total number of physical cores on the machine. Therefore, the host must have at least one inactive core to be able to move a mobile core to it.
  • If mobile cores are removed from a server but it cannot reclaim the cores because they are being used, they become unreturned cores.
    • DRO tries not to create unreturned cores. However, under certain circumstances, such as out-of-band operations, DRO operations might result in unreturned cores. In this case, DRO tries to detect and return the unreturned cores.
    • DRO does not use CoD operations if the host has any unreturned cores.

Activating mobile memory

When enabled, DRO tries to claim available mobile memory from an Enterprise Pool and activate that memory on violated hosts. If there is no memory available in the pool, DRO reclaims memory from other unviolated NovaLink managed hosts and returns it to the pool for use by the violated host. However, memory is not reclaimed if doing so would exceed the memory threshold for that host. Additionally, mobile memory is never reclaimed from HMC managed hosts. If a threshold cannot be satisfied by mobile memory operations, DRO migrations will occur instead, assuming they are enabled.