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Subtleties of power capping
Avoiding power capping conflicts within groups
Insight Control is a powerful tool for setting and managing power caps across defined groups of servers, including
SIM collections. It is important to remember, however, that except for Enclosure Dynamic Power Capping, power
caps are ultimately set at the individual server level. Insight Control apportions a group power cap as individual
power caps to all of the servers in the group.
Power capping is also completely non-hierarchical. A power cap set using Insight Control has no precedence over a
power cap set using the iLO interface. Each server conforms to the last power cap that it received, regardless of the
method used to set it.
For example, consider this situation: A server is a member of two distinct SIM collections. An administrator applies a
group power cap to each of these collections. The individual power caps of servers in both collections will be the
last cap applied to either collection. Later an administrator uses the iLO interface to set a power cap on an
individual server within one of the collections. For that server, the new cap will replace the previous power cap that
had been set through Insight Control.
A similar situation can occur when administrators use Insight Control to set enclosure dynamic power caps for
groups of enclosures. Setting a new cap on a single enclosure through its OA can overwrite a group power cap
previously apportioned to the individual enclosures.
This potential overlap in setting power caps is important to consider when planning and implementing an overall
power capping/capacity management strategy. Choosing a single consistent method for setting power caps is the
best way to avoid conflicts. For example, a system administrator could define a set of non-overlapping SIM
collections used specifically for power and cooling management.
Powering up groups of servers when using Dynamic Power Capping
HP Dynamic Power Capping is a powerful tool for controlling the steady-state power consumption of servers in real
time. However, it does not control the power consumption of servers at start-up when their maximum power
consumption is measured. If a group of servers on the same PDU power-up simultaneously, there will be a window
during start-up before Dynamic Power Capping is active. During that window, the servers will draw close to their
maximum power at roughly the same time. If this peak is too large, it may cause problems. Therefore, it is important
to manually power on these server groups in a staggered manner. Administrators using the iLO interface or the
enclosure OA for BladeSystem servers can stagger auto power-ups by enabling the servers’ Power On Delay and
setting it to “Random up to 120 seconds.”
Setting low or unattainable power caps on servers
In theory, administrators can set a power cap to any value above the minimum power consumption for a given
server or a group of servers. However, setting power cap values close to the minimum power consumption is not
good practice. Maintaining a power cap at or near this level prevents the server from accomplishing meaningful
work. Also, a server’s minimum power consumption level may rise over time for a number of causes, including
increased fan activity as data center temperatures rise or hot-adding disk drives to the server. A power cap
becomes unachievable if the minimum power consumption of the server rises above it. In this situation, the capping
subsystem will cap the power consumption as low as it can and report that the power cap is unachievable.
Best practice is to re-evaluate the cap value after adding any hardware to the server, and to use power caps that
are at least halfway between the minimum and maximum power consumption for a server or server group. Both the
iLO and Insight Control interfaces provide a warning when a power cap value is lower than this.
Enclosure Dynamic Power Capping has resolved much of this issue. It disallows enclosure power caps that are too
close to the aggregate minimum power for the enclosure and its server blades. However, setting a very low cap
value for the enclosure can prevent blades in the enclosure from accomplishing meaningful work.