HP Capacity Advisor 7.2 User Guide

you have a system with four cores where you never want utilization to exceed 75%, and peak
utilization is 1.75 cores, then headroom is 1.25 cores.
Optimum headroom varies depending on size of system. While a single processor system might
require 50% headroom to preserve reasonable response times, a 16-way system might have
reasonable response times when loaded at 80%.
Adequate headroom can also depend heavily on the characteristics of the loads; highly interactive
systems require much more headroom than those that can tolerate delays in response time; batch
systems may get by with very little headroom at all.
Headroom star rating
Various reports and results show headroom star rating. Use the following chart to understand the
headroom rating system.
MeaningStar image
Not only do all resources fit, but double the resource usage for any single workload could fit and
no utilization limits would be exceeded.
All resources fit, no utilization limits are exceeded, and at least 75% headroom for any single
workload is available.
All resources fit, no utilization limits are exceeded, and at least 50% headroom for any single
workload is available.
All resources fit, no utilization limits are exceeded, and at least 25% headroom for any single
workload is available.
This star arrangement (one green star) means that all resources fit in the system and no utilization
limits are exceeded; however, no or little headroom is available.
One or more resources do not fit in the system; the utilization limits are exceeded.
Data is not available for this system.
where
• Resources can be CPU cores, memory, network I/O, and disk I/O. In the case of a virtual
machine, the number of CPU cores considered are those assigned to the VM, not the total
number of cores on the VM host. The VM host clock speed, network capacity, and disk capacity
are all inherited by the VM guest when it is moved onto the VM Host.
• fit means the utilization limits (see “Utilization limits ” (page 28)) are met
• Headroom means “room for growth”
Interpreting the headroom star rating
Headroom star ratings for a host are a weighted average of all of the star ratings of the workloads
on that host. The weighting tends to give the highest weight to the lowest rating. One low rating
can dramatically lower the rating for the entire host.
In the case of a VM host, the star ratings account for how well the workloads fit into their virtual
machines, as well as how well the virtual machines fit on the VM host. The rating for the VM host
will be low if any of the virtual machines are too small for their workloads.
More examples — relative headroom
Understanding a Five-Star rating
If the headroom rating of a workload is five stars, then the relative headroom will be at least 100%
for all of the resources having utilization limits. This means that utilization could have been doubled
26 Key Capacity Advisor concepts