HP Capacity Advisor 7.2 User Guide

CPU Virtualization Overhead %
Meaning
The percent change in CPU utilization due to the overhead (or the absence of overhead) incurred
by running an application in a virtual machine.
When using virtual machines, the CPUs on the VM host do some of the work normally done by
I/O cards. This work is the virtualization overhead accrued by running an application in a virtual
machine.
You can expect that applications that require significant I/O may experience a CPU virtualization
overhead of 20%, and applications that do lots of calculations, but that require little I/O, would
have an overhead of about 10%. When in doubt about I/O usage, use 15%. If your measured
values in growth of CPU usage due to virtualization are greater than 20% for a particular workload
or set of workloads, virtualization may not be the appropriate solution for that server.
Default
The default value is 0% (0% change).
Where you might use this multiplier
• When changing a virtual machine to a server
• When changing a server to a virtual machine
Example: Making a server become a virtual machine
If your virtualization software would cause a 10% increase in CPU utilization due to the overhead
for virtualization software, enter 10 for the CPU Virtualization Overhead % to account for the
additional demand on the CPU core(s) when changing a server to a virtual machine.
Example: Making a virtual machine become a server
If your virtualization software requires a 10% increase in CPU utilization due to the overhead for
virtualization software in the virtual machine, enter -10 for the CPU Virtualization Overhead % to
account for the gain in CPU availability when changing a virtual machine to a server.
Hypervisor memory overhead
The following section presents the scaling factor that you can use to more accurately simulate the
impact of including a hypervisor in your scenario. With the addition of a hypervisor, memory
utilization increases due to the operation of the hypervisor. The Hypervisor Memory Overhead
helps you to account for this in a scenario.
Hypervisor memory overhead
Meaning
The amount of memory used by the virtualization platform to host virtual machines. The size of the
memory overhead varies for each virtualization platform.
Default
Capacity Advisor calculates the value for you.
Where you might use this percentage adjustment
• When moving workloads from one system platform to another different system platform in a
scenario, where one or both system platforms are VM hosts.
Doing the math for hypervisor memory overhead
When you encounter this adjustment factor in altering a scenario, you have a choice to supply
your own values. To help you with this, the following calculation examples are provided.
HP Virtual Machine
To compute the memory overhead of the hypervisor, use the following formula:
750 MB (.73 GB) + 7.5% of (Total Physical Memory – 1 GB)
188 Calculation assistance