In the physical world, capacity planning was done on a per (physical) server basis and took into account things such as processor count, processor speed, core count, the amount of RAM, NIC count, NIC throughput, disk count, RAID types and per unit power consumption. Again, this was per physical server and usually assigned to a single application or workload.
In the virtual world, the server is merely a host which offers up resources for the virtual machines to use. Capacity planning now takes into account how the resources are assigned to specific applications or workloads on virtual machines. You must take into account what the resource consumption per VM will be with respect to the total resources of the host (physical server) or group of hosts. VMware will do it's best to balance resource usage with components such as Dynamic Resource Scheduling (DRS) but you must still plan for the aggregate capacity needed to run all of your workloads efficiently. Monitoring the environment becomes key in order ensure that the environment is not experiencing resource contention at any level.
The other reason for capacity planning is to predict usage by business unit and be able to charge back depending on that usage. Naturally, those applications and workloads with the highest predicted (and actual) resource consumption will cost more to support over time. Capturing all of this data in the planning stage and beyond will make the ongoing maintenance and support of your virtual infrastructure much easier and more efficient.
For basic capacity planning, VMware's vCenter CapacityIQ is an excellent resource for analyzing your environment and planning your capacity needs. CapacityIQ:
- Eliminate waste by identifying any unused or over-allocated capacity
- Reduce operational overhead by automating routine capacity monitoring and management tasks
- Minimize business risk of outages or failures resulting from capacity bottlenecks and shortfalls
VMware vCenter CapacityIQ is a value-add component of the VMware vCenter family of management solutions, providing capacity management capabilities for virtualized datacenter or desktop environments. CapacityIQ integrates seamlessly with VMware vCenter Server, ensuring that your virtualized infrastructure capacity is always predictable and efficiently used.
Note that there are many other tools from vendors such as VKernel that go into further detail and automate many processes such as charge back and optimization. The VMware Guy specializes in virtual infrastructure capacity planning, management, charge back and optimization.