Chapter 1: System Considerations

Prentice Hall

As an example, a customer wanted a 20:1 compression ratio for virtualization of their low-utilization machines. However, they also had networking goals to compress their network requirements at the same time. The other limiting factor was the hardware they could choose, because they were limited to a certain set, with the adapters precisely limited. The specifications stated that with the hardware they could do what they wanted to do, so they proceeded down that path. However, what the hardware specification states is not necessarily the best practice for ESX, and this led to quite a bit of hardship as they worked through the issues with their chosen environment. They could have alleviated certain hardships early on with a better understanding of the impact of ESX on the various pieces of hardware and that hardware's impact on ESX. (Whereas most, if not all, of the diagrams and notes use Hewlett-Packard hardware, these are just examples; similar hardware is available from Dell, IBM, Sun, and many other vendors.)

Basic Hardware Considerations

An understanding of basic hardware aspects and their impact on ESX can greatly increase your chances of virtualization success. To begin, let's look at the components that make up modern systems.

When designing for the enterprise, one of the key considerations is the processor to use, specifically the type, cache available, and memory configurations; all these factors affect how ESX works in major ways. The wrong choices may make the system seem sluggish and will reduce the number of virtual machines (VMs) that can run, so it is best to pay close attention to the processor and system architecture when designing the virtual environment.

Before picking any hardware, always refer to the VMware Hardware Compatibility Lists (HCLs), which you can find as four volumes at http://www.vmware.com/support/pubs/vi_pubs.html:

  • ESX Server 3.x Systems Compatibility Guide

  • ESX Server 3.x I/O Compatibility Guide

  • ESX Server 3.x Storage/SAN Compatibility Guide

  • ESX Server 3.x Backup Software Compatibility Guide

Processor Considerations

Processor family, which is not a huge consideration in the scheme of things, is a consideration when picking multiple machines for the enterprise because the different types of processor architectures impact the availability of ESX features. Specifically, mismatched processor types will prevent the use of VMotion. VMotion allows for the movement of a running VM from host to host by using a specialized network connection. VMotion momentarily freezes a VM while it copies the memory and register footprint of the VM from host to host. Afterward, the VM on the old host is shut down cleanly, and the new one will start. If everything works appropriately, the VM does not notice anything but a slight hiccup that can be absorbed with no issues. However, because VMotion copies the register and memory footprint from host to host, the processor architecture and chipset in use needs to match. It is not possible without proper masking of processor features to VMotion from a Xeon to an AMD processor or from a single-core processor to a dual-core processor, even if it is the same family of processor that was introduced in ESX version 2.5.2. If the Virtual Machine to be moved is a 64 bit VM, then the processors must match exactly as there is no method available to mask processor features. Therefore the processor architecture and chipset (or the instruction set) is extremely important, and because this can change from generation to generation of the machines, it is best to introduce two machines into the virtual enterprise at the same time to ensure VMotion actually works. When introducing new hardware into the mix of ESX hosts, test to confirm that VMotion will work.


Best Practice - Standardize on a single processor and chipset architecture. If this is not possible because of the age of existing machines, test to ensure VMotion still works, or introduce hosts in pairs to guarantee successful VMotion. Different firmware revisions can also affect VMotion functionality.

Ensure that all the processor speed or stepping parameters in a system match, too.


Note that many companies support mismatched processor speeds or stepping in a system. ESX would really rather have all the processors at the same speed and stepping. In the case where the stepping for a processor is different, each vendor provides different instructions for processor placement. For example, Hewlett-Packard (HP) will require that the slowest processor be in the first processor slot and all the others in any remaining slots. To alleviate any type of issue, it is a best practice that the processor speeds or stepping match within the system.

Before proceeding to the next phase, a brief comment on dual-core (DC) versus single-core (SC) processors is warranted. ESX Server does not differentiate in its licensing scheme between DC and SC processors, so the difference between them becomes a matter of cost versus performance gain of the processors. The DC processor will handle more VMs than an SC but also cost more and has support only in the later releases of ESX. In some cases, it is possible to start with SC processors and make the first upgrade of the ESX Servers to be DC processors in their effort to protect the hardware investment. If performance is the issue, DC is the way to go. Nevertheless, for now, the choice is a balance of cost versus performance. Due to current shared-cached mechanisms for DC, an eight-core or four-processor server has the same processing power as if there were seven physical processors, and once shared cache goes away there is a good chance the efficiency of the DC will match that of a true eight-way machine.

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