Finding out that vSphere is picky about hardware support can happen fairly quickly. VSphere works from a highly-tuned kernel environment, where the virtual machines (VMs) have to communicate with specific drivers in vSphere to send instructions to the hardware in the physical host. To guarantee that this process is happening in the most optimal way, vSphere works with specific pieces of hardware only. For these specific hardware devices, vSphere provides a driver that is thoroughly tested and optimized.
The XenServer 5.5 approach is completely different, especially when using paravirtualization. In paravirtualization, the way a VM communicates to the hardware in the physical host is more direct than in vSphere. That means there is no need for very specific drivers. XenServer 5.5 just works directly on the hardware in the server. Therefore, there is no hardware compatibility list with specific network cards or hard disks -- the basic idea is that any server grade hardware works.
With regard to the CPU in the virtualization host, you'll need 1.5 GHz minimal. Be aware that on your customer's physical CPU, multiple machines are hosted. To make sure these machines run smoothly, you definitely want to use multicore processors. The exact amount of cores you need depends on the amount of VMs you are going to run. If you want to host five VMs, and each VM hosts one not too busy Web server, you can probably host all of the servers on one core only.
On the contrary, if you are planning to host a few busy databases in your customers' VMs, you may need multiple cores per VM. My best advice is to make an accurate hardware calculation -- add the CPU requirements for each of the VMs you are planning to run on a physical host and plan your CPU power based on that.
What is true for CPUs goes for RAM as well. The minimal requirement is 1 GB, but, in reality, this is hardly ever sufficient. Typical hosts for XenServer 5.5 start with 8 GB of RAM, and 16 GB, or even 32, is becoming more and more common.
For sufficient storage, you need at least one disk that serves as the system disk for XenServer 5.5. This can be a local disk, or a Fibre Channel boot disk with a minimal size of 16 GB. The recommended size, however, is 60 GB. Apart from that, you need space to store the VMs. Typically, this will be on the storage area network, and again, what you need here depends on the storage space that is required by the individual VMs.
The last part of the configuration you need to consider is the network card in your customer's server. In theory, a 100 Mb network card is sufficient. Realistically, you'll never want to use this minimal configuration. To ensure that your customer's virtual environment performs well, you need at least a gigabit network connection. It's even better to trunk two (or more!) gigabit network cards to allow for increased throughput. A high bandwidth ensures that users can reach the VMs, you can manage them and, if required, you can also do a live migration of your customers' VMs.
Apart from the configuration that is required on the server, you also need a workstation in your customer's Xen environment. On this workstation, you can use the Xen management interface, XenCenter. The requirements for this management environment are not too big; you need Windows 2000 or later, 512 MB of RAM and 500 MB of available disk space.
Compared to the hardware requirements of some of the other virtualization products on the market, the XenServer 5.5 hardware requirements are a delight. There is no specific hardware compatibility list to take into consideration, and as long as you calculate the total requirements of all your customers' VMs, their XenServer 5.5 environments should work.
About the expert
Sander van Vugt is an independent trainer and consultant living in the Netherlands. Van Vugt is an expert in Linux high availability, virtualization and performance and has completed several projects that implement all three. He is also the writer of various Linux-related books, such as Beginning the Linux Command Line, Beginning Ubuntu Server Administration and Pro Ubuntu Server Administration.
This was first published in October 2009