There is a lot to be said about oversubscribing to virtual machines. People might think that the ideal ratio of users to virtual machines would be 1:1. Think about it, if you have ten employees and you get ten virtual machines, you can be sure that everybody would have a virtual machine to use when they need it. However, think of the converse. Often times, a majority of these virtual machines would be unused.
This is very similar to treadmills in a gym. If you had 10 members to your gym and you buy 10 treadmills, then most of the time, these treadmills will sit unused. This is because not everyone will be at the gym all the time. Your gym members would come sporadically at different times. But you do know that there are peak times and down times. So if you have 10 treadmills, it is only logical that you accept more members. Say, you get 20 more members; it would be more accurate to say that these treadmills have the chance to be 30% occupied at all times. Or you could expect that at any given time, three treadmills will be in use. At peak times, you will have somebody waiting for a treadmill to use, but for most times, your gym members would be have enough treadmills to use, without having too many idle treadmills around.
Now think of treadmills as CPU cores, and you get the idea. With virtual desktop infrastructure, your users will have very minimal CPU use for most times. It makes sense for systems administrators to allocate many VMs to one physical CPU, so that their hardware investments would be optimized.
On the other hand, what happens when all 30 gym members show up at the same time? Well, it would certainly be a problem with 30 members lining up for only 10 treadmills. In the same way, performance will be very poor if there is a chance that too many users are using their VDIs and you oversubscribed your CPU resources.
That is you have to be aware that there is a trade-off. You cannot just oversubscribe your CPU resources, or else it will run into performance problems. CPU processes are scheduled by the host. A VM that thinks it has two cores will expect two cores to always be open on the host. With other VMs using these two cores, the VM would have to queue up for the next available CPU cores. Much like people lining up to use a treadmill at an overcrowded gym.
What is the optimum or ideal? It is all up to you and it all depends on your users’ utilization of their virtual machines. And if you need help, call Four Cornerstone. We can help you set up your virtual machine environments in such a way that you can minimize any risk and incidences of a slow-down.
Plus, we offer Oracle consulting in Dallas and we can help you set up your virtualized environments using Oracle software and hardware. So call us at 1 (817) 377 1144 today.
Photo by Ars Electronica.