Best Practices for Oversubscribing of CPU, Memory and Storage


Best Practices for Oversubscribing of CPU, Memory and Storage in VMware vSphere environments. New document which explains all about oversubscribtion. When managing VMware vSphere environments, you can easily oversubscribe memory, CPU or disk resources. But meaning that is easy to do does not mean that it’s the best for your infrastructure.

How to size the vCPUs and what’s the metrics to watch on the host, but also inside of the VM. There are some good guidelines, which you can follow, in this paper.

Oversubscription, it can be done without much risk. But there are metrics to watch otherwise when a particular resource is finally exhausted, the performances will drop significantly. If of course, that particular resource, has been oversubscribed and all VMs are affected which runs on that particular host.

Here are some examples:

  • If a host has two eight core processors and hyperthreading is either not supported or not ena-bled, that host has sixteen physical CPUs (8 cores x 2 processors).
     
  • If a host has two eight core processors and hyperthreading is enabled, that host has thirty two physical CPUs (8 cores x 2 processors x 2 threads per core).

With an understanding of how physical resources are represented on a vSphere host, the discussion turns to how those processing resources are presented to virtual machines. In a virtual machine, processors are referred to as virtual CPUs (vCPUs). When an administrator adds vCPUs to a virtual machine, each of those vCPUs is assigned to a pCPU, although the actual pCPU may not always be the same. There must be enough pCPUs available to support the number of vCPUs assigned to an individual virtual machine or that virtual machine will not boot.

However, that doesn’t mean that administrators are limited to just the number of pCPUs in the host. On the contrary, there is no 1:1 ratio between the number of vCPUs that can be assigned to virtual machines and the number of physical CPUs in the host. In fact as of vSphere 5.0, there is a maximum of 25 vCPUs per physical core and administrators can allocate up to 2,048 vCPUs to virtual machines on a single host.

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