My understanding is that one of the big benefits of using a VM is that you can share resources between the virtual machines on a host. So you can have a host with 120 CPUs and put 5 VMs on it with 32 CPUs each. The extra CPUs are "shared" between the VMs with the host assigning CPUs dynamically based on need. Same for memory.
I also understood that this is a big no-no for VMs that house SQL Servers but my VM admins disagree. Does anyone have any evidence or documentation one way or the other?
I don't know if it matters but we are using VMWare.
Best Answer
What you're asking is, "Is it okay if we overcommit CPUs?"
It's okay until you start experiencing performance bottlenecks that are related to CPU consumption. The same answer holds true with network overcommitment as well - it's not like you're putting 5 separate 10Gb Ethernet cards in the host, and dedicating one card per VM. Virtualization is all about overcommitment, and walking the line between resource availability and resource requirements.
Generally speaking, you don't want to overcommit on memory, though, since SQL Server will actively use whatever memory you present to it. Rather than letting the VMs page to the host's disk, you're better off doing VM-by-VM analysis to figure out which VMs could live with less memory, and then configuring them with less memory to begin with.