A Tale of Licensing – Part 2: Optimizing vCenter for Windows Licensing

See Part 1 here

For part 2, I would like to focus on what we are doing to simplify licensing and save some money. To recap, we have 160 server VMs spread across 28 hosts which means we would have to license 28 hosts for System center AND Windows Server OS. Among those 28 hosts, we also have many more VMs that are Linux.

Initially, we wanted to create a new Dev cluster and a new Prod cluster to split up the Linux and Windows guests but our VM admin advised against that – stating it would be too much administrative overhead for him to add and manage additional clusters (we already have 6).

So we started exploring using business rules (aka DRS rules). We figured to cover our VMs (about 95 prod and 65 dev) we would need 5 hosts for dev and 7 for prod. This would leave us with plenty of room to grow.

Keep in mind – these numbers will vary greatly when factoring in host specs, guests demands, N+1 or N+2, etc, etc. For the purposes or this write up, I am providing you with the numbers we came up with.

Next, on the dev cluster, we created a Host DRS Group that contained 5 hosts. Then we created a VM DRS Group that contained all of our Dev servers (65). And lastly, we created a DRS rule stating that the VMs in the VM DRS Group must run on the Host DRS Group. You will immediately see VMs start migrating after you apply these changes. We were concerned that they would all try and migrate at once but DRS does a good job of only allowing so many VMs migrate at one time.

In the end, we hope to take this setup and apply it to prod as well. If testing goes well (meaning, DRS behaves like we expect) we will only need to license 12 hosts for System Center and Windows Server OS using the Datacenter licensing model. This means we will save money on having to purchase licenses and pay for SA’s on 16 hosts. The savings could add up greatly!


*A few IMPORTANT notes*

  • We did not create rules stating that the Linux guests had to stay on the other 16 hosts. They can freely use any of the 28 hosts that they want. If a host becomes too overcrowded it should migrate a Linux guest off since the Windows guests have to stay there. We determined the license savings on Linux wasn’t enough to worry about isolating them.
  • Don’t forget to add new deployments to your DRS groups!
  • We are in the process of converting our standard licenses into datacenter licenses (we still need to keep some for our physical servers
  • The licenses allow for temporary failover. If there is a failover event, the VMs can reside on another host (#13) for up to 30 days before moving them back to where they need to be. Just don’t abuse this!
  • If you decide you want to manage devices and/or desktops you will have to purchase ‘client’ licenses as well for each device. I won’t go into details on this but it is something to keep in mind since I focused this discussion on only managing servers.
  • We experienced a strange DRS issue with VM-VM affinity rules combined with VM-Host rules. I have forum post outlining this issue here if you are curious.


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