When planning your VM workloads, you should be aware of what level of availability each server will need. By splitting the amount of users across multiple VM’s you can provide a level of availability to each set of users based upon your SLA with each business unit in your company. Looking back to the availability pyramid you can choose which level of availability for each mailbox server is needed. For example if you have an executive group that needs to be up with a 24/7 uptime and only limited downtime then level 3 should be your selection on a separate mailbox server. If all of your business units require the same level of availability and have the same SLA in place then you will split your mailbox servers according to usage. Using the chart from section one we can split the users based upon the type of user. For example if you have 1,000 heavy users we would assign 2 vCPU’s to the virtual machine. Always follow Microsoft best practices when deploying the amount of users per core or vCPU.
If we look at Figure 1 we can see that that we have 4 active VM’s spread across two servers with 2 vCPU’s assigned to each. Looking at the example chart above and using figure one we can see that this design example would support 4,000 “Heavy users”. We achieve this by allowing our storage groups on each mailbox VM to support 1,000 “Heavy” users.
Let’s take a look at a basic design with 3 separate types of users spread across 4 servers. We have an executive mailbox store, a mid-management store, and a general user store. In looking over what each teams HA requirement is we have come to the following, the executive team needs 24/7 up time with no downtime except for a maintenance window once a month. The mid-management team can handle some downtime, but only a few minutes each week. The general users have no HA requirement they can be down for an hour a week if needed. So how do we decided what level of availability we would like to use, it’s easy we simply look at the application availability pyramid and we put the appropriate mailbox store at each level:
By using this simple plan you can simplify you’re HA strategy for Exchange. By distributing the mailbox stores across multiple servers on the same hardware you can save rack space as well as provide individual levels of availability based upon different business unit needs.