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Many SMEs run all Exchange roles on a single physical machine because using a database availability group (DAG) requires another machine and thus would be more expansive because of the extra hardware and licenses required.
Using virtualization allows even SMEs to create a DAG and replicate their databases to multiple disks. This of course increases availability in the event of system outages and also allows you to patch your Exchange system without affecting users. Exchange server 2010 or 2013 running on Windows Server 2012 allows you to use the Standard Edition of Windows, thus providing DAG functionality without extra Windows license fees.
With a DAG, you can have database copies on another exchange server. The minimum requirement for a DAG is two Exchange servers, but you can add as many as 16 exchange servers to a DAG. However, from the financial perspective in SMEs, you may want to consider two Exchange servers to gain full high- availability features for the lowest cost.
You can take advantage of using a DAG instead of implementing hypervisor- based high availability (HA) such as vMotion or Live Migration. The benefit is that a DAG is application aware, whereas hypervisor-based HA is not. A DAG also does not require shared storage, such as a storage area network (SAN), and thus can save money and provide sufficient HA for your users.
Virtualization should also be considered if a company wants to use physical servers for its main datacenter and plans to have a second disaster recovery datacenter. The benefit of virtualizing Exchange at the disaster recovery site is being able to run a multi-site DAG without deploying a lot of hardware in the disaster recovery location. Some best practices you should consider include:
• Implement a virtual infrastructure with at least two host machines, and make sure the Exchange VMs do not run on the same physical host at the same time.
• Deploy at least two multi-role Exchange servers (including all Exchange roles) as VMs and use them for the DAG.
• Multi-role servers do not allow Windows Network Load Balancing (NLB) to be used. Thus you either need to consider a physical network load balancer, a virtual network load balancer or a DNS approach such as round-robin.
Using a single DNS record for your CAS server that points to one server is an inexpensive solution—especially for SMEs—and during an outage you can manually change the DNS record to point to the second Exchange server.
This is the least expensive way to implement failover. If you want to consider a network load balancer, this provides real load balancing but has the disadvantage of requiring an additional piece of software or hardware.
• When you want to implement a DAG, don’t forget to consider other components, such as storage devices and network interfaces, with regard to HA. Make sure these components are highly available on your host system, so an outage does not take all the VMs down.