Virtualization is one of the hottest trends, especially to combat server sprawl and increase operational efficiencies, and it can reduce TCO. However, database workloads like SQL Server can be some of the most troublesome to virtualize. Here are commonly asked questions about virtualizing SQL Server.

Is virtualization the only way to consolidate servers?
Although virtualization is one of the best ways to perform server consolidation, it’s not the only way. For instance, you can consolidate database servers simply by adding additional databases and their workloads onto a single server. In addition, you can run multiple instances of SQL Server on the same system. These are examples of how to consolidate your workload without virtualization.

How does virtualization help with server consolidation and operations in general?
First, because virtualization technology abstracts the server or OS layer from the underlying hardware, it’s easier to move a server to different hardware for better scalability or for disaster recovery. Next, not all workloads or applications are compatible with one another and server consolidation using virtualization enables you to keep all your databases on completely separate servers each running under their own OS.

How much overhead does virtualization impose?
The actual overhead depends primarily on the virtualization infrastructure and the I/O characteristics of the workload that’s running. However, a general rule of thumb is to expect a 10 to 15 percent reduction in performance between a physical machine and a virtual machine (VM). Obviously, if you’re running in a server consolidation environment, other active VMs will make an impact on overhead, too. What types of SQL Server implementations are best suited to virtualization?

Because virtualization adds overhead, the raw performance of a VM won’t match that of a physical implementation. Virtualize smaller or departmental SQL Server workloads where there is excess CPU and I/O capacity. Be wary of virtualizing database workloads that have high resource requirements. However, many enterprise databases can be virtualized and still meet their service level agreements. It just takes more planning.

Does virtualization make SQL Server administration harder or easier?
Virtualization is not a panacea for server administration. Although they’re on a common hardware platform, each virtual server still has the same management requirements as a physical server—services need to be started and stopped, queries need to be tuned, databases need to be backed up. You can still use management tools such as Enterprise Manager and SQL Server Management Studio just as you would to manage a standard physical server.

SQL Server is licensed per processor—how does that work if you’re running multiple SQL Server VMs on an 8-way host?
Microsoft licenses SQL Server per virtual CPU, not by the number of host CPUs. If you have a SQL Server VM that uses one virtual CPU and the VM runs on an 8-way host, then you only need one CPU license for SQL Server. Microsoft allows an unlimited number of virtual instances for SQL Server 2005 Enterprise Edition, with no additional licensing costs.