In continuation of our virtualization performance tests with large enterprise application platforms running on XenServer, we recently completed an extensive series of performance and scalability tests with Microsoft SQL Server 2008 on XenServer 5. In the past, we successfully demonstrated the enormous server consolidation and management benefits of virtualizing Citrix XenApp 4.5 and 5.0 as well as Microsoft Exchange 2007 using XenServer. Test results for these can be found on the white paper section of XenServer/Essentials at

The results of tests for SQL Server 2008 on XenServer were equally as impressive.

Testing was done with readily-available load test programs, DBHammer for SQL and SQLIO, using three hardware configurations we described simply as small, medium and large. We compared results of XenServer with that of native, physical SQL servers running anywhere from one to eight instances of SQL Server 2008, all on single servers. Multiple instance SQL servers were compared to virtual host servers running an equal number of SQL Server VMs. Finally, we did comparison tests using the latest available release (as of Q1 2009) of another virtualization platform provider, described as Vendor X. The following is a highlight of the results: 

Small Configuration:

  • When comparing a single instance native server with a single XenServer VM, XenServer’s virtualization overhead was less than 8%. By comparison, overhead for Vendor X was 36%.
  • In the two instance native vs. two VM tests, XenServer out-performed both the native server and Vendor X by a considerable margin.

Medium configuration:

  • XenServer continued to virtualize with less than 8% overhead in the single instance/single VM test and outperformed both the native server and Vendor X in the two and four instance/VM tests.
  • Note: Vendor X did not participate in the single instance/single VM test due to published maximum VM size limitations.

Large configuration:

  • Native servers outperformed XenServer in the two instance/VM test, however XenServer outperformed native servers in the four and eight instance/VM tests.
  • Vendor X outperformed XenServer in the four instance/VM test, the only test in the entire SQL Server 2008 test series where they did so. However, Vendor X performance degraded considerably once the number of VMs doubled from four to eight, while XenServer’s performance actually improved.     
  • Note: XenServer did not participate in the single instance/single VM test due to published maximum VM size limits. Vendor X did not participate in both the single and two instance/VM tests due to published maximum VM size limits.


Once again, XenServer continues to demonstrate the ability to virtualize the most demanding of enterprise application platforms with relative ease. The key benefit of using XenServer virtualization with Microsoft SQL Server lies in the low virtualization overhead, making XenServer an effective server management tool in SQL Server 2008 farms running primarily single instance servers. However, the benefits of virtualization and the strength of XenServer specifically are most pronounced with multiple instances of SQL Server. Multiple XenServer VMs are increasingly more productive than physical SQL servers as the number of instances increase. 

The results for all tests in the project along with a detailed description of the tools and methodologies can be found at: