As part of my current project I’ve spent some time over the past couple of months trying to determine the best (cheapest) SQL Server configuration to support web servers running in a virtualised environment. As a quick disclaimer, the following are my thoughts on the subject and should be used as guidance for further research only!
Firstly you need to figure out whether you are going to license using the “per user” or “per processor” model. For most typical configurations a “break even” point can be determined when it becomes cheaper to switch to the per processor licensing model instead of the per user model. It is important however to plan for future growth as it can be very expensive to try and switch from one model to another once a system has been deployed. It is not possible to convert SQL “user” CALs into a processor license.
If you are developing a web application that will be exposed via the internet to external customers then it would probably make sense to use the web edition, which only comes with “per processor” licensing. The Microsoft definition of what is a user is critical when selecting the web edition as this can not be used for intranet based applications used by company employees. See the licensing section for more information.
To aid in the selection of the correct edition Microsoft have put together the following SQL Server 2008 Comparison Table.
It is also worth considering the environment that is going to host the SQL Server instance? If you intend to host SQL Server in a virtualised environment things can quickly become confusing and potentially (yet again) expensive. Even the Microsoft FAQ on SQL Licensing appears to contradict itself – the answer to the question “What exactly is a processor license and how does it work?” appears to state that you only need to buy a license per physical processor even for virtualised environments. However, the answer to a later question “How do I license SQL Server 2008 for my virtual environments?” then contradicts this by giving a more detailed answer highlighting that in a virtualised environment the definition of the “per processor” model changes depending upon which edition of SQL Server you have purchased.
You then need to dig around the Microsoft site a little more to investigate the licensing model a little more – the Licensing Quick Reference PDF provides a lot more information from page 3 onwards. I won’t duplicate the information held in that document, because if nothing else it may be updated over time but it is worth noting the differences in Data Centre / Enterprise editions and Standard edition. For the standard edition the document then moves on to describe the formula that should be used to determine the number of per processor licenses that should be purchased for each virtualised instance of SQL Server. At the time of writing you divide the number of virtual processors with the number of cores on the physical processor (rounding up) to get the number of SQL licenses needed. This does mean that if you have a 4 core physical CPU and you expose each core as a virtual CPU to the instance of SQL Server, you still only need one “per proc” license. A common misconception is that you must have a “per proc” license for each virtualised CPU exposed to SQL Server, but this hopefully clears up that potentially costly misunderstanding.