Quoted from http://blogs.msdn.com:
User Instances (also known as RANU) available with SQL Server Express Edition, are NOT RECOMMENDED for hosting environments.
I have a server running Server 2008 R2 Web Edition with IIS 7.5 and Sql Server 2008 R2 Express installed.
I have only 3 friends as my hosting clients. Each has a restricted Local User Account on the server.
What is the drawback of using user instance in production environment?
Rather than considering the limitation in memory usage and database size that I know, let us consider the user instance only, why isn't it recommended?
I think that the answer to your question comes with the definition of the RANU
and also read the http://msdn.microsoft.com/en-us/library/bb264564(SQL.90).aspx
The RANU starts a seperate instance Process of SQL Express for every user, so in a share enviroment with let say 30 sites that needs access to SQL, then SQL run 30 instances at the same time, that is takes as you understand a lot of recource/memory etc. Also as say in the article the first time SQL Express run for every user is make copies of all system database to create this user instance.
In a single user machine to run one process is normal, but if SQL Express starts to run many instances then you take a lot of resource, memory, and this is not easy to administrate, update etc.
In the article on the link there are more detail informations about this.
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