I don't have an answer based on experience. We run standard edition so running multiple instances is not an option. I have some opinions.
1) It should be easier to implement a security model with multiple instances. If you have several enties (companies, divisions) that need SQL Server but don't share data, multiple instances of SQL could be used to segregate them.
2) It would be easy to implement a test environment that duplicates the production environment.
3) I don't know how there could be any performance improvement with multiple instances. I would think performance would be hurt with multiple instances on the same server. Perhaps someone with experience could enlighten me about this issue. Terry L. Broadbent
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