This is a question I was asked multiple times over the last week: the Microsoft guidelines for database mirroring say not to mirror more than 10 databases per instance - why is that and is it true? The answer is my favorite 'it depends!'. The number 10 is a rough guess at the sweet-spot for the majority of customers. The factors that need to be considered are: How much memory do the principal and mirror instances have? (hopefully the same) How much processing-power do the principal and mirror instances...(
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Read the complete post at http://www.sqlskills.com/blogs/paul/2007/11/09/ConferenceQuestionsPotPourri4HowManyDatabasesCanYouReallyMirrorPerInstance.aspx