Bruce W Cassidy (8/13/2009)
You can, of course, create multiple Oracle instances. There used to be a great deal of overhead in doing this, but with modern versions of Oracle that's far lower. The only real overhead is in the amount of system database area per instance.
Unfortunately statement above does not reflects reality.
Multiple instances DO add a lot of overhead no matter Ora version, anything in between 17 to 30 daemons running per instance plus all SGA/PGA instance specific memory segments.
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Pablo (Paul) Berzukov
Author of Understanding Database Administration available at Amazon and other bookstores.
Disclaimer: Advice is provided to the best of my knowledge but no implicit or explicit warranties are provided. Since the advisor explicitly encourages testing any and all suggestions on a test non-production environment advisor should not held liable or responsible for any actions taken based on the given advice.