(For the other posts in this series, see my Misconceptions blog post category.)
Resource governor is a great feature in SQL Server 2008, but there are some misconceptions about what it can do...
Myth #22: resource governor allows governing of IO activity.
FALSE
Resource governor does not govern IO activity in any way - hopefully that will be something that's added in the next major release of SQL Server. It'll be a lot more useful once you can use to prevent run-away queries doing huge table scans, or spills into tempdb.
Some other things that resource governor does *not* do in the first version:
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Allow governing of buffer pool memory. The memory governing it performs is for query execution memory grants only - not for how much space is used in the buffer pool by pages being processed by a query.
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Allow two instances of SQL Server to cooperatively govern CPU and memory resources. Multi-instance governing has to be done with Windows Server Resource Manager, and then with resource governor for each instance.
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Allow a connection to be notified that it has been governed in some way.
Don't get me wrong - it's great, but it will be a lot better with these additions too.
Our friend and fellow-MVP Aaron Bertrand (twitter|blog) and SQL PM Boris Baryshnikov wrote a comprehensive whitepaper that you should read for more details: Using the Resource Governor.