Please read the Xsan guidelines for volume expansion, specifically the part about storage vs bandwidth expansion (excerpted below)
ref: http://support.apple.com/kb/HT2769
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A “storage expansion” occurs when LUNs are added to newly created storage pools. When adding capacity to an Xsan volume is the goal, consider storage expansion first.
A “bandwidth expansion” occurs when you add LUNs to existing storage pools. A bandwidth expansion should be considered when the primary goal is to improve the bandwidth of a storage pool. Avoid bandwidth expansions if your goal is only to increase capacity.
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Did you use the Xsan 2 setup assistant? What kind of XSAN did you specify? Video or file server, etc How many LUNs did it ask for in each storage pool, 2 or 4?
If I was really pressed for time, or just wanted sotrage expansion, then I would create a new storage pool and add the two new LUNs.
If I had the time I would back it up, blow it away and create one new storage pool with four LUNs. That would be right for video, but maybe this is not a video workflow based SAN.
What is it used for?