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Currently (according to BaBar experts who are much further along in their VM plans) multi-core VMs don't perform nearly as well relative to single-core machines as their hardware counterparts, so the better strategy is to instantiate many single-core machines. But each one needs a license. That many rhel6 licenses would be prohibitive, so the went to Scientific Linux instead. We may want to do the same. Or things may have changed by the time it matters to us.