This is one of the most frequent asked topic, and misunderstood concept in my opinion. When it comes to design and giving decisions on how to architect your environment , you need to realize the
IOPS you will get from your active and passive DB copy in Exchange 201 are the same ( almost). The misconception is the passive DB should not be required such IOPS as no user activity is expected on the passive copy and this assumption turns out not to be true. Perhaps the very aggressive pre-reading Exchanged 2010 does to maintain the passive DB integrity and low check point for fast fail over design in Exchange 2010 architecture contributes the “fact”.
In Exchange 2013 where does this falls into ? the passive copy IOPS are reduced %50 percent, Exchange 2013 is offering huge IOPS reduction over Exchange 2010. The aggressive pre-reading is no longer being done within the passive copy, due to changes have been done to ESE logging.
Exchange 2013 is looking real promising, Although the product is not fully ready IMO , “yet” once pieces put together and given little time I am pretty sure it will offer some compelling business reasons to move into Exchange 2013 version.