Good read this:
It is virtually impossible for a cloud vendor to offer a strong SLA for two reasons. First, the cost advantage of the cloud is based on shared resources, although IBM (nyse: IBM - news - people ) is now pushing the idea of creating and running private clouds for its large customers. But even if a cloud is private, the fact that many applications are running on a shared infrastructure increases the risk of catastrophe.
The second reason is that offering a really strong SLA, one that covers the lost revenue from an outage, is just too risky. It means putting the vendor's entire enterprise at risk, essentially selling a form of insurance on the cheap. That's why in the cloud and elsewhere, SLAs will never be that strong.
Without SLAs to provide much comfort, the real remedy to managing catastrophic outages is redundancy. If your cloud infrastructure fails for a critical system, you must be able to bring up a redundant infrastructure that performs the same functions in order to keep your business running. In this respect, the cloud is coming to its own rescue in the form of multi-cloud offerings.
RightScale has recently announced multi-cloud support for its system, which helps rapidly deploy and scale applications on cloud infrastructure. Multi-cloud support means that the configuration you use to define a one-cloud computing infrastructure, say Amazon Web Services, will not only set up a working system there, but also on Eucalyptus, Flexiscale and GoGrid as well. This provides a way out of a catastrophic failure of one of the cloud vendors. If you replicate the data stored in one cloud to another on a continuous basis, if one cloud fails, you can rapidly recreate the same infrastructure on the second cloud.
This is much better than an SLA in terms of reducing risk. As more cloud vendors expand their offerings, more and more of the cloud will become redundant. For example, Mosso, RackSpace's cloud computing division, just purchased JungleDisk, a consumer- and work group-oriented cloud storage service based on Amazon's S3. Mosso now intends to transform JungleDisk into a multi-cloud offering.