In Part 1 of this topic (here) I suggested that cloud computing has the ability to be elastic… to expand and maybe contract the infrastructure as CPU, memory, or storage requirements change. I also suggested that the workload on an EDW is intense and static to point out that there was no significant advantage to consolidating non-database workloads onto an over utilized EDW platform.
But EDW workload does flex some with the business cycle… quarter end reporting is additive to the regular daily workload. So maybe an elastic stretch to add resources and then a contraction has value? It most probably does add value.
The reason shared-nothing works is because it builds on a sharded model that splits the data across nodes and lets the CPU and I/O bandwidth scale together. This is very important… the limiting factor in these days of multi-core CPUs is I/O bandwidth and many nodes plus shards provides the aggregate I/O bandwidth of all disk controllers in the cluster.
What does that mean with regards to building an elastic data warehouse? It means that with each elastic stretch the data has to be re-deployed across the new number of shards. And because the data to be moved is embedded in blocks it means that the entire warehouse, every block, has to be scanned and re-written. This is an expensive undertaking on disk… one that bottlenecks at the disk controller and one that bottlenecks worse if there are fewer controllers (for example in in a SAN environment). Then, when the configuration is to shrink it process is repeated. In reality the cost of th I/Oe resources to expand and contract does not justify the benefit.
So… we conclude that while it is technically possible to build an elastic EDW it is not really optimal. In every case it is feasible to build a cloud-based EDW… it is possible to deploy a shared-nothing architecture, possible to consolidate workloads, and possible to expand and contract… but it is sub-optimal.
The real measure of this is that in no case would a cloud-based EDW proof-of-concept win business over a stand-alone cluster. The price of the cloudy EDW would be 2X for 1/2 the performance… and it is unlikely that the savings associated with cloud computing could make up this difference (the price of SAN is 2X that of JBOD and the aggregate I/O bandwidth is 1/2… for the same number of servers… hence the rough estimates). This is why EMC offers a Data Computing Appliance without a SAN. Further, this 4X advantage assumes that 100% of the SAN-based cluster is dedicated to the EDW. If 50% of the cluster is shared with some other workload then the performance drops by that 50%.