There's an interesting article by storage consultant Henry Newman on Enterprise Storage Forum explaining in detail what I've been trying to get across for over a year now: too much server virtualization too quickly could wreak havoc on your existing storage network.
Newman spells out very clearly the dangers of trimming a 1,000-server farm to, say, 100 without upgrading even a relatively fast 1 Gb Fibre Channel infrastructure. Assuming you're using dual-port HBAs, you're faced with a 10-fold decrease in available ports, which means that even if you upgrade to 4 Gb Fibre Channel, you're still seeing total server bandwidth drop from about 195 GBps to 78 Gbps.
Even worse, he calculates that this scenario will likely see a 3.6-times increase in IOPS, which will be tough to meet even with a 10-fold increase in both CPU performance and storage capacity.
Newman offers a number of methods to address the situation, urging that you pay particular attention to how your file system will allocate data from multiple I/O requests.
However, the underlying point is that things like IOPS and CPU performance do not necessarily scale 1:1. It's going to take a bit of work to figure out exactly how changes on one side of the data center will affect the others.
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