The most important thing to remember is that storage tiers are really only inherently storage specific, so different...
tiers can coexist in the same fabric. Accordingly, any of the ways you have outlined the tiers would be fine. You can have as many as your business needs demand, based upon combinations of availability, performance and costs.
It is common in larger enterprises to use of three tiers (one -- high performance/high availability/with DR, two -- high performance/high availability/no DR, and three -- low performance/medium availability, no DR).
However, you should understand the risk of have multiple tiers coexisting on the same servers. So, to that end, consider VMware or Virtual Server partitions supporting different applications on different tiers.
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