Oversubscription has long been a contentious topic. In the mainframe world, popular wisdom dictates that you dedicate ports on a 1-to-1 ratio (no oversubscription). The reality is that we live in an oversubscribed world. Consider a server that cannot push 4 Gbit but if we aggregate the bandwidth or I/O operations across multiple servers to a storage port, some people might suggest that will lead to oversubscription. It might and it...
might not. We have to look at the I/O and response time, queuing delay, bandwidth and workload characteristics of the servers and applications. The network must satisfy those demands and keep the data flowing.
There are many potential rules of thumb when it comes to oversubscribing but it all depends on the server having the bandwidth and I/O that it needs to meet a particular level of service. If a server or application requires optimum performance for peak demand, I would probably dedicate a port with full bandwidth. Other applications may tolerate oversubscription better depending on the applications' particular needs -- which you should consider carefully.
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