I've noticed datastores clusters in the environment I'm supporting with I/O thresholds set below the 30ms default. I'm looking into why this was done but, most of the recommendations I have seen are to generally keep it at the default.
The datastore clusters I have seen are set anywhere from 15-25ms. That seems low to me but, the people who configured this are no longer available to question their justifications.
The default setting for SDRS is 15 ms (vSphere 5.0 Storage Features Part 6 &#8211; Storage DRS &#8211; Balance On I/O Metrics | VMw... ) In some environments with heavy load and not much cache I have seen 25 ms used to avoid triggering SDRS during backup's and other I/O times. Really it comes down to your environment you have to know your environment and vendor best practices before you can figure out why they set it. Assume that 25 ms was set for a reason since it's not the default. In my environment we ended up with 10 ms and it was great... but it really depends on your environment. Let me know if you have additional questions.