Viktor van den Berg explains how vSphere storage latency should be determined, and why blocksize is an important matter:
So, let’s say we have a latency that’s bigger then 30ms, do we have problem?
Not necessarily, the SCSI block size also determines the latency value. The SCSI block size is depending of the type of IO’s the Guest OS is sending to disk. In a “normal” situation you might observe 4k, 8k, or 16k IO blocks. But in some situations the Guest OS might be sending bigger IO sizes, Windows 2008 might send 1 MB (!) SCSI blocks to disk. The thing is that vSphere will measure IO latency for the whole IO SCSI block. This actually means that the size of an IO block will impact the latency value proportionally; thus the bigger the IO block, the higher the latency.