For example, last night we had a VM experience unusually high write latency which caused our monitoring software to trigger a page. When I review the data, I find that VMAN and vSphere are completely different. Right now I'm doing a side-by-side comparison and see that vSphere shows 2428KBps "Write Rate" at 8:27am while VMAN shows 639KBps "Write Throughput" at 8:27am. Someone please help me understand what I'm missing here.
Probable explanation is the unusual high write throughput was for a short amount of time. While vman shows the average data for the collection interval (10 mins by default), vSphere client shows average data for 20 seconds interval, so the values can vary quite a bit. You can eventually compare the maximum values of Write Rate from vSphere with Peak Write Throughput chart in VMan - data from these counters should be the same.