The Frontend (Guest) Latency graph shows vSAN disk group (Cache tier disk) Frontend (guest) latency. Frontend Read Latency Metrics displays the vSAN disk group (cache tier disk) Frontend read latency. For more information, see https://kb.vmware.com/s/article/2144493?language=en_US
Thanks, but checking VMware KB was first thing I did. Their explanation does not tell me anything.
Was waiting for my SR to be closed before replying here; this is a bug in reporting. Same "latency" was seen in both Web Client (under %host% > Monitor > Performance > Virtual SAN - Disk Group > "Frontend(Guest) Latency", and in VSAN Observer (on LSOM/lsomhost, that's VSAN Disks (deep-dive) > %host% > Host disk-layer aggregate stats > Full graphs > Look in the first graph - latency). Value was reporting around 200.00 - 500.00, when it should have been 0.2 - 0.5.
Good news is that it's resolved in the latest 6.0 U3-release (build 6921384).
From the release notes / VMware Knowledge Base
"The Host disk-layer aggregate stats full graphs view under the vSAN Disk (deep-dive) tab might report wrong latency values, because the latency values are generated in nanoseconds, but wrongly interpreted as milliseconds."
Nanoseconds.. well.. based on the numbers (500 to 0.5) it's microseconds. Anyways, it's now fixed! :-).