At which level is Datastore latency mesured ?

Hello,

I have a question concerning datastore latency. I see that, in general, 20 - 30 ms is the latency limit before having performance issue, but at which level is this limit ? is this the latency see from the VM ? see from Vsphere ?...

In Vcops, when I select a datastore and go to Operation - Details to see Read/Write Latency : Do I see Guest AVG or Device AVG ? (see image below)

When I select a VM in Vcops to see the latency "from the VM", what is the difference between the vDisk latency and the datastore VM Latency ?

I know it's a lot of questions in one thread, so thank you in advance to people who will take time to answer.