I have a question:
- If i have six nodes ESXi (2CPU for node) and i would like to use only 3 nodes for storage-only and 3 nodes compute-only, i need buy the VSAN license for 3 nodes only?
Similar to ESXi, vSAN licenses are required on a per-CPU basis so this will apply regardless of whether the hosts are contributing storage capacity to the vsandatastore or not.
"Virtual SAN Standard, Advanced, and Enterprise editions are licensed per-CPU (socket). These
are standalone licenses (i.e. not included with any other VMware license such as VMware
vSphere®, vSphere with Operations Management™, VMware vCloud Suite®, and so on)."
As an aside, there is no way to configure vSAN nodes as "storage-only" (can be "compute-only" though of course).
I wouldn't recommend doing this. Ideally you want all the nodes in the cluster to be as similar as possible. Also, only having 3 nodes for storage, limits your availability in case of maintenance/failures. Having all nodes contributing to storage may increase your performance and will increase your failure domains, allowing you to use something like FTT=2 if you wanted.