Hello ictadminrbassi
"I believe we need to do this to allow us the use of ROBO functionality which is a feature in 6.5."
ROBO has existed since 6.0 (6.1), 'ROBO' as a 2-node cluster not using ROBO-25pack licensing is possible, are you setting this up as a 2-Node Direct Connect? - This is 6.5 onward only.
Virtual SAN licensing / pricing
As far as I am aware there would be no issue with running a 6.5 Witness VM on a 6.0 cluster, provieded the Witness Appliance itself is managed by a 6.5 vCenter.
"In each node we will have 1 x 400GB cache and 2 x 600GB capacity tier disks."
Unless you plan to use very little percent of the storage I would advise larger (and multiple if possible) capacity drives, so that full repair in the event of a single disk on host is possible - your cache:capacity ratio would still be okay with far more than the current capacity per Disk-Group.
"How much usable capacity would this give us after the creation of 1 VSAN Data store with the disks from the 2 hosts?"
Assuming you are running it as FTT=1 mirrored on both date-nodes this will give you 1.2TB usable space (e.g. 1.2TB of vmdks) - however guidelines in general are to not utilize more than ~70-75% of vsandatastore as resyncs and reconfigurations of vSAN Objects temporarily needs space to do this (plus for other factors such as snapshots and/or backup temporarily storing data on vsandatastore). That utilisation recommendation is generally also advised for the purpose of recovery resync to rebuild failed components, but this isn't possible in the event of a node failure in a 2-node configuration (FTT=1 requires 3 usable Fault Domains), and for full recovery of a single disk failure you would require whatever % space the disk makes up of the Disk-Group(s) on that node.
"If we expand in the future is it just a simple case of adding disks into both hosts and expanding the data store?"
Yes, drives added to existing Disk-Groups add this storage to the vsandatastore automatically.
Do make sure you know how many slots these servers come with available and in what SFF so that you know you can expand by adding disks as opposed to having to replace existing disks with larger ones.
"Is that correct or is there a specific license key for the witness VM?"
Witnesses typically are run as a VM on one site/cluster and then are registered as a Witness Appliance in the same vCenter as the vSAN cluster - these 'hosts' come with their own special free licence.
"The witness states it needs a cache and capacity tier, we don’t have any SSD in the environment but I believe for the witness you don’t need SSD. Would this be correct?"
These don't require physical local disks as data-nodes do - only vmdks for the VM backing the Witness Appliance (stored wherever, do not need to be flash for cache etc.),
Here are the sizing guidelines for these
https://storagehub.vmware.com/t/vmware-vsan/vsan-stretched-cluster-2-node-guide/witness-host-sizing/
And the connection requirements from Witness to Data-nodes:
https://blogs.vmware.com/virtualblocks/2015/09/11/vmware-virtual-san-robo-edition/
Witness Network Connectivity Requirements
- 1.5 Mbps connectivity
- 500 milliseconds latency RTT
- Layer 3 network connectivity without multicast to the nodes in the cluster
"I assume we won’t have the issue with the VSAN for ROBO licensing which is limited to 25 VMs, would that be correct?"
Only ROBO per-VM licensed packs have VM quantity limitations AFAIK (I work in GSS - Licensing is handled by another group - if you have existing S&S I am sure you could get further clarification on this from them by opening a Support Request)
I hope this answers your questions.
Bob
