Welcome to Communities and vSAN.
What are you trying to configure the ESXi install on? Two SSDs in a RAID1? SD-cards/BOSS are a more probable install location, are these not available?
vSAN can only consume drives to make Disk-Groups that have no partitions on them - this is configured when creating the Disk-Groups on each node.
If you are considering using SSDs in a RAID configuration for boot then please ensure these are not on the same controller as the vSAN disks as it is not supported to mix modes
Edit: No - vSAN doesn't do anything with the ESXi 'OS' device, this is unrelated to vSAN.
Thanks for the reply.
No SD or BOSS. 3 Dell R940 and are currently raw. Nothing installed anywhere. I have 16 SSD's per server and only one controller per server. So it looks like I cant do a RAID for the OS as I would have a mixed mode on the only controller.
What would be the best approach?
The boss card will allow you to have 2 M.2 devices that can be mirrored for the OS. This approach allows for redundancy, does not use the controller(s) for vSAN, does not take drive slots away, AND it also allows you to have logs locally as opposed to SD card where you will need to ship the logs out.
Also, You stated that you have 16 drives but only 1 controller. This will most likely be a hurdle as many controllers do not support that many devices. For example, the Dell HBA330 supports up to 8 devices. It is also recommended the you have 2 or more controllers for redundancy and performance. We also recommend to use HBA over PERC Controllers as we don't need the PERC capabilities and HBAs are way cheaper. You can probably buy 2 HBAs for the price of 1 PERC Controller.
Hope this helps