I've not used Starwind VSA but the point of a VSA is that it handles the disk RAID - you just throw it the RAW disk, the RAW'er the better
(there were some issues on VMware VSAN if you used RAID-0 on some RAID cards so a dumb HBA was a better choice)
I assume the Linux VM is just the admin piece but completely guessing here...
Perhaps with SOME VSA's, but not the ones I've used. You would typically present it out with VMDK's and it would present that out as storage. You may have used ones that work the way you describe - I know they exist, so I'm not debating that. But most I've done, haven't been.
I am not sure but isn't the point of Starwind VSAN is to raid the disks between servers, not within single server?
that is how I am using it at work.
I do not think it can raid the disks within single server.
I don't know that I would think of, nor refer to it, as "RAID between servers". Starwind VSAN gets you a virtual SAN - NFS, ISCSI, SMB3, etc. You share out your volume to the hosts. You can additionally make it HA by creating a replication partner, so that if one node fails, you're still up. In some ways, that may be similar to a RAID1 mirror between hosts. But you can have a synchronous or an asynchronous lagged node somewhere else as well.
The VSA is just a Linux version of the Windows software. The VSA is the management/OS disk. You add disks (VMDK/VHDX/RAW) to the VM as needed. You then configure the disk inside of the OS to be shared out to ISCSI or other initiators. You configure that disk, with some SSD cache - by way of adding a VMDK disk that resides on a local SSD disk, or some RAM cache. That SSD could be standalone, could be PCIe, could be mirrored, could be a single disk RAID0, whatever. If it's like the Windows one, it only needs one disk/volume/partition, and it places a cache file of 4-8-32GB whatever you configured, on it, and attaches/combines it with the main "HDD" disk.
So you'd spin up the VSA, give it a 2TB VMDK, and a 128GB SSD based VMDK, and 24GB RAM. You could then make two LUN's with it of 1TB each - one might have 8GB SSD cache and 4GB RAM, the other might have 96GB SSD and 12GB RAM. But it would appear as 2x 1TB LUN's to anything that's accessing it.