RAID – Which RAID Level to Run for Virtual Machine Storage

This is a terrible idea...

It sounds as though you're going to be exporting a single drive over iSCSI to your virt host. That SSD does not sound as though it is part of a RAID group.

The real solutions here are:

• Mirror your SSD... That will reduce the impact of a drive failure. (You could place it/them inside of your virtualization host - why bother with a NAS?)
• Choose a more appropriate backup solution. If that means configuring each guest, that my be the way to go.
• Do you have another server/NAS or additional hardware you could backup up to or replicate to?

For the NAS side, there are some nicer options out there. Something like the Community Edition of NexentaStor at least allows replication of NFS shares to another host/destination... but it definitely provides an automated snapshotting service that could be useful in your case. Snapshots aren't backups, but they're one form of protection that could come in handy.

To answer your question directly, the thing that can best improve your system's tolerance to hard drive failure is to use your extra hard drives to hot spare parts. With such a low disk count, the chances of having two simultaneous drive failure is low enough that you can rely on backup. Having a hot spare, however, will allow the system to recover much more quickly (and safely) from a single drive failure. You'll need two spares, though: one of each size.

I have to add, however, that it sounds like your heading in the wrong direction here. First, you need to understand that RAID is not a backup system. There are many things that can go wrong with your RAID that will cause total data lose: manipulation error, controller failure, viruses, fire/water on the server itself. You need to use a real backup system.

Second, the best way to increase redundancy for an AD is not to improve the disk subsystem or have better backup but to add a second domain controller, if possible at a different physical location. It will improve uptime much more than anything you will do for a single server setup.