Is the server limited to 6 internal disks or does it have additional storage?
Why would you need a different RAID for virtualization than other applications?
Sorry for the late reply, but I have finally got some extra information from my teacher.
So I need 2 raids: 1 for performance (virtualisation, big databases, ..) and 1 for the protection of data (raid 5 was good according to my teacher).
Both raids need to be tolerant for failures (backup, ..).
For your environment (presuming that the server's RAID controller includes BBU or NVC cache memory) I'd implement a RAID 1 (requires 2 identical hard disks) and a RAID 5 (requires 4 identical hard disks).