In Server 2012, Storage Spaces has been added. It allows you to pool multiple physical disks together (regardless of disk size and connection type) as one big logical disk. For those people familiar with Windows Home Server (WHS), the concept is similar to Drive Extender. For more information read the Storage Spaces FAQ or this blog post.
After the Storage Pool has been created, you can create one or more Storage Spaces (virtual disks) and configure them as either a simple volume, mirror volume or parity (software raid).
Even though this seems great in theory, in reality the write performance on a parity volume is painfully slow at about 25-30 MB/s. Besides the performance, there are also other aspects that you might not appreciate. So before implementing this in a production environment, test it extensively in a testing environment. You can also play around in Microsoft’s virtual labs.
It’s a pity Storage Spaces didn’t meet my expecations, because I was really looking forward to replace my Windows Home Server with Drive Extender. But for now I would recommend sticking with hardware RAID solutions.