Wondering what ever happened to RAID-2, RAID-3, and RAID-4? You can look in history books for the details, but they were to be hybrids of mirroring and striping. Ways to include a parity with the data, for protection, but still staying away from mirroring each disks in a normal "one-to-one" mirror. One RAID type would have problems, so they would build another. RAID5, if you hadn't guessed, was the agreed upon solution. RAID-2 and RAID-3 died and burned and scattered into the sea of absolution. However, RAID-4 found a home with our friends at NetApp (www.netapp.com). See, RAID-4 is faster than RAID-5 because the parity is written to a dedicated disk, rather than scattered around with the data. If you had 4 disks, you'd have 3 data disks, and 1 parity disk. But what happens if you loose your parity disk? You've lost all protection. There is no parity, and therefore no way to calculate the data you lost, meaning that if any of the actual data disks die, your hosed. For this reason RAID4 is considered dangerous and isn't used. However, NetApp found a way to make this work and is used in their WAFL filesystem for their product line of NetApp Filer's (tm), and NetCache's (tm). This is one of the reasons NetApp's are as fast as they are, RAID-4 is the technology behind it.
(It's unfortunate, but NetApp Filer's are the worlds fastest NFS servers. But one day Sun is going to kick their ....... never mind.)