Q: In a RAID-10 configuration with four drives, can you recover data if two drives fail? Can you configure a RAID-1 system with three drives to tolerate a double drive failure?

A: With RAID-10, two drives can fail without data loss or loss of availability, depending on how it's configured; for example, if one drive from each mirror fails. But if both drives in a mirror set were lost, the RAID-10 volume would be compromised. If you want to avoid double drive failure, look at a six-drive scenario (three drives per mirror) or a hybrid model, such as a vertical and horizontal stripe across controllers using multiple drives.

With RAID-1, it's possible to have more than two drives in a mirror set, depending on whose RAID hardware system, controller, adapter or software you're using and what they support.

--Greg Schulz, founder and senior analyst, The StorageIO Group