Dear Ms. Meade,
I am charged with architecting a backup system without any single points of failure. Obviously, tape is SO failure-prone that I am not including it at all. How do you think I should configure such a system?
Tape is Doomed
You are doomed if you rely solely on disk for your data backup. A possible interpretation of your question may be “How much disk does it take to replace tape?” The answer is “unobtainium”—that is, you can’t replace tape using disk.
Further, the very concept of single point of failure is terribly funny in a terribly dark way. Failure is inevitable, unless you plan to address human imperfection? What about acts of natural and man-made disaster that may affect the national power grid? Switch problems? What about loose screws, including any screwed-up (or self-perceived screwed over) employee?
Instead, consider asking a question that does have an answer—“How can I reliably protect data?” The answer is “disk and tape.”
Ms. Meade is a major fan of disk with RAID 6, offered in Spectra’s nTier disk. With RAID 6, up to three disks can fail without affecting data integrity. Go disk and go RAID. However, disk (even with RAID 6) can’t be considered failure-proof because it has its own Achilles’ heel (aka single point of failure): the RAID controller. You can have all the data you want on all the spinning disk you want—but if the controller fails, the brains are gone, and the bits and bytes you’ve carefully protected are toast. Whither goest the RAID controller, so goeth the data. Dead controller= permanently decomposed data. So disk alone, even with the marvels of RAID, is not enough to provide true disaster recovery and continuity of operations.
Further, please note that your information about tape as failure-prone is completely wrong. Tape is, it turns out, incredibly reliable. With tape’s reliability increase of 700% over the last decade, multiple layers of ECC protection, and smart Spectra libraries tracking media and drive health, tape meets and beats disk in terms of reliability. If you’re worried about a single point of failure, make sure you get two tape drives. Consider the T950 and TFinity libraries’ global spare feature—which is an installed drive that can be directed to take over in case of a drive failure.
Ms. Meade admits that she is curious about the pointy-haired boss who directed you to create the no single point of failure unobtanium backup environment….