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a very good question about data backup

(Second entry in less that twenty-four hours! woohoo!)

In a comment to one of recent entries, Elgan asked a very good question:

You are the person to ask about this dilemma we are having at my house. My husband and I have decided that it would be a very good idea to back up our data in some fashion. He wants to buy an external hard drive to store it on. Another computer-nerd friend of mine says not to do that, as external hard drives have moving parts and eventually wear out. He says to back up the data on DVDs and keep them in a safe place away from temperature extremes and sunlight, and they should last for a very long time. What do you say?

Curiously enough, this is a question that people in the computer industry have been asking themselves for the last ten years or so. I recall attending a storage conference shortly before I finished my Ph.D in 2000 where this very question was discussed. An IBM researcher got up to discuss the latest advances in tape storage technology and a colleague of mine got up to ask "why use tape? why not just use a few disk drives?" during the Q&A that followed. That is essentially the same question that is now being asked here.

The really short answer is that it doesn't matter. The most important thing is that you need to have more than one copy of your data stored on two different kinds or pieces of media that can be physically separated. So that way, if a meteorite strikes one corner of your office, you can have a copy of that symphony, sermon, ground-breaking graphic novel draft, or soon-to-be earth-shaking dissertation draft in another corner, ready to go. With computer hard drives sized in the gigabytes these days (and computers being sold with little in the way of re-installation media), data backups are tremendously important no matter what method is used.

The more long-winded answer is that each method has its pluses and minuses and you need to do what makes the most sense for you. Let me lay out my thoughts on each type of media. You can decide what you want to do.

The biggest advantage of CD and DVD media is that they are potentially quite stable (like the aforementioned tape media above). I haven't looked at the estimates of how long they last recently, but you're very probably safe for at least 3-5 years if you keep them in a dark, cool, dry place. Why not longer? Unlike the mass-manufactured CDs and DVDs you buy for entertainment purposes, the ones you burn at home are thin sheets of metal, covered with light sensitive dye, and sandwiched between two pieces of plastic. That dye is pretty crisp when you first burn the disc, but it can potentially spread out, and no one is entirely sure how long it's going to take before the discs become unreadable.

The other disadvantage of CDs and DVDs is size. As recently as the late 1990's, hard drive size and CD/DVD media size enjoyed relative parity. I remember getting a 300MB hard drive in my computer in the mid-90's and that is half the amount of data that can be held on a single CD. Even as recently as 2002, you could fit most PC hard drives onto a few (less than 5) burned DVDs. I upgraded the hard drive in the machine that melted down over the weekend with a 160GB SATA hard drive that was only about $70. That is potentially what... 30-40 DVDs worth of information? That's a lot of discs to label and keep track of.

The big disadvantage of hard drives is, as Elgan's geek friend pointed out, that hard drives have moving parts. Moving parts can fail. Disk densities are also so huge now (lots of information on little bitty platters means the data stored is stored as tiny tiny dots) that manufacturing defects can cause data corruption the first time you try to lay down data on a part of the disk. So hard disks would seem to be the less stable medium for long term backups. Except that isn't quite true. Modern external hard drives can be turned off, and left off most of the time. Parts don't wear when they're not moving.

The biggest advantages of hard drive storage are size and flexibility. You can copy all the contents of one hard drive onto another hard drive. If you make a sufficiently faithful copy of your whole system, you can even start your system from the copy. You can also re-write data on the hard drive to your heart's content -- something you can't always do with disc media.

Me, I think that hard drives have the backup edge at the moment. That's what Mrs. Geek and I use for whole-system backups at home. Drives are relatively cheap and backups are fairly easy to do with the right software. Whole-system backups are what we do... just because the added security of knowing that our applications as well as our data are all safe.

So Elgan, I think the answer to your question revolves around what you want to do. If you just want to save a few documents or data files, make a few CDs or DVDs (more than one copy of all your data, please). If you are interested in doing a backup of your whole system (data and applications), then an external hard drive is the way to go.

And if you do want to do a backup of an OS X system, please get an external drive with a Firewire interface (not USB 2.0 -- OS X likes Firewire much better, and Firewire is often faster than USB 2.0). Try to make sure that the hard drive is by a decent manufacturer, if you can. I generally prefer drives from Seagate or Hitachi. I have a well known bias against drives by Western Digital. I can recommend a program called SuperDuper. It's fairly cheap and seems to do the job.

said drgeek on 2007-09-13 at 10:35 a.m.


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