DevExpress Newsletter 8: Message from the CTO

12 August 2009

My Message from the CTO in our eighth newsletter:

Back in 2007 Google released a study on the lifetimes of hard disk drives. They are obviously in a pretty unique position to do so since they run huge server farms that run the majority of the world's searches, emails, and other cloud-like services.

Although they didn't single out any particular manufacturer or brand as being better or worse than the norm, they did say that a new disk drive has about a 2% chance of failing in a year. Drives that are over 2 or 3 years old the rate is significantly higher, over 8%.

Let's take that lower probability, and run with it.

So, the disk in your newish laptop has a one in 50 chance of dying this year. Doesn't sound too bad, surely? That's like shuffling a pack of cards thoroughly, placing the pack on the table, cutting, turning over the top card and it being the ace of spades.

Taking it further: in our house we have several PCs, containing in all 8 newish disk drives spinning all the time. The probability that at least one of those 8 dying this year according to Google's probability is 15%.

One in 7. That's in between tossing three heads in a row with a coin or throwing a six on a single die. Suddenly it doesn't seem that remote any more. (If they were older, that becomes 1 chance in 2 that at least one drive would fail.)

So, when was the last time you backed up? Given that, like me, you're the free tech gopher for all your relatives' and friends' machines, when did they back up?

OK, so a message only tangentially related to development this time, but, as you might have guessed, it came up for me recently in the "tech gopher" sense. Ever since my wife lost a lot of data because of a crashed hard disk and I discovered the latest backup was a couple of months old, I've been fairly obsessive about backups (you would be too, if you'd heard the words in the Bucknall household that day). For friends and relatives, I've been trying to coerce them to using something like JungleDisk or Mozy so at least their documents would survive.

(By the way, for those who wonder how to calculate these probabilities, here's the math. If a given disk drive has a probability of 0.02 of failing this year, it has a probability of 0.98 of staying alive. The probability that all 8 drives in our house staying alive this year is 0.98^8 or about 0.85. So, the probability of at least one failing is 0.15 or 15%. For the 0.08 probability of dying per drive, it's a 0.92 probability that it would survive, so for 8 drives that's 0.92^8 or 0.51 for them all staying alive, or 49% that one will die this year. A coin toss. (I'll note that Google's paper gives 0.017 as the probability of a drive failing in the first year; I just rounded up to make the point.))

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