Skarecrow77
100+ Head-Fier
- Joined
- Nov 9, 2010
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Quote:
The chances of you losing more than one drive at a time are 1 in a number I can't even fathom. It's not a back up really, but it is a very safe method of storage.
Actually, the chances are a lot higher than you think.
The MTBF rating of a drive is not determined by taking a couple drives, running them until they fail, and then taking the average of how long it took for them to fail. No, they'll take an absurd number of drives, run them for a short period of time (relativly speaking), and however many failed in that batch is factored against how many total "drive hours" were run for the entire batch, and the MTBF is calculated for that.
What does that mean? It means that drive failures are not random and you can't take the MTBF as an estimate of how long your batch of drives will last. Stuff wears out. Especially stuff under heavy use. Drives from the same batch VERY often die en mass in a short period of time. Since most people don't build raid arrays with drives they purchased months apart, but instead buy 3-5 drive simultaniously and then build from that all at once, it means that once one drive goes, you've got a fairly short period of time (months if you're lucky, weeks most of the time, days or even hours if you're unlucky) before another drive from the same batch fails. If you buy 4 tires at the same time, and drive until one of them bursts from normal use (as opposed to a manufacturing defect), what are the chances the other 3 are in good shape? very slim.
To give you an example. I've got a server in the server room with 16 drives in it in multiple RAID arrays (Three raid 6s and a raid 5). I've had two fail in the past 2 months. It's not uncommon at all, and is actually quite a well known problem.
RAID arrays buy you time before complete data loss, but just time. Take it from me. Once that first drive goes, don't just replace it, get a spare. In fact, if you can swing it, get a spare -now- so it's not from the same batch as the next spare you buy.