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by bell-cot·1mo ago·view on hn ↗
>> The odds of this happening go way up the larger your individual drives are.

> This is a common claim, but honestly, citation needed.

FWIW, here's the quick & dirty argument for that:

Assume:

- We're working within a given generation of disk technology, with comparable drive and interface tech.

- The drives' I/O speed is relatively constant, not scaling with the drive capacity.

- The chance of any given bit (of data on a disk) going bad, in a given time interval, is relatively constant.

- The drives, regardless of size, will be some fixed-ish percentage full. (If nothing else, the boss won't sign off on overspending for unneeded capacity.)

From there - the time needed for resilvering scales linearly with the drive size. As does the number of non-redundant bits that could go bad in any given second of the resilvering. Linear time linear - so the chance of data loss during resilvering rises with the square of the disk size.

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> - The chance of any given bit (of data on a disk) going bad, in a given time interval, is relatively constant.

We can't just assume this, though. Especially for data that's being checked regularly, because the drive can see how many pre-ECC bits have gone bad and rewrite iffy sectors.

And even if it's true, it's normal to go years without errors. If you have a 15% chance of getting an error each year, and your degraded array has 4 drives without redundancy, then the chance of getting an error in 4 hours is .02% and your chance of getting an error in 48 hours is .3%. Having that risk show up once every few years is pretty negligible compared to everything else that could go wrong with your array.

On top of that, if you don't have a hot spare and it takes 5 days to get a new drive, the difference in resilvering speeds is now 5.2 days versus 7 days and that barely matters.

The main situation where you have to worry about extra drives failing is when you think the resilvering process will greatly accelerate drive deaths. And that depends very much on the specific numbers you have in mind.