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by bensyverson 20 days ago
> This is a common claim, but honestly, citation needed.

This is based on the data I had available 10 years ago when I set up this array… If you have data that points to drives having fewer read errors per GB in recent years, I'd love to see that citation.

> And how many times did you have a read error on one of your other disks during a resilver?

I don't know, because I have RAIDZ2, so that read error would probably be corrected without it being reported to me.
1 comments
Dylan16807 19 days ago
> This is based on the data I had available 10 years ago when I set up this array

Data you collected or data you found? Any idea where it was?

I've only seen oversimplified math, not real data, and different versions of the math make different assumptions.

> I don't know, because I have RAIDZ2, so that read error would probably be corrected without it being reported to me.

Zpool status gives you a per-disk error count. I kind of assumed someone doing a resilver would use that command.

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bensyverson 19 days ago
> Data you collected or data you found? Any idea where it was?

I think it was a lengthy blog post citing manufacturer data, found via the FreeNAS forums. The added safety of RAIDZ2 was a good tradeoff. Someone else on this thread reports they lost two SSDs in rapid succession, which I hear happens sometimes with drives from the same batch. In any event, I felt no need to launch an independent investigation of the math and underlying assumptions.

> Zpool status gives you a per-disk error count. I kind of assumed someone doing a resilver would use that command.

Yes, I did indeed use zpool to do the resilver… I don't recall if there were any read errors on other drives at the time of those failures, but probably not, as the scrub probably would have flagged it. They've all been replaced with higher capacity drives, so at the moment I see "No known data errors"

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Dylan16807 19 days ago
Okay, interesting. Also to be clear I do think Z2 makes sense in a lot of situations, but I suspect that it's not very dependant on drive size like a lot of people think. If I wanted high reliability I'd use Z2 even with small drives.

Also to reply to your other comment: Personally one extra drive is a small price to pay for the peace of mind.

I achieve peace of mind via backups. For each copy, once I get the AFR below 2% I'm not too fussed about pushing it lower.

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