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As someone who runs 3 large arrays with 8TB, 16TB, and 21TB drives respectively, know that:
Less disks is simpler, but more disks is safer. 6 disks is the perfect sized array IMO. If you don't need more space, I'd buy a 2TB hot spare and call it a day. But if space is a concern, Z2 with 4 disks.
Edit: Those three arrays mirror each other in different locations, and the fear was still there when the Z1 had an issue. Mostly due to the headache, but still.
The reason I went RAIDZ2 in my current setup was because of the number of disks increasing the chance of multi failures. But with fewer disks that goes down. I'm not at all worried about data loss, as I said I have good backups so I can always restore. So if the remaining disk dies during a rebuild, that's unfortunate, but it only affects my uptime, not my data.
Hate to be that guy, but those maths aren't mathing.
Less drives does not equal less chance of multiple failures. The statistical failure rate of one drive has no impact on another. In fact, analysis of Backblaze's data showed that larger drives were more prone to failure (platter density vs platter count).
Who has more chance of a single disk failing today: me with 6 disks, or Backblaze with their 300,000 drives?
Same thing works with 6 vs 2.
Backblaze of course, but we aren't talking about the probability of seeing a failure, but of one of your disks failing, and more importantly, data loss. A binomial probability distribution is a simplified way to see the scenario.
Let's pretend all disks have a failure rate of 2% in year one.
If you have 2 disks, your probability of each disk failing is 2%. The first disk in that array is 2%, and the second is 2%. If 2 disks fail in Z1, you lose data. This isn't a 1% (half) chance, because the failure rate of one disk does not impact the other, however the risk is less than 2%.
So we use a binomial probability distribution to get more accurate, which would be .02 prob in year one with 2 trials, and 2 failures making a cumulative probability of .0004 for data loss.
If you have 6 disks, your probability of each disk failing is also 2%. The first disk in that array is 2%, the second is 2%, so on and so forth. With 6 disk Z2, three must fail to lose data, reducing your risk further (not to .08%, but lower than Z1).
So with a binomial probability distribution, this would be .02 prob with 6 trials, and 3 failures making a cumulative probability of .00015 for data loss.
Thats a significantly smaller risk. The other interesting part is the difference in probability of one disk failing in a 6 disk array than a 2 disk array is not 3x, but is actually barely any difference at all, because the 2% failure rate is independent. And this doesn't even take into account large disks have a greater failure rate to start.
I'm not saying mirroring two larger disks is a bad idea, just that there are tradeoffs and the risk is much greater.