Is there a strong reason why I would want to use Raid 5/6 over Raid 10 anyway though? I guess Raid 10 is a bit more expensive, but my (naive) understanding was that everything else was almost pure upside.
RAID6 is safer: any two drives can fail. RAID 10 can sustain more drive failures, but can tolerate only 'certain' drives to fail. And the drive that really should not fail is taxed during a rebuild.
Do you need the performance or capacity? What are your needs? How many drives can your chassis hold. How much space do you need?
And so on. Maybe RAID10 is the best option for you but it's not a straight rule-of-thumb.
> And the drive that really should not fail is taxed during a rebuild.
That is also true for RAID 5/6 (since all drives are stressed during a resilver of a RAID 5/6).
> RAID6 is safer: any two drives can fail. RAID 10 can sustain more drive failures, but can tolerate only 'certain' drives to fail
You should consider two-way mirrors as having the same redundancy as RAID5, but with faster resilver times (a byte-for-byte copy is faster than parity calculations -- and you should want your pool to be in a degraded state for as short a time as possible). You might survive more disk failures, but you shouldn't count on it.
If you want RAID6-like redundancy, use 3-way mirrors. But that's obviously more expensive than RAID6 (even though I would strongly argue that 3-way mirrors are far safer than RAID6).
To rebuild RAID10, only one disk needs to be read completely; to rebuild RAID6, n-1 disks.
For RAID10 to fail, two disks need to fail, and they need to be a mirrored pair, so it's a conditional probability. It's possible to lose up to n/2 disks and for the array to stay up.
For RAID6 to fail, three disks need to fail, but once three disks fail, that's it, you're out.
This all means that whether RAID6 is better than RAID10 is dependent on the number of disks and the actual failure rate. The more disks you have in your array, the more likely RAID10 is to be safer than RAID6.
RAID10 is much, much safer than RAID5. They are not similar in reliability.
Most of the time, RAID6 is safer than RAID10, but RAID10 gets you a lot better performance.
I didn't say that RAID5 and RAID10 are similar in terms of reliability (most of my comment said that RAID10 had many upsides over RAID5 in terms of reliability). I said they you should consider them to have the same level of redundancy -- unless you like to play Russian roulette with your data. Yeah, if you have more drives there are less bullets in the revolver but I'd prefer to not play that game in the first place. If you need a system that can survive 2 independent disk failures, use 3-way mirrors.
