Practical RAID Decision Making
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@GUIn00b said in Practical RAID Decision Making:
Though both RAID levels can sustain 2 drive failures, the caveat with RAID-10 is as long as it's not the same member from each mirrored set. With RAID-6, ANY 2 drives could fail and still be operational and recoverable. I guess it would have to be a very specific concern to opt for the parity overhead in favor of the "added protection" over a statistically very rare potential failure scenario of 4-drive RAID-10.
Even then, statistically RAID 6 is much more dangerous. RAID 10 has a reliability rating so high that it never matters, RAID 6 does not. RAID 6 has a rebuild time hundreds of times longer than RAID 10; and it has 300% higher URE risks during a rebuild (that is chance of hitting one in a four drive scenario).
Remember the rule of thumb in determining RAID risk: always ignore the false security of "how many drives can you lose." That's not what matters. That's one of many factors, and almost never a significant one, in determining actual risk. URE risks are orders of magnitude more significant and factors like rebuild intensity and rebuild time make "chances to lose another disk" generally more significant than "how many disks can you stand to lose."
At four drives, I know of no scenario where RAID 6 is faster or more reliable than RAID 10. It's always worse. At 5+ drives it starts to have capacity advantages that once in a while make it a good choice. But the rule is at four drives, RAID 6 is a "never" because it's slower and riskier without any offsetting benefits.
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@GUIn00b said in Practical RAID Decision Making:
I guess it would have to be a very specific concern to opt for the parity overhead in favor of the "added protection" over a statistically very rare potential failure scenario of 4-drive RAID-10.
It's a specific failure scenario that even when it happens, there's no way to know if the same scenario would have been protected under RAID 6 because most scenarios where RAID 10 would fail, RAID 6 would also fail during its recovery mode (nearly 100%.) But the chances that it would face that recovery scenario are higher.
The complexity comes from choosing single unpredictable failure scenarios. After a failure has occurred, if we had the ability to pick how to have protected against it in the past, yes, RAID 6 would be chosen sometimes. There's a known example to explain why you can't use this in real life. It's the seatbelt problem.
Seatbelts save lives. On average, by far, wearing a seatbelt protects you. But there are special cases where the seatbelt can be what causes you to die. Yet statically, you never skip wearing a seatbelt because it is a one in a million chance that the seatbelt will cause a death rather than preventing one. And at the time that you choose to wear or not to sear wear your seatbelt you have no idea which type of accident you will have.
So we know that wearing the seatbelt is the safer bet. Seatbelts are like RAID 10. You can't know how things will go wrong, and in this scenario, RAID 10 protects you much more often than RAID 6 does.
