November 29, 20214 yr I'm considering using unRAID for my home NAS, but reading about how the parity drives, I'm concerned that adding older drives that I don't quite trust would lead to a greater risk of data loss. For example if I have: - New NAS-grade parity drive - HDD - HDD I could recover by losing any 1 of the drives, however, if I want to add an older 1TB drive I have laying around that I think might be less reliable than the other drives, I risk losing data of my other more reliable drives. So I guess my question(s) about this is: - Is my understanding of how parity/recovery with unRAID correct? - Would this be any different with other options such as running ZFS? - How would adding a 2nd parity drive change this? Is it literally just storing the same parity info, or do they get different cross-sections of drives so 2 could be lost? Edited November 29, 20214 yr by 7thFox
November 29, 20214 yr The 2 parity drives are different algorithms and are independent, so 2 drives can be rebuilt with dual parity. 7 minutes ago, 7thFox said: an older 1TB drive I have laying around In my opinion, it doesn't have enough capacity to bother with. You should at least run extended SMART test on old disks before trusting them in your array.
November 29, 20214 yr Author Do you have anything you can link to explaining the 2-drive parity? Also still want to know if I'm right in understanding the risk with adding drives. It's by no means a deal breaker (ZFS demanding same-size drives is enough to drive me off), but it would obviously change how I configure it hardware-wise. Originally I was thinking "well, if I lose my crap drive, I can always recover it", but if that puts my actually decent drives at risk then it's 100% not worth it.
November 29, 20214 yr Parity has none of your data. All bits of parity PLUS all bits of all other disks must be reliably read to reliably rebuild a disabled disk. Dual parity just makes it possible to rebuild 2 disks, or rebuild one disk with another disk still disabled. 1 hour ago, 7thFox said: puts my actually decent drives at risk An unreliable disk in the array might not reliably rebuild another disk.
November 29, 20214 yr A good explanation of how parity and dual parity works in Unraid is given here by SpaceinvaderOne. https://www.youtube.com/watch?v=HybwCOVDg9k&t=1s * Start at the 9:15 time mark for the parity/dual parity explanation. Edited November 29, 20214 yr by peterg23
November 29, 20214 yr Parity, as always, is just an extra bit that allows a missing bit to be calculated from all the other bits. This wiki explains single parity: https://wiki.unraid.net/Manual/Overview#Parity-Protected_Array Parity2 is somewhat more complicated, but it is still just an extra bit that allows a missing bit to be calculated from all the other bits: https://wiki.unraid.net/Parity#Dual_parity https://mirrors.edge.kernel.org/pub/linux/kernel/people/hpa/raid6.pdf
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