Thanks, @JorgeB. for your reply. I would love to understand more about how parity works.
As I read some old threads in Unraid and as you clarified already, Parity can not help anything in bit-rot, bad sector situations and parity just simply helps recover data of one malfunction data disk (suppose only 1 parity disk) while other disks in an array MUST be healthy to participate in recovery.
So does this feature also create a consequence that when the data disk encounters data corrupted, the parity will also sync the wrong corrupted bit string and replace the good bit string of the previous parity-sync ? Is there any solution within Unraid array to protect array against bit-rot, bad-sector except off-side backup ? oh wait, even if I run off-side backup on Unraid OS, there still exists corruption potential as well.
If this is the case, Unraid eventually exists a drawback against Raid due to the fact that Unraid has no redundancy and can not protect data from corruption, while Raid does by discovering corruption by BTRFS scrub job and repair corruption by comparing bit strings among raid disks.
Please help me to clarify this. Much appreciated.