Parity Question

[Tom_808DE]

I'm sure this is documented or answered somewhere but I couldn't find it.  It seems that parity protection requires a hard drive failure with some kind of detected error, otherwise there's no way to repair the error.  So are drives today just that reliable that they never have an undetected error?

 

[trurl]

There doesn't have to be a drive failure, or even an I/O failure due to some other part of the system such as controller or cables.

 

Parity allows you to rebuild a disk from the parity calculation by reading parity plus all the other disks. Simple as that.

 

This allows you to, for example, remove a disk that has nothing wrong with it and has never had any error associated with it, and rebuild its contents onto another disk, perhaps for the purpose of replacing it with a larger disk so that you have more capacity.

 

[Tom_808DE]

Thanks for the reply.  You make an excellent point that there are more than just drive failure modes, I hadn't considered that.   Actually this is scary, a system controller failure could take out multiple drives.  But as I've read on this board, unraid isn't a substitute for backups, which is why I eventually want a second unraid system just for backups.

 

Still, a parity error says that a bit is wrong -- but parity doesn't tell you which bit or drive, there has to be some other mechanism to point to the failure.  Which was my question, I assume drives just never fail to write a bit correctly unless there is some kind of failure? 

[trurl]

4 minutes ago, Tom_808DE said:

excellent point that there are more than just drive failure modes

Bad connections are by far the most frequent cause of problems.

 

5 minutes ago, Tom_808DE said:

drives just never fail to write a bit correctly unless there is some kind of failure? 

There is a lot going on at the lower levels in different parts of the system that try to prevent bad data. But there is a lot of reliability throughout computer systems generally, not just with disks. If there weren't civilization would come crashing down.😉