Lets say I lose two drives, the party will replace the first drive but not the second, is there a way to tell what data was lost on the second drive? Is this something we should document?

Lets say I lose two drives, the party will replace the first drive but not the second, is there a way to tell what data was lost on the second drive? Is this something we should document?

If you lose two drives simultaneously, you will lose the contents of both. The parity drive has no sensible information on its own, it requires all the other drives to reconstruct the single failure.

Lets say I lose two drives, the party will replace the first drive but not the second, is there a way to tell what data was lost on the second drive? Is this something we should document?

If you lose one drive then it can be recovered.  The recovery process requires all other drives plus the parity drive to be good for it to be successful.  If you lose two drives then the contents of both drives are lost.

 

When dual parity is available and you use it then you can lose two drives without data loss, but a third drive would lead to all three being lost.

 

This is one reason why you should always have backups and never rely on unRAID as the only copy of important data. 

 

There is no way to know exactly what is on each disk unless you either have user shares set up in such a way that you know, or you take periodic listings of what is on each disk.

 

In practise it is often not as bad as mentioned above even with multiple simultaneous disk failures as the way that unRAID works means each drive is a discrete file system, so unless a disk physically fails (which is relatively rare) data can still be recovered from it.

If you think about it a little bit, it is obvious that the parity drive by itself cannot have the capacity to contain all of the data for a single failed disk. How would the system know which disk would fail?

 

The reason the people who responded on this thread know the answers they gave is not from personal experience, but because they understand how parity works. See this wiki. It really isn't very complicated, and if you get it then many things about unRAID will make a lot more sense and it may even keep you from making mistakes that could cost your data.

Thanks, I do understand how parity works, my question was more about those unusual instances where lose a drive that is unrecoverable due to stupidity or some other anomaly whereby it cannot be recovered the normal way using parity data, I should of been more specific in my question, apologies.

 

My UnRaid server is my backup server, so its not my primary data repository.

 

Thanks.