Broken boot drive w/o backup - preventing data loss

[transmogrifiedtiger]

Hey all, the documentation pointed me this way so I'm hoping you all can help me understand my options.  I was a dummy and forgot to back up my boot drive, which is now a problem since I accidentally broke off the flash drive I was using when moving my Unraid server.  

 

My setup is only two identical 10 TB drives, and I don't have the config of the array
written down.  I've done some reading to try to understand how I can be sure to get this restored without risking data loss, and I have a couple questions.

 

1. From reading about how parity works, if I choose wrong when setting up my array again and map the two drives backwards, that would essentially wipe out my data, is that right?
2. If I put one of the drives into an external enclosure, would just reading that data damage anything in terms of restoring the array?  (I think no, but want to be cautious)
3. If using an enclosure doesn't have any risk, could I tell which disk was parity v. data in a 2 drive setup by looking at what files are in either of those drives?
4. Separately, if I can see the data drive's contents in an enclosure, is there any risk in copying the contents of that drive elsewhere, which would eliminate the risk of data loss if I mapped the array backwards in reconfiguring Unraid?

 

Apologies if these are answered elsewhere, I spent some quality time searching docs/forum/reddit since I first broke the flash drive Sunday but couldn't find a concrete answer.  Thanks!

[trurl]

The usual advice is to not install any disk to the parity slot and you won't have any risk of getting your data overwritten by parity.

 

In your case, if you only have a single parity disk and a single data disk then they are essentially mirrors of each other just because of the way parity works.

 

[transmogrifiedtiger]

34 minutes ago, trurl said:

The usual advice is to not install any disk to the parity slot and you won't have any risk of getting your data overwritten by parity.

 

In your case, if you only have a single parity disk and a single data disk then they are essentially mirrors of each other just because of the way parity works.

 

Appreciate the quick reply! 

 

It sounds like I might be misunderstanding Parity then, from doing some research I was led to believe that it wasn't a literal recreation of the data, although maybe that distinction is irrelevant in a 2 drive setup.   From your description, if I rebuild the array with one disk as the data drive, even if the drive I pick was previously the parity drive, that will work successfully?  Even if that wouldn't work exactly right, would that damage the ability to rebuild the array?  If it doesn't then using one drive like you're describing would be a faster option than my thought about incorporating an external enclosure to ID which was which.

[trurl]

Mounting a disk on another system will invalidate parity. And unless you are using linux, you won't be able to mount the disk anyway.

7 minutes ago, transmogrifiedtiger said:

I might be misunderstanding Parity

I don't know what your understanding of parity is, but parity is very common in computing and communications. It is just an extra bit that allows a missing bit to be calculated from parity and all the other bits.

 

Parity doesn't contain any of your data, but with a single data disk parity is identical to that data disk just because of the way the parity calculation works.

 

Here is the wiki on parity. If you work through the calculation for the case of a single data disk you will see what I mean.

 

https://wiki.unraid.net/UnRAID_6/Overview#Parity-Protected_Array

 

 

[transmogrifiedtiger]

29 minutes ago, trurl said:

 

I don't know what your understanding of parity is, but parity is very common in computing and communications. It is just an extra bit that allows a missing bit to be calculated from parity and all the other bits.

 

Parity doesn't contain any of your data, but with a single data disk parity is identical to that data disk just because of the way the parity calculation works.

 

Here is the wiki on parity. If you work through the calculation for the case of a single data disk you will see what I mean.

 

https://wiki.unraid.net/UnRAID_6/Overview#Parity-Protected_Array

 

I was mostly basing off SpaceInvader One's video talking about the array, which has an example just like that documentation.  I think I'm understanding you, but just to be super explicit since I don't have a fallback for getting this wrong:

You're saying if I reinstall Unraid with a single disk connected, it will work regardless of which disk I connect, since in the previous 2 disk array the data and parity disks are identical.  I can then add the other disk afterwards as a parity disk after confirming the data disk in the new installation is working properly.

 

Is that right?

 

[trurl]

3 minutes ago, transmogrifiedtiger said:

Is that right?

Yes

 

And in the more general case where you have more disks, you can always assign them all as data and none as parity. The only danger is accidentally assigning a data disk to the parity slot so it gets overwritten by parity.

 

1 hour ago, trurl said:

The usual advice is to not install any disk to the parity slot and you won't have any risk of getting your data overwritten by parity.

What we usually say is assign all disks as data and none as parity, and parity should be the only unmountable disk because it doesn't have a filesystem. That is how you can identify parity. Unless you have a corrupt filesystem on any data disks, then there might be more than just parity unmountable.