Data Rebuild?

[lowbiker]

This question is academic.

1. I lost a 6TB drive. It's just dead so I pull it out and it starts being emulated.

2. That feels weird and I have space so I run the Unbalancer to move the data to other free drives

3. I did this a couple times to make sure i got it all.

4. New drive comes in. Also 6 TB. Run the pre-clear. Takes forever

5. I shut down the array. Add the new drive back to the array.

6. Now it's doing a "data rebuild" with over 400,000,000 writes 30% done. 

 

What is it doing? I thought I moved the data off the emulation? What exactly is it rebuilding? What should I have done different?

Edited October 30, 2022 by lowbiker
Typo

[JorgeB]

Parity works with bits only, if doesn't know or care if the disks are empty or not.

 

https://wiki.unraid.net/Parity#How_parity_works

[itimpi]

17 hours ago, lowbiker said:

What is it doing? I thought I moved the data off the emulation? What exactly is it rebuilding? What should I have done different?

Moving the data off the drive was unnecessary (except perhaps as a safety measure).    When you do a rebuild of the drive the contents end up being the same as was showing on the emulated drive.  
 

The pre-clear was also technically unnecessary (except as a stress test of the replacement drive) as the rebuild process overwrites every sector on the drive so previous contents are irrelevant.

[trurl]

3 hours ago, itimpi said:

Moving the data off the drive was unnecessary (except perhaps as a safety measure)

If you do feel the need to save the data before rebuilding, perhaps because you don't have a backup, better to copy the emulated data off the array somewhere. Copying off the array would only require reading all array disks to get the emulated data. Moving to other disks in the array requires those same reads, but also requires writing to other disks in the array when the array is already unprotected, and it also requires writing to the emulated disk (updating parity) to remove the files from the emulated disk.