Looking for a good documentation on how to recover a raid

Hello,

I am new to this list and I would like to know if does an official FAQ 
and/or documentation exist on how to recover a broken raid?

I am in the situation that we have a broken WD EX4 NAS. There was one 
disk failure and after replacing the disk the system went into the 
rebuild mode. So far so good.

But Murphy's baseball bat hit us right in the face (yes, a raid does not 
replace a good backup, I know) and during the rebuild a second disk had 
reported a failure. However, the second disk seems to be fine as it is 
not that old and as it was replaced roughly half a year ago. SMART data 
for that disk is fine.

However, the first disk reported a failure is still readable and I was 
able to clone it using dd. So our hope is that we could bring the raid 
back to life again. As a test I connected all 4 drives (including the 
cloned disk) to a linux box and run RR-Studio over the set of disks 
(read only mode). I am able to extract some data.

Now I want to learn the necessary steps to reassamble the raid correctly 
on command line and get the rebuild mode completed. Unfortunately I only 
have very basic knowledge of RAID technology. There are some guides in 
the net but I would put more trust in a guide created by this community.

Best regards
Michael

On 14/05/2021 09:43, Michael Marré wrote:

Hello,

I am new to this list and I would like to know if does an official FAQ 
and/or documentation exist on how to recover a broken raid?

https://raid.wiki.kernel.org/index.php/Linux_Raid

This is as close to official as you'll get. Failures tend to follow a 
pattern, and I do try and document any variation I see, but it does tend 
to be a case of "either it's easy or it turns into a big recovery job".

Look especially at
https://raid.wiki.kernel.org/index.php/Linux_Raid#When_Things_Go_Wrogn
and if it's not an easy fix (it might be as simple as a forced-assembly 
followed by a fsck), post the requested info to the list and somebody 
should dive in and help.

Cheers,
Wol