Re: Assemble RAID on new machine but with missing devices

[antlists <antlists@youngman.org.uk>]

On 27/11/2020 23:10, c.buhtz@posteo.jp wrote:
> Hello,
> 
> I red some stuff about assembling mode for mdadm. But two points are
> unclear for me.
> 
> 1. Missing devices
> I have two HDDs in a RAID1 - only data, no OS, on them. I want to
> physical remove one HDD and plug it into a new server machine. Then I
> want to bring the RAID1 online again on the new machine but with one
> existing and one missing drive.
> Background: When finished I will plug in a new fresh HDD as the second
> one to the new server.

It's fairly certain that the array will - initially - refuse to run. 
That's a safety feature - if an array is damaged while shut down, it 
won't come back without intervention.

Put the single disk in your new system. As I say, it's unlikely it'll 
start straight away. "cat /proc/mdstat" and if the array is there in a 
failed state, stop it. Then you can re-assemble it and it should come up 
degraded. All you have to do after that is replace the non-existent 
drive with your new replacement, and you'll have a functioning array.
> 
> 2. mdadm.conf
> On the web I read sometimes about modifying mdadm.conf on the new
> machine. But I do not understand why. If so. Why and what do I have to
> modify in the mdadm.conf?
> 
If you've even got one ...

I don't know the history of it, but the early arrays did not have 
superblocks, so mdadm.conf - an ACCURATE - mdadm.conf was essential. Now 
they've got superblocks, it's optional and - to the best of my knowledge 
- none of my systems have mdadm.conf's.

Do you have a superblock? Preferably v1 (1.0, 1.1 or 1.2). If you do, 
don't worry about it, it's old advice, and while it's good to have an 
up-to-date mdadm.conf to tell *you* what's what, the system doesn't need it.

Cheers,
Wol