Re: RAID5 devices assemble into RAID0 array

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

On 26/11/17 17:29, Duane wrote:
> You're right I failed and removed device 3, then failed and removed
> device 2, which broke my array.
> 
> Let's assume I haven't messed things up any more than I already have.
> 
> Is there a method to reverse the above operations for device 2 and
> device 3?
> 
> Thanks, Duane
> 
My raid-fu is mostly theoretical, but what I would try is as follows.

It's assembled the broken array as md0, so

mdadm /dev/md0 --re-add /dev/device2

followed by

mdadm /dev/md0 --re-add /dev/device3

That won't cause any further damage, and may work fine.

Beyond that, I wouldn't like to suggest anything - if it doesn't work
you'll probably need to use things like --force, which could easily
break stuff. But re-add will probably work. And my syntax might not work
- you might need to specify the mode like --grow or --manage, I don't know.

Suck it and see, at least this won't do any damage, and if it doesn't
work we'll have to wait for further advice.

Cheers,
Wol
> 
> 
> On 2017-11-26 05:04 AM, Wols Lists wrote:
>> On 26/11/17 01:50, Duane wrote:
>>> I have 3 RAID5 devices. When I assemble them, I end up with a RAID0
>>> device.
>>>
>>> What is the cause? What is the solution?
>>>
>>> All I can think of is the fact that there is only 1 active device. I had
>>> 2 but then manually failed it. I want to reassemble a RAID5 array and
>>> then re-add the second device.
>>>
>> OUCH!
>>
>> Sorry. You have a 3-device raid-5. You only have 1 working device. Your
>> array is well broken.
>>
>> You can't "reassemble raid5 then readd the second device". You need to
>> readd the second device in order to get your raid5 back. I'll let
>> someone else tell you how, but you need a MINIMUM of two devices to get
>> your raid working again. Then you need to get your third device added
>> back otherwise your raid 5 is broken.
>>
>> DON'T DO ANYTHING WITHOUT ADVICE. I'm sorry, but your message says you
>> don't understand how raid works, and you are on the verge of destroying
>> your array irrevocably. It should be a simple recovery, *provided* you
>> don't make any mistakes.
>>
>> Cheers,
>> Wol