From mboxrd@z Thu Jan  1 00:00:00 1970
From: Veljko <veljko3@gmail.com>
Subject: Re: Linear device of two arrays
Date: Tue, 18 Jul 2017 10:58:01 +0200
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On 07/17/2017 12:16 PM, Veljko wrote:
> On 07/15/2017 02:12 AM, NeilBrown wrote:
>
>> So command should be
>>
>>>>   dd if=/dev/md2 of=SOMEWHERE/SAFE bs=1M skip=5626590
>>
>> and expect it to create a 3M file.
>>
>> Use this 'skip' number of the 'seek' number later.
>>
>> NeilBrown
>
> Thanks, Neil, now it makes more sense.
>
> I tried to create new linear device, but mdadm is complaining about
> data-offset:
>
> # mdadm -C /dev/md4 -l linear -n 2 --rounding=1M -e 1.0 --data-offset=0
> /dev/md2 /dev/md3
>  mdadm: invalid data-offset: 0
>
> I'm using Debian 8.8 if it makes any difference.
>
> # mdadm -V
>  mdadm - v3.3.2 - 21st August 2014
>
> What could be the problem?

I noticed that md2 and md3 use 1.2 metadata. Can that be the issue? 
Trying to create 1.0 metadata for md4?