I was trying to create RAID1 as per https://wiki.archlinux.org/index.php/RAID#Build_the_array and succeeded. However, I wasn’t happy about system creating some random number (like /dev/md127) for RAID instead of /dev/md0, so I decided to re-create it and this is where I fu**ed up.
I followed instructions about deleting existing mdadm RAID and failed. Then I found instructions how to mark RAID drives as failed to remove them, and succeeded with one drive only (other couldn’t be removed because “resource is busy”). As a result, I now have RAID with 1 disk and can’t do anything with it. In short, I want to re-create RAID1 using sda and sdb drives. They are empty, so I don’t mind format/data loss, etc.
My current cat /proc/mdstat output:
Personalities : [raid1] md127 : active (auto-read-only) raid1 sda1 976630464 blocks super 1.2 [2/1] [U_] bitmap: 8/8 pages [32KB], 65536KB chunk unused devices: <none>
Running mdadm --stop /dev/md127 succeeds.
Then running mdadm --remove /dev/md127 fails:
mdadm: error opening /dev/md127: No such file or directory
Running mdadm --zero-superblock /dev/sda (sdb) fails too:
unrecognised md component device - /dev/sda (sdb)
As a result, trying to re-create array using mdadm --create /dev/md0 --level=mirror --raid-devices=2 /dev/sda1 /dev/sdb1 fails:
mdadm: /dev/sda1 appears to contain an ext2fs file system size=976761540K mtime=Thu Jan 1 03:00:00 1970 mdadm: /dev/sda1 appears to be part of a raid array: level=raid1 devices=2 ctime=Wed Jun 13 13:21:20 2018 mdadm: Note: this array has metadata at the start and may not be suitable as a boot device. If you plan to store '/boot' on this device please ensure that your boot-loader understands md/v1.x metadata, or use --metadata=0.90 mdadm: super1.x cannot open /dev/sdb1: Device or resource busy mdadm: /dev/sdb1 is not suitable for this array. mdadm: create aborted