Recently I had to rebuild a CentOS5 VM host as CentOS6. My VMs were stored on a RAID1 pair (seperate from the OS disks). To avoid any possibility of impacting the VM data during the re-installation, I removed the VM data disks from the host during the reinstall.
After successfully re-installing CentOS6 on the host (having wiped out any previous OS data), I needed to restore the VMs. Re-installing the pair of VM data drives (sdc and sdd), I ran:
mdadm --assemble --scan
And expected a new md device to be discovered and available. It was not.
I discovered that I first had to remove /etc/mdadm.conf, before running the “assemble” command above, after which my device was successfully discovered.
[root@myhost ~]# mdadm --assemble --scan mdadm: /dev/md/2_0 has been started with 2 drives. [root@myhost ~]#
Then I needed to recreate /etc/mdadm.conf by running
mdadm --detail --scan >> /etc/mdadm.conf
And make any device numbering adjustments as required.