For this how-to I've preferred to use loopback devices in place of real hard drives. This allows to better play with the storage devices in a more destructive way.
We'll set up a raid1 device using two block devices and one spare device.
The two extra disks are detected by the system as
The disks are mirrored in RAID 1.
$ mdadm --create --verbose /dev/md0 --level=mirror \ raid-devices=2 /dev/sdb1 /dev/sdc1
Eventually one ore more devices can be added to the array as stand-by spare disks. Spare disks will automatially become part of the mirror if one of the active devices break.
$ mdadm --create --verbose /dev/md0 --level=mirror \ raid-devices=2 /dev/sdb1 /dev/sdc1 --spare-devices=1 /dev/sdd1
To examine the details
$ mdadm --detail /dev/md0
To stop a running raid device
$ mdadm --stop /dev/md0
To reassemle an already created raid
$ mdadm --assemble /dev/md0 /dev/sdb1 /dev/sdc1
For various reasons can happen that a raid device is showed as removed E.g.
$ mdadm --detail /dev/md0 ... Number Major Minor RaidDevice State 3 8 17 0 active sync /dev/sdb1 2 8 33 1 removed /dev/sdc1
In this case the device should be re-added "by-hand" to the raid array. First zero out the device super block
$ mdadm --zero-superblock /dev/sdc1
Then add the device
$ mdadm /dev/md0 --add /dev/sdc1
Once added, the device starts syncing the device with the array.
$ mdadm --detail /dev/md0 ... Number Major Minor RaidDevice State 3 8 17 0 active sync /dev/sdb1 2 8 33 1 spare rebuilding /dev/sdc1
The process can be monitored via the /proc/mdstat file
$ watch cat /proc/mdstat