I never remember the command to manage soft RAID on my servers.

Create

mdadm --create --verbose /dev/md0 --level=mirror --raid-devices=2 /dev/sdb1 /dev/sdc1

Status

cat /proc/mdstat

Replace disk in array

Remove:

mdadm --manage /dev/md0 --fail /dev/sdb1
mdadm --manage /dev/md0 --remove /dev/sdb1

Copy disk setup:

sfdisk -d /dev/sda | sfdisk /dev/sdb

Adding:

mdadm --zero-superblock /dev/sdb1
mdadm --manage /dev/md0 --add /dev/sdb1

Change raid size

This is to be done in rescue mode.

# backup
cp /etc/mdadm/mdadm.conf /etc/mdadm/mdadm.conf_orig
mdadm --examine --scan >> /etc/mdadm/mdadm.conf
mdadm -A --scan

The resize:

mdadm --grow /dev/md2 --size=max
resize2fs /dev/md2
e2fsck -f /dev/md2