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6.3.4. Extending a RAID Device

To add a new device to an existing array, use the command in the following form as root: 
mdadm raid_device --add component_device
This will add the device as a spare device. To grow the array to use this device actively, type the following at a shell prompt: 
mdadm --grow raid_device --raid-devices=number

Example 6.4. Extending a RAID device

Assume the system has an active RAID device, /dev/md3, with the following layout (that is, the RAID device created in Example 6.2, “Creating a new RAID device”): 
~]# mdadm --detail /dev/md3 | tail -n 3
    Number   Major   Minor   RaidDevice State
       0       8        1        0      active sync   /dev/sda1
       1       8       17        1      active sync   /dev/sdb1
Also assume that a new SCSI disk drive, /dev/sdc, has been added and has exactly one partition. To add it to the /dev/md3 array, type the following at a shell prompt: 
~]# mdadm /dev/md3 --add /dev/sdc1
mdadm: added /dev/sdc1
This will add /dev/sdc1 as a spare device. To change the size of the array to actually use it, type: 
~]# mdadm --grow /dev/md3 --raid-devices=3