Follow/Monitor mode also allows arrays to share spare
disks.
That means you only need to provide one spare disk
for a group of arrays or for all arrays.
It also means that system administrators don't
have to manually intervene to shuffle around spare disks when arrays
fail.
When Follow/Monitor mode is invoked, it polls arrays at
regular intervals.
When a disk failure is detected on an array without a
spare disk, mdadm will
remove an available spare disk from another array and insert it into
the array with the failed disk.
To facilitate this process, each ARRAY line in /etc/mdadm.conf needs to have
a spare-group defined.
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