Add spare disks

A special pseudo-vdev which keeps track of available hot spares for a pool.ZFS allows devices to be associated with pools as "hot spares". These devices are not actively used in the pool, but when an active device fails, it is automatically replaced by a hot spare. To create a pool with hot spares, specify a "spare" vdev with any number of devices.Once a spare replacement is initiated, a new "spare" vdev is created within the configuration that will remain there until the original device is replaced. At this point, the hot spare becomes available again if another device fails.If a pool has a shared spare that is currently being used, the pool can not be exported since other pools may use this shared spare, which may lead to potential data corruption.Spares cannot replace log devices.