When adding a spare disk to an existing MD region, select an available object that has the same size as the disks that are currently active in the MD region. If the MD region consists of objects with different sizes, use the smallest size.
Note that after adding a spare to a degraded MD region, the kernel MD code automatically starts the reconstruction of the MD array. When reconstruction finishes, the spare disks becomes an active disk.
If you want to reorganize disks and segments, you can remove an existing spare disk from the MD region. This is a safe operation because the spare disk does not contain any data.