Add hot spare to raid 5

Some RAID cards and / or NAS devices can connect one or more spare disks (spare disks). These disks are not used except in case of failure of one disk. We implemented a two-disk RAID 1 and we also added a spare disk (of course it requires equipment that supports hotspare). If one disk in the RAID comes in a crash, it will be immediately replaced with the spare disk. This increases fault tolerance of RAID because if you use a spare disk we can afford to lose an extra disk without losing data. That is of course expensive in terms of equipment on small RAID. By cons on large sets (5 discs and over) this could become very interesting.