Data Recovery Weekly

There are three levels of hot-something hardware repair capability in a storage system cold swap hot swap hot spare differing on two properties if a downtime is required to perform the repair if a human intervention is required Hence, the levels break down as follows: Cold swap: both downtime and human intervention required Hot swap: human intervention is required but no downtime Hot spare: no downtime and no human intervention The price of the system increases with the level of hotness , while the maintenance cost decreases. In certain applications, where the maintenance cost is high, it is cheaper to put in enough "hot spare" parts for the entire useful life of the system than to use a hot-swap and human intervention. While an unmanned spacecraft is the extreme example of the high-cost maintenance, we will probably see maintenance-free end-user systems soon after hard drive production finally comes back to proper rates.

What is the difference between hotspare and hotswap ? Hotswap is the capability to replace a failed part with a spare without having to shut down the system. To resolve a problem, somebody still has to bring the spare part and do the actual work replacing it. Hot spare is the spare part that is placed into the system at build time, in anticipation that something will eventually fail. The hot spare ties the part which could be otherwise used, but instead sits idly waiting for something to fail. On the bright side, when something fails, there is no need for a human intervention because the spare part is already there and ready.

Hot spares are a good addition to a fault-tolerant array. If a drive has failed in a fault-tolerant (RAID 1 or RAID 5) array, there is a vulnerability window. If another drive fails during this vulnerability window, the data is lost. Hot spare drives allow the controller to rebuild the array without administrator intervention, thereby reducing the vulnerability window. The need for a hot spare increases as the number of disks in array increases. Hot spares are most effective when a single hot spare drive is shared between several arrays. Consider for example an 8-bay NAS. If there is only one RAID 5 array in the NAS, then RAID 6 may be a better option than a hot spare. The hot spare drive just sits there idly. In a RAID 6 array, the same drive would be utilized to improve a read speed. However if you need two RAID 5 arrays, the hot spare drive is shared between these two arrays, reducing the disk space overhead.