That isn't what RAID6 means at all. What you are describing is a hot spare - which is essentially just an extra drive that stores no data, but is available in case of a drive failure to replace the failed drive - it applies to any raid level or SHR etc.
RAID6 is an extension of RAID5 but uses two parity stripes to store recovery information, so can tolerate any 2 drives within the array failing without data loss. The downside being that you "waste" 2 drives worth of space, and it's very computationally heavy both in writing as 2 sets of parity data have to be computed, and especially in the event of a drive failure, where the extra stress can often cause another drive to fail.
Depends entirely on how many drive bays you have available and ultimately the value of the data you are storing.
For a 2 bay NAS, your only option is RAID1.
For a 4 Bay NAS, your options are RAID5, RAID6 or RAID10 :
- RAID 5 uses the equivalent of 1 drive worth of space for parity information, and so can survive 1 drive failure. However due to the complexity and risk of a 2nd failure - it isn't recommended, unless the data isn't critical e.g. it would be ok for CCTV recordings or maybe ripped movies etc that can be replaced, and where capacity is more important than reliability
- RAID 6 uses 2 drives worth of space - meaning you are losing 50% capacity in a 4 bay NAS!
- RAID 10 is mirrored and striped. You always lose 50% capacity with RAID 10, however compared to RAID 6 it is much less complex, and a failed drive can be rebuilt much quicker with less risk (it's a 1:1 copy from the remaining drive, rather than reading all drives and doing parity calculations. This is generally the best choice for important data.
