Choosing a NAS by bay size

Why bay count is the first decision

Bay count sets the ceiling on everything else: total capacity, which RAID levels you can run, how much you lose to redundancy, and how easily you can grow later. It is worth getting right first, because adding bays usually means buying a whole new appliance, whereas adding capacity within a NAS is just a matter of larger drives.

The key idea is redundancy efficiency. Protecting an array means handing some raw capacity to parity or a mirror. With few bays that overhead is a big slice of the total; with more bays the same protection spreads across more drives, so a smaller fraction is lost. A two-drive mirror gives up half its raw capacity, while one parity drive in a six-bay array costs only about a sixth.

There is also the question of how many failures you want to survive. Single-parity layouts (RAID5, SHR) tolerate one dead drive; dual-parity layouts (RAID6, SHR-2) tolerate two, which matters as arrays grow, because a second drive can fail during the hours or days a large array takes to rebuild.

How bay counts compare

2-bay or step up to 4-bay

More bays still is not a backup