Should I upgrade to RAID 6 for large drives?

The case for RAID 6 / SHR-2 is entirely about the rebuild window, and it grows with drive size.

Why large drives change the maths

A single-parity array (RAID 5 / SHR) survives one failure, but during the rebuild it has no remaining fault tolerance. The bigger the drives, the longer that rebuild takes, a 16–20 TB rebuild can run over a day, and the rebuild reads every surviving drive end to end, which is exactly when a second marginal drive is most likely to fail. RAID 6 / SHR-2 keeps a layer of protection through the rebuild because it tolerates two failures.

The trade-off

Double parity costs one more drive's worth of capacity than single parity (two parity drives instead of one). On a large array that is a small percentage; on a small array it is a big one, which is why the guidance scales.

A rough rule

• Use RAID 6 / SHR-2 once you have roughly five or more bays, or drives around 12 TB and up, where rebuilds are long.
• Single parity (RAID 5 / SHR) is usually fine for four-bay arrays of smaller drives with a solid backup.
• RAID is never a backup either way: keep a separate copy.

Whatever the level, use CMR drives so rebuilds are not needlessly slow. The table lists in-stock CMR drives by price per terabyte.