RAID 5 with four 12 TB drives: longer rebuilds and when to step up to RAID 6

A rebuild reads every bit of the three survivors, 36 TB in total, and writes a full replacement drive. On a quiet array, one 12 TB drive takes at best 21 hours to read in full at the calculator's modeled HDD speed. Real rebuilds run slower when the array is busy.

That's longer than a 4 TB drive, which takes 7 hours at best. The 12 TB rebuild takes 3 times as long.

The longer read also has more chances to hit an unrecoverable read error. At a spec rate of 1 in 10^14 bits, the chance of at least one such error during that read is at most about 94.39%. That's an upper bound, since a spec rate is a ceiling and your drive may do better. Check your drive's datasheet for the rate it lists. At a spec rate of 1 in 10^15 bits, that bound is about 25.02%.

In this model, drive count sets the yearly risk, not drive size. At an assumed 2% yearly failure rate per drive, the chance that at least one of the four drives fails within a year is 7.76%. That 2% is a placeholder, not a measured rate for any drive. Size doesn't enter the formula, so four 4 TB drives carry the same figure.

At this size the real question is whether one parity drive is enough. RAID 6 and ZFS RAID-Z2 both keep two drives' worth of parity, so a second failure during a rebuild doesn't end the array. OpenZFS puts it this way: the raidz2 vdev type "specifies a double-parity raidz group." On four 12 TB drives, RAID 6 gives 24 TB usable and RAID-Z2 gives 24 TB, against 36 TB for RAID 5.

Four drives in RAID 6 give less space, 24 TB against 36 TB, and that's the price of the second parity drive.

Five 12 TB drives in RAID 6 give 36 TB, the same as four in RAID 5.

Keep RAID 5 on four 12 TB drives only when the data can be replaced or is backed up somewhere else. The layout gives 36 TB usable from 48 TB raw, with one drive of parity. A rebuild has to read 36 TB from three drives with no spare parity left.

The best case is 21 hours on a quiet array, and a busy array takes longer. A read error during the rebuild is possible, at most about 94.39% at a spec rate of 1 in 10^14 bits. Check your drive's datasheet for its rate.

For data that matters, RAID 6 or RAID-Z2 makes more sense at this size. Both keep two drives' worth of parity, so a second failure during the rebuild doesn't end the array. Five 12 TB drives in RAID 6 give 36 TB, and four give 24 TB.