RAID 6 with 25 disks — 23 usable

RAID 6

Two disks’ worth go to parity, so two simultaneous failures are survivable — a second disk failing during a rebuild does happen.

TB on the label, TiB on the screen

• 1 TB × 2523 TB · 20.92 TiB
• 2 TB × 2546 TB · 41.84 TiB
• 4 TB × 2592 TB · 83.67 TiB
• 6 TB × 25138 TB · 125.51 TiB
• 8 TB × 25184 TB · 167.35 TiB
• 10 TB × 25230 TB · 209.18 TiB
• 12 TB × 25276 TB · 251.02 TiB
• 14 TB × 25322 TB · 292.86 TiB
• 16 TB × 25368 TB · 334.69 TiB
• 18 TB × 25414 TB · 376.53 TiB
• 20 TB × 25460 TB · 418.37 TiB
• 22 TB × 25506 TB · 460.2 TiB
• 24 TB × 25552 TB · 502.04 TiB

One TB on the box is 10¹² bytes, while the operating system counts in units of 2⁴⁰ bytes. The names look alike but the units differ, so the same disk reads about 9% smaller. Nothing went missing — the ruler changed.

Same disk count, other levels

Same level, other disk counts

The rebuild is the dangerous part

When a single-parity array loses a disk, you put a new one in and every remaining disk is read end to end to rebuild it. That is precisely when the survivors work hardest and longest. The bigger and more numerous the disks, the longer the rebuild — days, sometimes — and a second failure during it ends the array. This is why RAID 6 or 10 is preferred over RAID 5 on large arrays.

How to read it

• Usable = total − parity × groups. Every level is that one line.
• RAID 5 spends one parity disk in a single group, so n−1; RAID 6 spends two, so n−2.
• RAID 50 and 60 run several such groups striped together, so the parity is multiplied by the group count.
• RAID 1 holds one copy on every disk, so one disk’s worth; RAID 10 mirrors in pairs, so half.
• Groups must be equal in size, so the group count can only be a divisor of the disk count.
• TB on the box and TiB on the screen are different units — about 9% apart.

RAID is not a backup

RAID protects against exactly one thing: a disk dying. A file deleted by mistake, a file overwritten, ransomware, a failed controller, fire or theft all propagate to every disk at once — a mirror deletes just as faithfully as it writes. A backup is a separate copy on separate hardware, and RAID is not doing that job.