How to use
- Choose a RAID level; a short description of each appears below it.
- Enter the number of disks and the size of each. For disks of different sizes, list them in the optional field.
- For RAID 50 and 60, enter the number of groups. Read the usable capacity, fault tolerance and efficiency.
Options explained
- Usable capacity
- RAID 0 = disks × size; RAID 1 = one disk; RAID 5 = (disks − 1) × size; RAID 6 = (disks − 2) × size; RAID 10 = disks ÷ 2 × size.
- Failures tolerated
- The worst case. RAID 10 can survive more failures if they hit different mirror pairs, but you cannot count on it.
- TB and TiB
- Drive makers use 1 TB = 10¹² bytes; operating systems often count in 2⁴⁰, so an 8 TB disk shows as about 7.28 TB.
Capacity, fault tolerance and speed at a glance
This table shows eight 4 TB disks at each level. RAID 50 and 60 use two groups of four disks. Speeds are theoretical sequential multiples of one disk, not benchmarks.
| Level | Minimum disks | Usable | Failures tolerated | Space efficiency | Read / write |
|---|---|---|---|---|---|
| RAID 0 | 2 | 32 TB | 0 | 100% | 8× / 8× |
| RAID 1 | 2 | 4 TB | 7 | 12.5% | 8× / 1× |
| RAID 5 | 3 | 28 TB | 1 | 87.5% | 7× / 7× |
| RAID 6 | 4 | 24 TB | 2 | 75% | 6× / 6× |
| RAID 10 | 4 (even number) | 16 TB | 1 | 50% | 8× / 4× |
| RAID 50 | 6 | 24 TB | 1 | 75% | 6× / 6× |
| RAID 60 | 8 | 16 TB | 2 | 50% | 4× / 4× |
For RAID 50 and 60, the number of disks must divide evenly into the groups, with at least 3 disks per group for RAID 50 and 4 for RAID 60. Tolerance counts failures in a single group, so RAID 50 survives one failed disk in each group at the same time but not two in one group.
If you list disks of different sizes, every disk is used only up to the size of the smallest one, and the page warns you with the smallest size.
Frequently asked questions
How much space do four 8 TB disks give in RAID 5?
(4 − 1) × 8 = 24 TB, shown by the OS as about 21.83 TiB, and one disk can fail.
Why is RAID 6 recommended for large disks?
Rebuilding after a failure reads every other disk, which takes longer as disks grow. If another disk fails during the rebuild, RAID 5 loses data while RAID 6 survives.
Does it cover Synology SHR?
No. SHR uses mixed-size disks more fully and works differently from classic RAID.
Privacy
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