Basic RAID levels - for the reluctant

The usable capacity of a RAID (Redundant Array of Independent Disks) configuration depends on the specific RAID level, as well as the number and capacity of the individual drives in the array. Here are typical usable capacities for some popular RAID configurations:

RAID 0 (Striping):

Usable capacity: Equal to the sum of the capacities of all drives in the array.
Data Striping: Data is evenly distributed across all disks in the array.
Redundancy: No data redundancy. If one drive fails, all data will be lost.

RAID 1 (Mirroring):

Usable capacity: Equal to the capacity of a single drive in the array.
Data mirroring: Data is duplicated on all drives in the array.
Redundancy: High redundancy because data is duplicated. It can survive a single drive failure.

RAID 5 (Striping with Parity):

Usable capacity: (number of drives – 1)
Data Striping: Data is spread across all drives.
Parity: Parity information is distributed across disks to provide error tolerance.
Redundancy: Can tolerate a single disk failure without data loss.

RAID 6 (Double Parity Striping):

Usable capacity: (number of drives – 2)
Data Striping: Data is spread across all drives.
Double Parity: Two sets of parity information are used to provide error tolerance.
Redundancy: Can tolerate two disk failures without data loss.

RAID 50 (combination of RAID 5 and RAID 0):

Usable capacity: (number of drives in each RAID 5 group – 1)
Data Striping: Data is spread over multiple RAID 5 arrays.
Parity: Parity is calculated in each RAID 5 group.
Redundancy: Can tolerate the failure of one drive in each RAID 5 group without data loss.

RAID 60 (combination of RAID 6 and RAID 0):

Usable capacity: (number of drives in each RAID 6 group – 2)
Data Striping: Data is spread over multiple RAID 6 arrays.
Double parity: Double parity is calculated in each RAID 6 group.
Redundancy: Can tolerate the failure of two drives in each RAID group 6 without data loss.

The one thing to keep in mind, especially in the world of high-capacity drives, is that the capacity you put on your drives refers to decimal capacity of 10, 16, 18, 20 TB, etc.

Basic data is stored in bits, which are not decimal numbers.

So a 20TB drive is actually "seen" by a RAID or HBA controller as 18.18TB.

TB: Terabajt
TB: tebibajt

1 terabyte is equal to (1012/240) tebibytes.

A terabyte contains 0.90949470177293 tebibytes.

1 TB = 0.90949470177293 TiB

The calculation is "easy", just 20 x 0.9 to get an approximate number

It sounds enigmatic, but in the case of large devices such as 60 x 20 TB drives, the difference is noticeable: 1200TB = 1090.80 TiB