RAID

RAID

• RAID 0 (Striping): Data is split and written across multiple disks for high performance, but offers no redundancy. If one disk fails, all data is lost.
• RAID 1 (Mirroring): Data is duplicated across disks for high redundancy. Tolerates one disk failure, but usable storage is halved.
• RAID 5 (Striping with Parity): Data is striped, and distributed parity allows for single disk failure tolerance with good storage efficiency and balanced performance.
• RAID 6 (Striping with Dual Parity): Similar to RAID 5 but with two distributed parity blocks, tolerating two simultaneous disk failures at the cost of more write overhead.
• RAID 10 (Striping & Mirroring): Combines RAID 1 and RAID 0 for excellent performance and high redundancy. Tolerates multiple failures as long as they are not in the same mirrored set, but is more expensive.

RAID 0

Striping is a data storage technique where data is broken down into blocks and spread sequentially across multiple physical disks.

Analogy: Dealing Cards

Think of it like dealing a deck of cards to several players. The first card goes to Player 1, the second to Player 2, the third to Player 3, and then you circle back to Player 1. In this analogy:

• The deck of cards is your file.
• Each card is a block of data.
• Each player is a hard disk.

Striping offers high performance because it enables parallel data access.

RAID 5

• At least 3 disks
• Single Parity
• Single disk failure tolerance.

RAID 5 doesn’t replicate data in the sense of making an identical copy; it uses a more efficient mathematical calculation to allow for data recovery from a single disk failure.

$$\text{Disk usable capacity} = (N-1) \times \text{(size of smallest disk)}$$

• Parity calculation:

$$ A \oplus B = P$$

$$ A \oplus P = B$$

$$ P \oplus B = A$$

XOR $\oplus$ table

A	B	P
0	0	0
0	1	1
1	0	1
1	1	0

As long as $N \ge 3$, any one disk fails, any block on that disk can be calculated from all the other disks. For example, A, B, C, D blocks are across 4 disks, $P=A \oplus B \oplus C \oplus D$, when the disk contains C fails, C can be calculated by $C=P \oplus A \oplus B \oplus D$.

RAID 6

• At least 4 disks.

• Dual Parity

• Two disks failure tolerance.

• Sum1 = Data1 + Data2 + Data3 (like P parity).

• Sum2 = (Data1 * X) + (Data2 * Y) + (Data3 * Z) (like Q parity, using different coefficients or a different operation).

• if Data1 and Data2 are lost, two equations (Sum1 and Sum2) with two unknowns (Data1 and Data2). With two independent equations, you can solve for both missing numbers.

RAID 10

• At least 4 disks
• Mulitple disks failure tolerance
• Performance comes first cases