Redundant Array of Independent Disks - RAID Levels - RAID 0 , RAID 1 , RAID 5 , RAID 6 , RAID 10

RAID stands for Redundant Array of Independent Disks. It's a technology that combines multiple physical disk drives into a single logical unit to achieve data redundancy, performance improvement, or both.

The common RAID levels include RAID 0, RAID 1, RAID 5, RAID 6, and RAID 10.

Every RAID is having their own advantages and disadvantages and so the best choice depends on your specific requirements for performance, redundancy, and storage efficiency.

1. RAID 0 (Striping)

• Description: Data is split (striped) across multiple disks.
• Minimum Disks: 2
• Fault Tolerance: None (if one disk fails, all data is lost)
• Performance: High read and write speeds
• Storage Efficiency: 100% (all disk space is usable)
• Use Case: Best for non-critical applications where speed is prioritized, like gaming or video editing.

Key Note: RAID 0 (Striping): Focuses on performance with no redundancy. Data is split across disks, so failure of one disk results in total data loss.

 

2. RAID 1 (Mirroring)

• Description: Identical copies of data are written to two or more disks.
• Minimum Disks: 2
• Fault Tolerance: Can survive the failure of one disk
• Performance: Moderate read speed (can read from both disks), slower write speed
• Storage Efficiency: 50% (only half of the total disk space is usable)
• Use Case: Ideal for critical data where redundancy is essential, like operating system drives or important backups.

Key Note: RAID 1 (Mirroring): Focuses on redundancy by copying data identically on two disks. Good for critical data but results in only 50% usable storage.

 

3. RAID 5 (Striping with Single Parity)

• Description: Data is striped across disks with one disk used for parity (error-checking data).
• Minimum Disks: 3
• Fault Tolerance: Can survive the failure of one disk
• Performance: Good read speed, moderate write speed due to parity calculations
• Storage Efficiency: (N-1)/N, where N is the total number of disks (e.g., 4 disks = 75% efficiency)
• Use Case: Balanced solution for performance and redundancy, suitable for file servers and databases.

Key Note: RAID 5 (Striping with Single Parity): Balances performance, storage efficiency, and redundancy. Requires at least three disks, but data recovery is slow if a disk fails.

 

4. RAID 6 (Striping with Double Parity)

• Description: Similar to RAID 5 but with two disks used for parity.
• Minimum Disks: 4
• Fault Tolerance: Can survive the failure of two disks
• Performance: Good read speed, slower write speed compared to RAID 5 due to dual parity calculations
• Storage Efficiency: (N-2)/N (e.g., 6 disks = 66.7% efficiency)
• Use Case: High-availability systems where data protection is crucial, like large-scale storage servers.

Key Note: RAID 6 (Striping with Double Parity): Similar to RAID 5 but with extra redundancy. Can tolerate up to two disk failures, but has slower write performance due to dual parity calculations.

 

5. RAID 10 (1+0, Mirroring + Striping)

• Description: Combines RAID 0 (striping) and RAID 1 (mirroring) for speed and redundancy.
• Minimum Disks: 4
• Fault Tolerance: Can survive up to one disk failure per mirrored pair
• Performance: High read and write speeds
• Storage Efficiency: 50% (similar to RAID 1)
• Use Case: Best for applications requiring both high performance and redundancy, like virtualization and high-traffic databases.

Key Note: RAID 10 (1+0, Mirroring + Striping): Combines the speed of RAID 0 with the redundancy of RAID 1. Requires at least four disks, and storage efficiency is 50%.

 

Here's a detailed comparison of the main RAID versions available in Linux, presented in a tabular format:

Feature	RAID 0 Striping/Split	RAID 1 Mirroring	RAID 5 Single Parity	RAID 6 Dual Parity	RAID 10 Striping+ Mirroring
Minimum Disks	2	2	3	4	4
Data Striping	Yes	No	Yes	Yes	Yes
Data Mirroring	No	Yes	No	No	Yes
Parity	No	No	Single parity (1 disk)	Double parity  (2 disks)	No
Fault Tolerance	None	1 disk	1 disk	2 disks	1 disk per sub-array
Read Speed	High	Moderate to High	High	High	High
Write Speed	High	Moderate	Moderate to High	Lower than RAID 5	High
Storage Efficiency	100%	50% (N/2)	(N-1)/N	(N-2)/N	50% (N/2)
Use Cases	Performance (no redundancy)	High availability, small setups	Balanced performance and redundancy	Enhanced redundancy over RAID 5	High performance with redundancy
Common Scenarios	Video editing, gaming	OS boot drives, critical data	File servers, databases	High-availability systems	Virtualization, large databases

 

What is Single Parity and Dual/Double Parity in RAID 5 and RAID 6

In Linux RAID systems, single parity and double parity are methods used to provide data redundancy and fault tolerance, primarily in RAID 5 and RAID 6 configurations, respectively. Here's a detailed explanation:

Single Parity (RAID 5)

• Definition: Single parity means that a single parity block is used to store redundancy information for a set of data blocks. This parity block is used to recover data if any one disk in the array fails.
• How It Works:
  • Data is striped across multiple disks.
  • For each set of data blocks (one block per disk), an additional parity block is calculated and stored on one of the disks.
  • The parity block is generated using an XOR (exclusive OR) operation on the data blocks. For example: $$\text{Parity} = \text{Data Disk 1} \oplus \text{Data Disk 2} \oplus \text{Data Disk 3}$$
  • If any single disk fails, the lost data can be reconstructed using the parity block and the remaining data blocks.
• Fault Tolerance: Can survive the failure of one disk.
• Storage Efficiency: The equivalent of one disk is used for parity, so storage efficiency is $(N-1)/N$, where $N$ is the total number of disks. For example:
  • 3 disks → 66.7% efficiency
  • 4 disks → 75% efficiency
  • 5 disks → 80% efficiency
• Use Case: RAID 5 is suitable for systems requiring a good balance between performance, storage efficiency, and fault tolerance, like file servers and read-heavy databases.

Double Parity (RAID 6)

• Definition: Double parity means that two independent parity blocks are used for redundancy. This allows for protection against the failure of two disks within the array.
• How It Works:
  • Data is striped across multiple disks, similar to RAID 5.
  • Two sets of parity information are calculated for each set of data blocks:
    • First parity (P): Calculated using XOR (similar to RAID 5).
    • Second parity (Q): Calculated using a more complex algorithm (e.g., Reed-Solomon code).
  • These two parity blocks are stored on different disks.
  • If one or two disks fail, data can be reconstructed using the two sets of parity information.
• Fault Tolerance: Can survive the failure of two disks.
• Storage Efficiency: The equivalent of two disks is used for parity, so storage efficiency is $(N-2)/N$. For example:
  • 4 disks → 50% efficiency
  • 5 disks → 60% efficiency
  • 6 disks → 66.7% efficiency
  • 8 disks → 75% efficiency
• Use Case: RAID 6 is ideal for high-availability systems where data protection is critical, such as large-scale storage solutions and write-heavy environments where the extra redundancy is worth the performance trade-off.

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Comparison of Single Parity and Double Parity

Feature	Single Parity (RAID 5)	Double Parity (RAID 6)
Parity Blocks	1	2
Fault Tolerance	1 disk failure	2 disk failures
Minimum Disks	3	4
Storage Efficiency	(N-1)/N	(N-2)/N
Write Performance	Moderate (due to single parity calculation)	Lower (due to dual parity calculation)
Data Rebuild Time	Faster than RAID 6	Slower due to extra parity calculations
Best For	Balanced performance & redundancy	Enhanced redundancy, high data protection

Key Takeaways

• Single Parity (RAID 5): Good balance of performance, storage efficiency, and fault tolerance for environments where you can afford the risk of a single disk failure.
• Double Parity (RAID 6): Provides higher redundancy at the cost of write performance. Ideal for critical data storage where protection from dual disk failures is needed.

Both RAID 5 and RAID 6 are commonly used in Linux for different types of data storage setups, depending on the need for redundancy, performance, and cost efficiency.