RAID 6 Explained

5 min readJun 8, 2023

RAID 6 diagram — Photo by PITS Global Data Recovery Services

In the previous blog, we talked about RAID 5, which is an efficient data storage solution for situations where we need performance, redundancy, and space efficiency. For the most part, RAID 5 is the perfect solution for informational systems, but there are some other situations that RAID 5 cannot handle, and that’s where RAID 6 comes into play.

In this article, we will talk about what RAID 6 is, how it functions, and what kinds of problems it tries to solve.

What is RAID 6?

Photo by PITS Global Data Recovery Services

RAID stands for Redundant Array of Independent Disks. It is a technology that treats multiple disks as a single logical unit, providing higher performance, redundancy, and fault tolerance.

RAID 6 is a data storage technology that builds upon the principles of RAID, specifically RAID 5, while providing an additional layer of redundancy and fault tolerance. Similar to RAID 5, RAID 6 utilizes multiple drives and requires a minimum of four disks to implement. This extra disk allows for the dual distribution of parity information, providing increased data protection against multiple drive failures.

RAID 6 is commonly used in various storage systems, such as servers, network-attached storage (NAS) devices, and enterprise storage arrays. It is particularly suitable for applications that require high data redundancy, fault tolerance, and performance, making it popular in environments where data protection and availability are critical, such as data centers, enterprise-level storage solutions, and large-scale file servers. RAID 6 is also beneficial for applications that involve large amounts of data, such as video editing, multimedia storage, and database servers, where the loss of data or downtime can have significant consequences.

Structure

A RAID 6 array consists of four or more disks. Like in RAID 5, data is divided into blocks and striped across all the drives in the array. However, unlike RAID 5, RAID 6 uses two sets of parity information, both of which are distributed across the drives. This dual parity scheme ensures that even if two drives fail simultaneously, the array can reconstruct the missing data using the remaining drives and the parity information. Therefore, RAID 6 offers a higher level of fault tolerance compared to RAID 5, and that is one of the main advantages of this RAID level.

Space Efficiency

Despite the additional redundancy provided by the dual parity scheme, RAID 6 still offers efficient use of disk space. The usable capacity of a RAID 6 array can be calculated using the following formula:

U = (N — 2) * S

U — Usable capacity
N — Number of disks
S — Size of the smallest disk

For example, in a RAID 6 array with five 2 TB drives, the usable capacity would be 6 TB.

Redundancy and Fault Tolerance

RAID 6 provides robust fault tolerance by utilizing dual parity information. The array can sustain the failure of any two drives without data loss. In the event of multiple drive failures, the remaining drives and the dual parity data are leveraged to reconstruct the missing data.

The dual parity scheme in RAID 6 provides an additional layer of protection compared to RAID 5. While RAID 5 can tolerate the failure of a single drive, RAID 6 offers increased resilience against multiple simultaneous drive failures. However, it is crucial to promptly replace any failed drives to minimize the risk of data loss.

Performance and Capacity

RAID 6 strikes a balance between performance and capacity similar to RAID 5. Since data is striped across multiple drives, read and write operations can be executed concurrently, resulting in improved performance. The array’s capacity is determined by the total capacity of all the drives minus two drives reserved for dual parity. RAID 6 enables efficient capacity utilization, allowing for larger usable storage compared to RAID 1 (mirroring) or RAID 10 (striping and mirroring). However, like RAID 5, write operations can have a slight penalty due to the need to calculate and write dual parity information.

Write Penalty and Rebuild Time

The write penalty in RAID 6 refers to the additional time required to calculate and write dual parity information during write operations. This can impact the overall write performance compared to RAID levels without parity, such as RAID 0. The write penalty becomes more noticeable during random write operations since each write operation involves updating the corresponding data block and the associated dual parity information. Nevertheless, dedicated hardware RAID controllers or optimized software implementations can help mitigate the write penalty.

Rebuilding a failed drive in a RAID 6 array can be time-consuming, particularly for larger-capacity drives (20 TB or more). During the rebuild process, the data from the remaining drives and the dual parity information are utilized to reconstruct the missing data on the new drive. The rebuild time depends on factors such as drive size, array load during the rebuild, and overall system performance. It is essential to have a backup strategy in place and monitor the rebuild process closely to ensure data integrity and availability.

Pros of RAID 6

Offers a higher level of fault tolerance compared to RAID 5, with the ability to withstand the failure of any two drives.
Efficient use of storage capacity, even with the dual parity scheme.
Improved data protection against multiple simultaneous drive failures.

Cons of RAID 6

Write performance can be slightly impacted due to the need to calculate dual parity.
Rebuilding an array after drive failures can be time-consuming and resource-intensive.
If multiple drives fail or develop bad sectors during the rebuild process, data loss may occur.

Conclusion

RAID 6 provides an advanced level of fault tolerance, making it an excellent choice for applications that demand higher data protection and resilience against multiple drive failures. By implementing dual parity information, RAID 6 enhances the reliability of the array, ensuring data integrity and availability. It gives us a balance between performance and capacity, allowing for efficient storage utilization. Implementing RAID 6, along with appropriate monitoring and maintenance practices, can offer a dependable and efficient storage solution for a wide range of applications.