RAID 6, also known as double-parity RAID, is a type of redundant array of independent disks (RAID) that offers high fault and drive-failure tolerance. It is a data striping process that utilizes two disks as a backup, allowing for two disk failures within the RAID set before any data loss. RAID 6 uses two parity stripes on each disk, which allows for data to be divided across the set of hard disks or SSDs in a balanced way, improving performance.

The key features of RAID 6 are:

• Double Parity: RAID 6 uses two parity stripes, which provides an additional layer of redundancy compared to RAID 5.
• Minimum Number of Disks: RAID 6 requires a minimum of four disks to operate.
• Fault Tolerance: RAID 6 can handle up to two concurrent disk failures without losing user data.
• Space Efficiency: RAID 6 uses less storage than other RAID configurations, such as RAID 10, which can only store half of its total storage capacity in data.

RAID 6 is ideal for mission-critical applications, such as in the healthcare, banking, and defense sectors, due to its ability to tolerate simultaneous failures without loss of user data. It is also suitable for environments that need long data retention periods, such as archiving. However, RAID 6 requires special hardware, and it is important to use a controller specifically designed to support it. RAID 6 has a slower writing speed compared to other configurations, and for fewer drives, the space efficiency is very low.

In summary, RAID 6 is a type of RAID that offers high fault and drive-failure tolerance, and it is suitable for mission-critical applications and environments that need long data retention periods. RAID 6 requires a minimum of four disks to operate and uses less storage than other RAID configurations. However, it has a slower writing speed and is not very space-efficient for fewer drives.