Introduction to RAID : RAID Levels : RAID 6

RAID 6

RAID 6 is similar to RAID 5 (disk striping and parity), except that instead of one parity block per stripe, there are two. With two independent parity blocks, RAID 6 can survive the loss of any two drives in a virtual drive without losing data. RAID 6 provides a high level of data protection through a second parity block in each stripe. Use RAID 6 for data that requires a very high level of protection from loss.

In the case of a failure of one drive or two drives in a virtual drive, the Nytro MegaRAID controller uses the parity blocks to recreate all of the missing information. If two drives in a RAID 6 virtual drive fail, two drive rebuilds are required, one for each drive. These rebuilds do not occur at the same time. The Nytro MegaRAID controller rebuilds one failed drive, and then the other failed drive.

The following table provides an overview of a RAID 6 drive group.

Table 10. RAID 6 Overview

Uses

Use for office automation and online customer service that requires fault tolerance. Use for any application that has high read request rates but low write request rates.

Strong points

Provides data redundancy, high read rates, and good performance in most environments. Can survive the loss of two drives or the loss of a drive while another drive is being rebuilt. Provides the highest level of protection against drive failures of all of the RAID levels. Read performance is similar to that of RAID 5.

Weak points

Not well-suited to tasks that require many writes. A RAID 6 virtual drive has to generate two sets of parity data for each write operation, which results in a significant decrease in performance during writes. Drive performance is reduced during a drive rebuild. Environments with few processes do not perform as well because the RAID overhead is not offset by the performance gains in handling simultaneous processes. RAID 6 costs more because of the extra capacity required by using two parity blocks per stripe.

Drives

3 through 32

The following figure shows a RAID 6 data layout. The second set of parity drives is denoted by Q. The P drives follow the RAID 5 parity scheme.

Figure 14. Distributed Parity across Two Blocks in a Stripe (RAID 6)