Lenovo ThinkServer RD240 MegaRAID SAS Software User Guide - Page 53
RAID 6, Table 2.8, RAID 6 Overview
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2.5.6 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 two drives in a virtual drive without losing data. Provides a high level of data protection through the use of 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 RAID 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 controller rebuilds one failed drive, and then the other failed drive. Table 2.6 provides a graphic example of a RAID 6 drive group. Table 2.8 RAID 6 Overview Uses Strong Points Weak Points Drives 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. 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. Not well-suited to tasks requiring a lot of 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. 3 to 32 Figure 2.8 shows a RAID 6 data layout. The second set of parity drives are denoted by Q. The P drives follow the RAID 5 parity scheme. RAID Levels 2-21
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