RAID 6: Storage tech­nol­o­gy to minimize data loss

Combining multiple hard drives into a single logical drive isn’t exactly a new approach. Over the years, various setups have proven suc­cess­ful, and become stan­dard­ized as RAID levels (Redundant Arrays of In­de­pen­dent Disks). One approach that is rarely used, but is relevant thanks to its high fail-safety, is RAID 6. Find out what makes RAID 6 stand out, its strengths and weak­ness­es and when you should use it.

RAID 6 is a data storage approach that combines four or more hard drives into a single logical drive. Compared to in­di­vid­ual data carriers, it boosts re­li­a­bil­i­ty of the system and read speeds. The basis for this is the com­bi­na­tion of striping and parity, that also forms the basis of RAID level 5. It comes as little surprise then that RAID 6 is often referred to as “RAID 5 expansion”.

RAID 6 systems implement the “striping” approach in a classic manner. All data is divided into blocks and dis­trib­uted evenly to the par­tic­i­pat­ing hard disks. This gives users the option of accessing several disks at the same time and reading the sub-blocks of a data strip in parallel.

When it comes to parity, RAID 6 differs from other levels: the system always saves two sets of parity in­for­ma­tion. In that way, as­so­ci­at­ed data can be restored if one or two disks fail. For this purpose, a RAID 6 system can op­tion­al­ly use the XOR logic or a mix of XOR logic and multi-bit error cor­rec­tion using Reed-Solomon code. The latter is also required to transmit tele­vi­sion signals according to the DVB standard, where it improves the bit error rate of the received signal.

The total storage capacity of a RAID 6 drops sig­nif­i­cant­ly compared to that of in­di­vid­ual drives. The space available for user data can be easily cal­cu­lat­ed using the following formula:

For example, with four 1GB hard disks, only 50 percent of their potential memory would be available to store user data. However, as the number of disks increases, this re­la­tion­ship between capacity and parity improves.

Compared to RAID 5, the extended RAID 6 has one decisive advantage: parity in­for­ma­tion to recover lost data is saved in duplicate. Du­pli­cat­ed parity data is a more efficient way of creating re­dun­dan­cy, and also ensures higher re­li­a­bil­i­ty. In the RAID 6 system, up to two hard disks can fail at the same time without en­dan­ger­ing the operation of the system.

The ad­van­tages of RAID 6 are also closely linked to the strengths of RAID 5. The pos­si­bil­i­ty of parallel access ensures a better through­put rate when reading data. Because of double parity, this advantage is weaker compared to level 5 systems. The ratio between the capacity for storing user data and no storage capacity (for parity) is weaker compared to RAID 5. However, when five hard disks are installed in a RAID 6, the system proves more resource-efficient and can be scaled with every ad­di­tion­al hard disk.

Compared to single drives, the sig­nif­i­cant reduction in potential storage space is one of the system’s major dis­ad­van­tages. This is par­tic­u­lar­ly evident in a system of four disks where just 50 percent of storage capacity is available. The write per­for­mance is another dis­ad­van­tage in a RAID 6 network. If data is saved on the linked hard disks, parity is cal­cu­lat­ed and dis­trib­uted twice each time. This minimizes the write rate, which is evident when re­build­ing the system, i.e., when in­te­grat­ing new hardware as a re­place­ment for defective copies.

Because RAID 6 acts like a buffer between two possible hardware failures, its potential to store large amounts of data in a fail-safe manner long-term is huge. Ideal ap­pli­ca­tions are therefore server systems where large amounts of data are archived. Database or trans­ac­tion servers, which typically use RAID 5, can also benefit from a RAID 6 storage structure if the security of the data is the focus and a minimized write speed is com­pat­i­ble with the re­quire­ments of the re­spec­tive ap­pli­ca­tion.

RAID 6 is a fail-safe al­ter­na­tive to the more widely used RAID 5. But re­dun­dan­cy through parity isn’t the only option. Other standards such as RAID 1 and RAID 10 store data in a mirrored manner, i.e., always in duplicate. The latter approach combines two RAID levels with one another, in that it not only includes the mirroring tech­nol­o­gy of RAID 1, but also dis­trib­utes the data according to RAID 0 to all in­te­grat­ed hard drives.