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RAID-6 vs RAID-5 (The Case For RAID-6)

​RAID-5 is the most popular choice for storage arrays today, as it is relatively simple to implement at the hardware level and is able to continue to operate in the event of failure of a single device. For many years RAID-5 has provided adequate levels of protection for highly reliable operation, but the growth in data storage requirements and the corresponding increase in capacity of arrays has the potential to impact the reliability of RAID-5 systems.

It is easy to see that larger data storage requirements leads to arrays with more drives and with more drives comes a higher probability of one drive failing. However, another problem is now becoming apparent - the rate at which an otherwise healthy occasionally drive fails to read a sector from the disk. These errors are known as "non-recoverable read errors" and the rate at which they occur is typically specified by the drive manufacturer as "non- recoverable read errors per bits read". This rate ranges from one error per 10^14 bits (12.5 TB) read for consumer SATA drives such as the ​Seagate Barracuda 7200.10 to one error in 10^16 bits (1.25 PB) read for enterprise grade drives such as the​ Seagate Cheetah NS​.

​RAID-5 Array Rebuild​

​In order to rebuild the array after a drive failure, a RAID-5 array must read the entire contents of every remaining disk in the array. If the array consists of seven 1 TB disks, 6 TB of data would be read during this process, 1 TB from each of the six non-failed disks. If consumer SATA disks are used, one read in every 12.5 TB of data read is likely to fail, so there is almost a 50% chance of an unrecoverable read error during this process. When such an error occurs the RAID controller cannot continue the recovery process and the array will fail. When this happens, all is not lost. Our experienced data recovery experts can rebuild arrays which have failed in this way, often with same-day turnaround.

​Drive failures in a RAID ​array require immediate action. You need fast, secure results, and R3 offers the solutions you need to effectively maintain your data and system after any type of failure. Call us today at 0800 999 3282 to get started with your unique case.

​RAID-6 improves on the reliability of RAID-5 by being able to tolerate failures on two disks simultaneously, thus considerably enhancing the reliability of the array. This enhanced reliability comes at the cost of additional hardware in the form of the additional disk per array, as well as increased complexity in the RAID controller.

In an ideal implementation, RAID-6 is only marginally slower than a RAID-5 array with the same capacity and identical drives. The minor performance penalty comes from the need to wait for data on one additional disk to rotate to the read heads. The complexity of the calculations required when writing data mean that real world implementations are often far from ideal and are limited in performance by the rate at which parity can be calculated, rather than the speed of the disks.

​How RAID-6 Works

While a RAID-5 array contains one additional drive, over and above those required to store the data, a RAID-6 array contains two additional drives. The capacity of an array consisting of N drives with minimum capacity C is thus given by the formula (N - 2) * C. For example, a RAID-6 array of seven 1 TB drives would have a usable storage capacity of 5 TB.

The following diagram illustrates how the data and parity blocks are distributed across disks in a RAID-6 system.

RAID 6 Array

Posted In: RAID

What Our Customers Say About Us

I have recently used R3D to retrieve data from an external hard drive.

I called Andy at 11pm on a Saturday after my hard drive failed, he immediately ran through the symptoms and calmed my fears that all data was lost, but was honest that data recoveries are not always successful.

I required the data immediately as it was for a deadline the following Friday, the issue was I was based in London and the lab based in Sheffield.

After searching all data companies in and around London none would answer my call on a Saturday. In fact no one else would answer my call or say they could look at my case on the Sunday.

I took the drive myself to R3D and was very impressed with their facilities. Andy also explained the whole process and what they were doing at each stage.

Unfortunately the hard drive had a physical failure and needed to be rebuilt to have any chance of recovering the information.

After starting the rebuild at 4pm on the Sunday, Andy advised it would take him and his team until the morning to know if any information was recoverable - fantastic I thought. Andy and Mark got to work on my drive as soon as I left and by 10pm on Sunday they had retrieved almost the entirety of the information of my hard drive.

I cannot reccomend R3D highly enough, they are clearly experts in their field and have a passion for what they do. Through the whole process Andy kept me informed of progress and explained everything they were doing.

If you have a crisis or any data recovery needs these are the guys you need!

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