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What Is RAID 6 For?

The RAID 6 Areca ARC-1120 One-ups RAID 5 Controllers
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Today, storage systems based on the RAID Level 5 array are indispensable. But they are not perfect - they can only handle failure of a single hard drive. Although that doesn't seem problematic at first glance, in a worst-case scenario it can mean data loss. If a hard drive fails, it must be replaced as quickly as possible. Ideally, a reserve drive - a hot spare - should be ready. If it's not available, it's up to the administrators to react quickly. if no one is available on the weekend who can swap the defective drive and restore the RAID 5 array, the array will remain as vulnerable as a RAID 0 for a potentially significant length of time. Any further errors will invariably lead to total data loss - and restoring data at a data rescue company like CBL or Ontrack is very expensive.

Even when the defective drive is replaced, there is still a risk, because the RAID controller must restore the missing data blocks on the basis of the parity information and write it to the swapped drive. This process is called rebuilding and again makes use of the array drives. In practice, the problem with RAID 5 is that after an array has been used for a certain length of time, e.g. a couple of years, often several hard drives will crash almost simultaneously or one by one in succession.

RAID Level 6 Array In Detail

To prevent the nightmare of catastrophic data loss, a second set of parity information is recommended. And this is exactly what a RAID 6 array does: using the stripes of parity information already created, the controller can generate another parity set. This can be done with Reed-Solomon codes, which are commonly used during digital data transfer for forward data correction. However, this requires additional hardware. Areca takes a simpler route and creates the second parity set using XOR calculation, even though this requires adding a module of their own creation.

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