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RAID5 backup, why bother?

RAID 5 combines capacity and performance with fault-tolerance, a disk can fail and the RAID will keep on going, so does this mean that there is no need for backup? Is RAID 5 data recovery never going to be a requirement?

Some people seem to think so, but have been dangerously mis-informed. The error correction used in a RAID5 array is there to provide a level of protection for what will often be business critical data, but can still only survive the loss of a single hard drive from the array. Even RAID 6 with double error correction can only cope with a failure of two drives.

You might wonder how likely it is that multiple disks will fail in a RAID, the answer is that it is quite likely.

Environmental factors, for example heat, can have a severe impact upon the operation of a hard disk drive. With a RAID array the disks are in the same enclosure and so an environmental factor will influence all hard drives.

Electronics and connectivity must also be taken into account. In a RAID 5 array all disks are connected to a single controller, and may be on a single data bus. Instability on the bus can result in hard drives being determined to have failed even when they have not. We have received RAID for data recovery with multiple hard drive failures, but under closer examination it has transpired that one drive has failed and resulted in three drives disappearing from the bus and being logged as failed.

Failure with a RAID 5 array can result from minor problems. A single bad sector on a disk will often result in that disk being flagged as failed. This makes sense as the failed sector cannot be read nor written so the hard drive should no longer be used and must be replaced. But, this does mean that a failure of two disk sectors on separate drives, that is 1024 bytes becoming unreadable, can result in a RAID no longer being accessible.

So, whilst RAID 5 brings many benefits in terms of capacity, performance and data protection, the security given by the RAID’s fault tolerance should not be used as an excuse to relax other procedures for data protection such as tape backup.

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One Response to “RAID5 backup, why bother?”

  • BBG says:

    That was wonderful information. You have done wonderful work communicating your message. Keep up the good writing.

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