Tandberg Data Viking FS-1500 manual Tandberg DataStorage & File Server Management, Disk Array

Tandberg DataStorage & File Server Management

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3.1.1.1. Disk Array

With an array controller installed in the system, the capacity of several physical drives can be logically combined into one or more logical units called arrays. When this is done, the read/write heads of all the constituent physical drives are active simultaneously, dramatically reducing the overall time required for data transfer.

Notes

Depending on the storage server model, array configuration may not be possible or necessary.

Because the read/write heads are simultaneously active, the same amount of data is written to each drive during any given time interval. Each unit of data is termed a block. The blocks form a set of data stripes over all the hard drives in an array.

For data in the array to be readable, the data block sequence within each stripe must be the same. This sequencing process is performed by the array controller, which sends the data blocks to the drive write heads in the correct order.

A natural consequence of the striping process is that each hard drive in a given array contains the same number of data blocks.

Notes

If one hard drive has a larger capacity than other hard drives in the same array, the extra capacity is wasted because it cannot be used by the array.

3.1.1.2. Fault Tolerance

Drive failure, although rare, is potentially catastrophic. For example, using simple striping with several HDD, failure of any hard drive leads to failure of all logical drives in the same array, and hence to data loss.

To protect against data loss from hard drive failure, storage servers should be configured with fault tolerance.

3.1.1.3. Online Spares (Hot-Spares)

Further protection against data loss can be achieved by assigning an online spare (or hot-spare) to any configuration except RAID 0. This hard drive contains no data and is contained within the same storage subsystem as the other drives in the array. When a hard drive in the array fails, the controller can then automatically rebuild information that was originally on the failed drive onto the online spare. This quickly restores the system to full RAID level fault tolerance protection. However, unless RAID 6 is being used, which can support two drive failures in an array, in the unlikely event that a third drive in the array should fail while data is being rewritten to the spare, the logical drive still fails.

3-2 Tandberg Viking Series Storage Server Administration Manual