Tandberg Data NAS Hardware Viking FS-1500 - Manual (Page 22)

Tandberg Data Storage & File Server Management
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3-2 Tandberg Viking Series Storage Server Administration Manual
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.