For any configuration except RAID 0, further protection against data loss can be achieved by assigning a
drive as an
online spare
(or
hot spare
). This drive contains no data and is connected to the same
controller as the array. When any other physical drive in the array fails, the controller automatically
rebuilds information that was originally on the failed drive to the online spare. The system is thus restored
to full RAID-level data protection, although it now no longer has an online spare. (However, in the unlikely
event that another drive in the array fails while data is being rewritten to the spare, the logical drive will
still fail.)
When you configure an online spare, it is automatically assigned to all logical drives in the same array.
Additionally, you do not need to assign a separate online spare to each array. Instead, you can configure
one hard drive to be the online spare for several arrays if the arrays are all on the same controller.
More information
Fault-tolerance methods on page 86
Effects of a hard drive failure on logical drives
When a drive fails, all logical drives that are in the same array are affected. Each logical drive in an array
might be using a different fault-tolerance method, so each logical drive can be affected differently.
• RAID 0 configurations do not tolerate drive failure. If any physical drive in the array fails, all RAID 0
logical drives in the same array also fail.
• RAID 1 tolerates the failure of one drive.
• RAID 10 configurations tolerate multiple drive failures as long as no failed drives are mirrored to
another failed drive.
• RAID 5 configurations tolerate one drive failure.
• RAID 50 configurations tolerate one failed drive in each parity group.
• RAID 6 configurations tolerate two failed drives at a given time.
• RAID 60 configurations tolerate two failed drives in each parity group.
• RAID 1 (ADM) and RAID 10 (ADM) configurations tolerate multiple drive failures if no more than two
drives, mirrored to one another, fail.
Fault-tolerance methods
Several fault-tolerance methods exist. The methods most often used with Smart Array controllers are
hardware-based RAID methods.
Alternative fault-tolerance methods are also available. However, hardware-based RAID methods provide
a much more robust and controlled fault-tolerance environment, so these alternative methods are seldom
used.
More information
Alternative fault-tolerance methods on page 93
RAID 0
A RAID 0 configuration provides data striping, but there is no protection against data loss when a drive
fails. However, it is useful for rapid storage of large amounts of noncritical data (for printing or image
editing, for example) or when cost is the most important consideration.
86
Hardware issues
