Drive arrays and fault-tolerance methods 105
• 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. Those most often used with Smart Array controllers are
hardware-based RAID methods.
Two alternative fault-tolerance methods that are sometimes used are also described ("Alternative
fault-tolerance methods" on page 113). However, hardware-based RAID methods provide a much more
robust and controlled fault-tolerance environment, so these alternative methods are seldom used.
RAID 0—No fault tolerance
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.
Advantages:
• Has the highest write performance of all RAID methods.
• Has the lowest cost per unit of stored data of all RAID methods.
• All drive capacity is used to store data (none is needed for fault tolerance).
Disadvantages:
• All data on the logical drive is lost if a physical drive fails.
• Cannot use an online spare.
• Can only preserve data by backing it up to external drives.
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