User's Manual

Troubleshooting 119
Each logical drive in an array is distributed across all of the physical drives within the array. A logical drive
can also extend across more than one port on the same controller, but it cannot extend across more than one
controller.
Drive failure, although rare, is potentially catastrophic. For arrays that are configured as shown in the
previous figure, failure of any physical drive in the array causes every logical drive in the array to suffer
irretrievable data loss. To protect against data loss due to physical drive failure, logical drives are configured
with fault tolerance ("Fault-tolerance methods" on page 119).
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.
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 and RAID 10 configurations tolerate multiple drive failures if no failed drives are mirrored to one
another.
• 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. 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 127). However, hardware-based RAID methods provide a much more
robust and controlled fault-tolerance environment, so these alternative methods are seldom used.