HP Smart Storage Administrator - Manual (Page 112)

Troubleshooting 112
In each mirrored trio, the physical drives that are not busy answering other requests answer any read
requests that are sent to the array. This behavior is called load balancing. If a physical drive fails, the
remaining two drives in the mirrored trio can still provide all the necessary data. Several drives in the array
can fail without incurring data loss, as long as no three failed drives belong to the same mirrored trio.
This fault-tolerance method is useful when high performance and data protection are more important than the
cost of physical drives.
Advantages:
• This method has the highest read performance of any fault-tolerant configuration.
• No data is lost when two drives fail, as long as no two failed drives are mirrored to another failed drive.
• Up to two-thirds of the physical drives in the array can fail.
Disadvantages:
• This method is expensive, because many drives are needed for fault tolerance.
• Only one-third of the total drive capacity is usable for data storage.
RAID 5—distributed data guarding
In a RAID 5 configuration, data protection is provided by parity data (denoted by Px,y in the figure). This
parity data is calculated stripe by stripe from the user data that is written to all other blocks within that stripe.
The blocks of parity data are distributed evenly over every physical drive within the logical drive.
When a physical drive fails, data that was on the failed drive can be calculated from the remaining parity
data and user data on the other drives in the array. This recovered data is usually written to an online spare
in a process called a rebuild.
This configuration is useful when cost, performance, and data availability are equally important.
Advantages:
• Has high read performance.
• Data is not lost if one physical drive fails.