HP Array Configuration Utility User Guide (416146-001, January 2006)

Drive arrays and fault-tolerance methods 65
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No data is lost when a drive fails, as long as no failed drive is mirrored to another failed drive (up
to half of the physical drives in the array can fail).
Disadvantages:
• This method is expensive (many drives are needed for fault tolerance).
• Only half 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.
• More drive capacity is usable than with RAID 1+0—parity information requires only the storage
space equivalent to one physical drive.
Disadvantages:
• Has relatively low write performance.
• Data is lost if a second drive fails before data from the first failed drive is rebuilt.
RAID 6 (ADG)—Advanced Data Guarding
NOTE: Not all controllers support RAID 6 (ADG).
RAID 6 (ADG), like RAID 5, generates and stores parity information to protect against data loss caused by
drive failure. With RAID 6 (ADG), however, two different sets of parity data are used (denoted by Px,y