AMCC 9590SE-4ME User Manual

Understanding RAID Levels and Concepts

9

RAID Concepts

The following concepts are important to understand when working with a 
RAID controller:

. In the storage industry, the term “array” is used to 

describe two or more disk drives that appear to the operating system as a 
single unit. When you work with 3ware software, “unit” is the term used 
to refer to an array of disks that is configured and managed through the 
3ware software. Single-disk units can also be configured in the 3ware 
software.

. Mirrored arrays write data to paired drives simultaneously. If 

one drive fails, the data is preserved on the paired drive. Mirroring 
provides data protection through redundancy. In addition, mirroring using 
a 3ware RAID controller provides improved performance because 
3ware’s TwinStor technology reads from both drives simultaneously. 

. Striping across disks allows data to be written and accessed on 

more than one drive, at the same time. Striping combines each drive’s 
capacity into one large volume. Striped disk arrays achieve highest 
transfer rates and performance at the expense of fault tolerance. 

. Parity works in combination with striping on RAID 5. 

Parity information is written to each of the striped drives, in rotation. 
Should a failure occur, the data on the failed drive can be reconstructed 
from the data on the other drives. 

. The process of exchanging a drive without having to shut 

down the system. This is useful when you need to exchange a defective 
drive in a redundant array. 

The process of removing a unit from a controller and 

putting it back later, either on the same controller, or a different one, and 
having it recognized as a unit. The disks may be attached to different ports 
than they were originally attached to, without harm to the data. 

For definitions of other terms used throughout the documentation, see the 
“Glossary”.