3Ware 9550SXU-8LP User Guide - Page 15

Understanding RAID Concepts and Levels Understanding RAID Concepts and Levels 3ware RAID controllers use a Redundant Array of Inexpensive Disks (RAID) to increase your storage system's performance and provide fault tolerance (protection against data loss). The following concepts are important to understand when working with a RAID controller: • Arrays and Units. 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. • Mirroring. 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. 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. • Distributed Parity. Parity works in combination with striping on RAID 5 and RAID 50. 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. • Hot Swap. The process of exchanging a drive without having to shut down the system. This is useful when you need to exchange a degraded drive or a bad drive in a redundant array. • Array Roaming. The process of swapping out or swapping in a configured unit without having to shut down the system. This is useful if you need to move the unit to another controller. • Disk Roaming. 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 can be attached to different ports than they were originally attached to, without harm to the data. The disks may be attached to the same ports or different ports on the controller. For definitions of other terms used throughout the documentation, see the "Glossary". www.3ware.com 5