Understanding RAID Levels and Concepts
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10
RAID Concepts
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 working with a 3ware RAID controller, “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 (RAID 1) 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 (RAID 0) achieve
highest transfer rates and performance at the expense of fault tolerance.
•
Distributed Parity
. Parity works in combination with striping on RAID 5,
RAID 6, 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 defective
drive in a redundant unit.
•
Array 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 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”.
