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SSA RAID Adapter, SSA, and RAID
• Only one pair of adapter connectors can be connected in a particular SSA loop.
RAID Functions
Hot Spares
RAID-5
Array States
Redundant Array of Independent Disks (RAID) technology gives yoU:
• Larger disk capacity
• Immediate availability and recovery of data
• Redundancy of data at a level that you can choose.
RAID technology stores data across groups of disk drives that are known as arrays.
Subject to the level of RAID that you are using, this method of data storage
provides the data redundancy that is needed for a secure system, and can allow
faster retrieval of data through multiple channel access. Also, if a disk drive fails,
you can normally exchange that disk drive without interruption of normal system
operation.
The arrays can provide data redundancy that ensures that no data is lost if one
disk drive in the array fails. The method that is used to write data to an array is
related to the level of RAID that you are using. Arrays are contained in array
subsystems. You can configure your subsystem with one or more arrays.
A hot spare disk drive is a disk drive that is defined for automatic use if a disk drive
in an array fails. The hot spare must have a storage capacity that is greater than,
or equal to, that of the smallest member of an array. You can define as many hot
spares as you want. RAID-5 arrays can use hot spare disk drives.
If a disk drive in an array fails, the adapter automatically uses a hot spare instead
of the failed disk drive, and rebuilds the data from the failed disk on the hot spare.
RAID-5 arrays write strips of data across all but one members of the array. Parity
for the other strips is written on the last member. The members of the array are
used in rotation for the parity data.
RAID-5 arrays offer both data protection and increased throughput.
An array can be in one of several states. The state of an array is shown in the
status column of some of the SSA configurator menus. Those states are described
here.
Good State
An array is in the Good state when all the member disk drives of that array are
present.
Exposed State
An array enters the Exposed state when a member disk drive becomes missing
(logically or physically) from that array. When an array is in the Exposed state, you
can reintroduce the missing disk drive, or exchange it for a new one. If necessary,
the array management software rebuilds the data that was on the original disk drive
before it became missing, then writes that rebuilt data to the reintroduced, or
Chapter 7. The SSA RAID Adapter, SSA, and RAID
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