User`s guide

StorageWorks RAID Array 200 Controller Installation and Standalone Configuration Utility
1–8 EK–SWRA2–IG. C01
1.2 Key Concepts
The following sections define key terminology used to describe and operate
RAID arrays, or StorageWorks RAID Array 200 Subsystems.
1.2.1 Disk array
Disk array, or array refers to a set of disk drives and a specialized array
controller. The array controller keeps track of how data is distributed across the
drives.
1.2.2 Drive group
Drive group refers to a set of physical drives defined and addressed by the
controller as a single unit.
1.2.3 Logical RAID drive
A logical RAID drive refers to an amount of storage space presented to the host
operating system as a single storage device. Logical RAID drives differ from
conventional storage devices in that the storage space of a logical drive is usually
spread out over multiple disk drives (a drive group). In addition, logical RAID
drives can provide redundancy. RAID 1, RAID 0 + 1 and RAID 5 configurations
provide redundancy.
1.2.4 Write Caching
Write caching determines what procedure the logical drive controller uses to
write data to the logical drive. StorageWorks RAID Array 200 Subsystems
support the following write caching methods:
• Write-Back caching
• Write-Through caching
1.2.4.1 Write-Back caching
Write-Back caching refers to a caching policy in which the controller
acknowledges that a write operation has completed successfully before data is
written to the disks. If you choose this caching policy, you may increase the I/O
performance of your RAID subsystem, but if there is a power failure with no
battery backup, you risk losing the data in cache that is not yet written to the
disks.