User`s guide

StorageWorks RAID Array 200 Controller Installation and Standalone Configuration Utility
1–2 EK–SWRA2–IG. C01
1.1.1 Implementing RAID through Hardware, or through Software
RAID can be implemented in storage subsystems through dedicated hardware, or
through software. In a hardware implementation the RAID algorithms run on the
controller board attached to your server I/O bus. In a software implementation,
the RAID algorithms run on your server CPU in concert with the operating
system.
Hardware-based RAID promises the highest performance over the widest range
of application loads. With Software-based RAID, the overhead of the RAID
software increases as the load on the system increases. (This is especially true
after a disk failure, when the original data must be reconstructed.) Therefore, the
application software and the RAID software end up competing for host CPU
cycles.
With software RAID, maximum request rates during reconstruction will be
significantly lower, and reconstruction may take many more hours than with
hardware-based RAID. Hardware-based RAID keeps reconstruction overhead off
the CPU, minimizing the impact of drive failure on server operations. The
possibility that software-based RAID will cost less is a compelling factor only if
your system loads are consistently light, with few or no peak periods.
1.1.2 RAID Levels
This section describes only the RAID levels that StorageWorks RAID Array 200
Subsystems support. For information about RAID levels not described here, see
The Digital Guide to RAID Storage Technology, EC-B1960-45.
StorageWorks RAID Array 200 Subsystems support the following RAID levels:
• RAID 0
• RAID 1
• RAID 0 + 1
• RAID 5
• Just a Bunch of Disks (JBOD)
1.1.2.1 RAID 0
RAID 0 writes data across the drives in the array, one segment at a time, as
shown in Figure 1–1. This is known as striping.