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30 Introduction to RAID
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RAID Definition
RAID is an array of multiple independent hard disk drives that provides
high performance and fault tolerance. The RAID array appears to the host
computer as a single storage unit or as multiple logical units. Input/output
(I/O) improves because several disks can be accessed simultaneously.
Although hard drive capabilities have improved drastically, actual
performance has improved only three to four times in the last decade. In
comparison, computing performance has improved over 50 times during the
same time period. Implementing RAID improves the performance of the
disk subsystem.
RAID systems also improve data storage availability and fault tolerance.
Data loss caused by a hard drive failure can be recovered by reconstructing
missing data from the remaining data and parity drives.
PERC 3 Host-Based RAID Solution
PERC 3 is a peripheral component interconnect (PCI) adapter card that is
installed in any available PCI expansion slot in a host system. It is a host-
based RAID solution, meaning that the RAID controller puts all of the
RAID intelligence on an adapter card that is installed in a network server.
The array controller resides on the bus (for example, a PCI or EISA bus) in
the host computer and has its own central processing unit (CPU) to
generate the parity and handle other RAID functions. A bus-based
controller can transfer data at the speed of the host bus (PCI, ISA, EISA,
VL-Bus), but is limited to the bus it is designed for. PERC 3 resides on a
PCI bus, which can handle data transfer at up to 528 MB/s. With PERC 3,
each channel can handle data transfer rates up to 160 MB/s per SCSI
channel. The available sequential data transfer rate is determined by the
following factors:
• The sustained data transfer rate on the motherboard PCI bus
• The sustained data transfer rate on the i960RN (i960RM for the PERC
3/SC) PCI to PCI bridge
• The sustained data transfer rate of the SCSI controller
• The sustained data transfer rate of the SCSI devices