Red Hat ENTERPRISE LINUX 3 - INTRODUCTION TO SYSTEM ADMINISTRATION Administration Manual page 91

Chapter 5. Managing Storage
5.6.2.1.2. RAID Implementations
It is obvious from the previous sections that RAID requires additional "intelligence" over and above
the usual disk I/O processing for individual drives. At the very least, the following tasks must be
performed:
Dividing incoming I/O requests to the individual disks in the array
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For RAID 5, calculating parity and writing it to the appropriate drive in the array
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Monitoring the individual disks in the array and taking the appropriate action should one fail
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Controlling the rebuilding of an individual disk in the array, when that disk has been replaced or
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repaired
Providing a means to allow administrators to maintain the array (removing and adding drives, initi-
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ating and halting rebuilds, etc.)
There are two major methods that may be used to accomplish these tasks. The next two sections
describe them in more detail.
5.6.2.1.2.1. Hardware RAID
A hardware RAID implementation usually takes the form of a specialized disk controller card. The
card performs all RAID-related functions and directly controls the individual drives in the arrays
attached to it. With the proper driver, the arrays managed by a hardware RAID card appear to the host
operating system just as if they were regular disk drives.
Most RAID controller cards work with SCSI drives, although there are some ATA-based RAID con-
trollers as well. In any case, the administrative interface is usually implemented in one of three ways:
Specialized utility programs that run as applications under the host operating system, presenting a
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software interface to the controller card
An on-board interface using a serial port that is accessed using a terminal emulator
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A BIOS-like interface that is only accessible during the system's power-up testing
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Some RAID controllers have more than one type of administrative interface available. For obvious
reasons, a software interface provides the most flexibility, as it allows administrative functions while
the operating system is running. However, if you are booting an operating system from a RAID con-
troller, an interface that does not require a running operating system is a requirement.
Because there are so many different RAID controller cards on the market, it is impossible to go into
further detail here. The best course of action is to read the manufacturer's documentation for more
information.
5.6.2.1.2.2. Software RAID
Software RAID is RAID implemented as kernel- or driver-level software for a particular operating
system. As such, it provides more flexibility in terms of hardware support — as long as the hardware
is supported by the operating system, RAID arrays can be configured and deployed. This can dramat-
ically reduce the cost of deploying RAID by eliminating the need for expensive, specialized RAID
hardware.
Often the excess CPU power available for software RAID parity calculations greatly exceeds the pro-
cessing power present on a RAID controller card. Therefore, some software RAID implementations
actually have the capability for higher performance than hardware RAID implementations.
However, software RAID does have limitations not present in hardware RAID. The most important
one to consider is support for booting from a software RAID array. In most cases, only RAID 1 arrays
can be used for booting, as the computer's BIOS is not RAID-aware. Since a single drive from a RAID
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