Managing RAID on Linux - Derek Vadala [20]
Choosing the right components can be the hardest decision to make when building a RAID system. If you're building a production server, you should naturally buy the best hardware you can afford. If you're just experimenting, then use whatever you have at your disposal, but realize that you may have to shell out a few dollars to make things work properly.
Several factors will ultimately affect the performance and expandability of your arrays:
Bus throughput
I/O channels
Disk protocol throughput
Drive speed
CPU speed and memory
Computer architecture is a vast and complicated topic, and although this book covers the factors that will most drastically impact array performance, I advise anyone who is planning to build large-scale production systems, or build RAID systems for resale, to familiarize themselves thoroughly with all of the issues at hand. A complete primer on computer architecture is well beyond the scope of this book. The Bibliography section of the Appendix contains a list of excellent books and web sites for readers who wish to expand their knowledge of computer hardware.
One essential concept that I do want to introduce is the bottleneck. Imagine the filtered water pitchers that have become so omnipresent over the last ten years. When you fill the chamber at the top of the pitcher with ordinary tap water, it slowly drips through the filter into another cache, from which you can pour a glass of water. The filtering process distributes water at a rate much slower than the pressure of an ordinary faucet. The filter has therefore introduced a bottleneck in your ability to fill your water glass, although it does provide some benefits. A more expensive filtration system might be able to yield better output and cleaner water. A cheaper system could offer quicker filtration with some sacrifices in quality, or better quality at a slower pace.
In computing, a bottleneck occurs when the inadequacies of a single component cause a slowdown of the entire system. The slowdown might be the result of poor system design, overuse, or both. Each component of your system has the potential to become a bottleneck if it's not chosen carefully. As you will learn throughout this chapter, some bottlenecks are simply beyond your control, while others begin to offer diminishing returns as you upgrade them.
An Organizational Overview
All systems are built around a motherboard. The motherboard integrates all the components of a computer by providing a means through which processors, memory, peripherals, and user devices (monitors, keyboards, and mice) can communicate. Specialized system controllers facilitate communication between these devices. This group of controllers is often referred to as the motherboard's chipset. In addition to facilitating communication, the chipset also determines factors that affect system expandability, such as maximum memory capacity and processor speed.
When an application needs data, the CPU first checks to see if the data is stored in memory. If the data is no longer in memory, the CPU asks the chipset to request the information from disk. The chipset sends a request to the data bus, where it is picked up by the appropriate disk controller and sent across the disk bus to the drive containing the data. The drive sends the information back to the controller card, which in turn passes it back to the CPU and main memory. Figure 2-15 illustrates the connections between various components of a modern PCI motherboard.
Figure 2-15. When disk I/O occurs, data travels over both the data bus and the disk bus, each a potential bottleneck.
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The BIOS
Another important component of every computer system is the BIOS (basic input/output software). The BIOS is a chip on the motherboard that contains a simple set of drivers and instructions.