Managing RAID on Linux - Derek Vadala [9]
Hardware Versus Software
Although RAID is built directly into the Linux kernel, some users might find it advantageous to buy custom drive controllers that have built-in RAID capabilities. Some users might even find it worthwhile to purchase custom RAID systems that have been preconfigured. The choice between using software (kernel-based) or hardware (controller-based) RAID or buying a turnkey RAID solution can be difficult, but this book will help you determine which option is best suited for your needs.
Chapter 2. Planning and Architecture
Choosing the right RAID solution can be a daunting task. Buzzwords and marketing often cloud administrators' understanding of RAID technology. Conflicting information can cause inexperienced administrators to make mistakes. It is not unnatural to make mistakes when architecting a complicated system. But unfortunately, deadlines and financial considerations can make any mistakes catastrophic. I hope that this book, and this chapter in particular, will leave you informed enough to make as few mistakes as possible, so you can maximize both your time and the resources you have at your disposal. This chapter will help you pick the best RAID solution by first selecting which RAID level to use and then focusing on the following areas:
Hardware costs
Scalability
Performance and redundancy
Hardware or Software?
RAID, like many other computer technologies, is divided into two camps: hardware and software. Software RAID uses the computer's CPU to perform RAID operations and is implemented in the kernel. Hardware RAID uses specialized processors, usually found on disk controllers, to perform array management functions. The choice between software and hardware is the first decision you need to make.
Software (Kernel-Managed) RAID
Software RAID means that an array is managed by the kernel, rather than by specialized hardware (see Figure 2-1). The kernel keeps track of how to organize data on many disks while presenting only a single virtual device to applications. This virtual device works just like any normal fixed disk.
Figure 2-1. Software RAID uses the kernel to manage arrays.
Software RAID has unfortunately fallen victim to a FUD (fear, uncertainty, doubt) campaign in the system administrator community. I can't count the number of system administrators whom I've heard completely disparage all forms of software RAID, irrespective of platform. Many of these same people have admittedly not used software RAID in several years, if at all.
Why the stigma? Well, there are a couple of reasons. For one, when software RAID first saw the light of day, computers were still slow and expensive (at least by today's standards). Offloading a high-performance task like RAID I/O onto a CPU that was likely already heavily overused meant that performing fundamental tasks such as file operations required a tremendous amount of CPU overhead. So, on heavily saturated systems, the simple task of calling the stat[1] function could be extremely slow when compared to systems that didn't have the additional overhead of managing RAID arrays. But today, even multiprocessor systems are both inexpensive and common. Previously, multiprocessor systems were very expensive and unavailable to typical PC consumers. Today, anyone can build a multiprocessor system using affordable PC hardware. This shift in hardware cost and availability makes software RAID attractive because Linux runs well on common PC hardware. Thus, in cases when a single-processor system isn't enough, you can cost-effectively add a second processor to augment system performance.
Another big problem was that software RAID implementations were part of proprietary operating systems. The vendors