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RAID is a very cool, very powerful way of deploying hard disk drives in your Linux system Using RAID allows you to:
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Speed up your hard disk system Protect your data against hard disk failures
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Depending on how you implement your disks, RAID can do either of these things or both at the same time Back when I first started working with computers in the late 1980s, RAID was something only the server guys were concerned with RAID arrays were too expensive to be implemented in desktop systems That has all changed, however RAID is still the backbone of server hard disk storage systems However, the price of RAID adapters and the price of hard disk drives have dropped enough to where it is feasible to implement a RAID solution in desktop systems as well In this part of this chapter, we re going to introduce you to RAID We ll cover the following concepts and tasks:
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Let s start by introducing you to basic RAID concepts
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Before you can effectively set up a RAID array, you need to be very familiar with basic RAID concepts You ll also need to be familiar with these concepts for your Linux+ exam To provide you with the background you need about RAID, we re going to discuss the following in this part of this chapter:
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Basic RAID concepts RAID levels Hardware vs software RAID
Let s begin by reviewing some key RAID concepts
Basic RAID Concepts
When RAID was first developed many years ago, the acronym stood for redundant array of inexpensive disks Today, it s generally acknowledged to stand for redundant array of independent disks Regardless of which definition you choose, RAID still works in the same way: you implement two or more hard disk drives in your system and then use a special hard disk controller or software in RAM to group the disks together into an array The disk controller board (or software in RAM) presents the array to the operating system as a single hard disk drive This is shown in Figure 12-23 When grouping disks into an array, you can choose from a variety of schemes that define how the disks relate to each other and how data is stored on them The first of
12: Con guring Hardware
FIGURE 12-23
A RAID array
RAID Array Appears as a Single Hard Drive
RAID Controller
RAID Array Composed of Three Hard Drives
these schemes is called striping When striping is employed, the hard drives in the array are joined together into one big disk The size of the array is equal to the sum of the sizes of all the disks it contains For example, if you have two 100GB hard disk drives in your array, the operating system would see the array as a single 200GB hard disk drive When striping is used, any data that is to be written to the array is split up and written to all the disks in the array at the same time In Figure 12-24, a hypothetical file is being written to a striped RAID array The file is divided up into chunks determined by the stripe size of the array and written to all three disks in the array at the same time Likewise, if the file were to be retrieved from the drive and loaded into RAM, its pieces would be read from all three drives in the array at the same time and reassembled by the RAID controller board and delivered to the operating system in its original, intact state Because of the way striping works, a striped array can dramatically increase the overall speed of your disk system The more disks you stripe in the array, the faster it can read and write data Striping is frequently used in server systems, video-editing systems, and so on where large quantities of data need to be handled very quickly The downside
FIGURE 12-24
Striping data between disks in an array
File Is Split and Written to All Drives in the Array Concurrently
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to striping is the fact that all of your files are spread across multiple hard drives If one of the drives in the array fails, you will lose parts of all of your files, which isn t a good thing Essentially, striping provides increased disk performance while increasing your risk of data loss Remember earlier that we said RAID arrays could also protect data in addition to making the disk system run faster This is accomplished through another array scheme called mirroring In a mirrored array, we group multiple hard drives together just like we did in the striped array However, instead of spreading data across the drives, as striping does, mirroring writes complete data files redundantly to all of the drives in the array This is shown in Figure 12-25 Each disk in the array is a mirror copy of the others The cool thing about mirroring is the fact that if one of the disks in the array fails, the others can immediately take over There s no downtime; the data you need remains available The downside of mirroring is that it doesn t provide any performance gain The size of a mirrored array is equal to the size of the smallest disk in the array For example, if you create a mirrored array that contains three 100MB hard drives, the total size of the array is 100MB There s another version of mirroring called duplexing Notice in Figure 12-25 that mirrored drives are connected to the same RAID controller board Duplexing mirrors data between hard drives connected to two or more different RAID controllersThis protects the system from data loss if the RAID board itself were to fail The last RAID concept you need to understand is parity As we mentioned, one of the key weaknesses associated with striping is the fact that one failed disk in the array destroys all of the data in the array That s a risky situation Hard disk
FIGURE 12-25
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