DIRECT-ATTACHED STORAGE and Network Attached Storage have a great many components that work exactly alike. At this point in the discussion, I want to move to the storage stack itself and leave the implications of moving data across a network. In this chapter, you'll see the options that are available to a Storage Area Network for protecting and preserving data, as well as strategies for minimizing the performance penalty paid for that protection.

Specifically, I'll cover RAID levels from the perspectives of how they work and where you would use them. Then moving beyond the management of physical disks, we'll look at the role of the file system, and some of the options that are available.

This chapter will introduce some of the performance penalties to be paid for ensuring access to disk resources. The relative impact of these penalties will be illustrated in Chapter 10, where I review the performance of a specific 1GB file set and the performance metrics that various storage configurations yield. I'll introduce logical volumes as supported in both Linux and Windows. The relationship of logical volumes to the hard disk devices and the file system is also explained in this chapter.

RAID Levels and What They Mean

The definition of RAID levels was introduced in Chapter 1. Given the review you've had for disk and networking technology, let's take a closer look at RAID levels. Several terms are frequently used either interchangeably with RAID levels or in place of them. As they are descriptive and relevant, I thought it best we start with those concepts and then evaluate how they are implemented in various settings. Striping-aka RAID Level 0

You can see a representation of several hard disk drives with a physical volume cut away in each. Striping refers to the strategy of writing a given file to blocks on different volumes in a parallel fashion. The benefit results from minimizing the amount of time the heads are required to move over the platters and to put as many of the disk resources as possible to work concurrently. Although read/write performance is enhanced with striping enabled, the cost is increased exposure to a head crash. Losing one of the disks of a stripe set results in the loss of the striped drive as a whole.

Spanning: When Only the Very Biggest Will Do

The logical drive spans all four disks and is represented as one storage pool. The advantage is that file sets larger than any one disk can then be stored intact.

A spanned drive may not take advantage of having four sets of read/write heads to make reading and writing more efficient; instead it may write sequentially, first filling up one drive, and then "spilling over" to the next. However, the risk or cost introduced by striping is also inherent in this storage strategy. Loss of one drive results in loss of access to all of the others.