Chapter 6. Disk Arrays

I/O certainly has been lagging in the last decade .

”Seymour Cray, 1976

Most of the improvements in disk technology have been made with the aim of increasing the capacity/price ratio. Although these changes have made mass storage much more affordable, they have brought about two fairly serious problems:

• When a single disk can store tens of gigabytes of data, the reliability of an individual disk becomes a serious concern, as the failure of an individual disk results in the loss of large amounts of data.

• Disk performance has lagged drastically behind capacity/price improvements.

To solve these problems, a great deal of effort has been put into designing methods of organizing sets of disks to enhance both reliability and performance. This has come to be known as RAID, which either stands for "Redundant Array of Inexpensive Disks" or "Redundant Array of Independent Disks," depending on who you listen to. There are seven levels of RAID; each takes a different approach to solving these problems. The types of RAID are summarized in Table 6-1.

Table 6-1. A summary of RAID levels

RAID level	Organization	Strengths	Weaknesses
RAID 0	Striping	Very fast, simple	Low reliability
RAID 1	Mirroring	Fast, simple	Expensive
RAID 2	Reliability viaHamming codes	Reliable	Inflexible
RAID 3	Parity	Fast sequential access, reliable	Slow random access, implementation difficult
RAID 4	Parity	Average performance, reliable	Bottlenecks on dedicated parity disk
RAID 5	Parity	Good performance, reliable, cheap	Slow writes , heavy cache requirements
RAID 10	Striped mirrors	Very fast, simple	Expensive

There is a fundamental tradeoff in configuring disk arrays -- in fact, it is the classical example of the one of the principles of performance tuning (see Section 1.2.2). The tradeoff is stated in the following note.

Fast, cheap, safe.

Of these three attributes, you may pick only two.

In this chapter, I discuss some of the fundamentals of assembling multiple disks into a single logical unit: the basic terminology used, the differences between software and hardware disk arrays, recipes for desiging disk arrays, and many other aspects of modern array implementation.