6.4 A Summary of Disk Array Design

Disk array design is fundamentally a matter of creating a list of the most critical attributes for the environment at hand. As a result, understanding the characteristics of the environment is absolutely necessary. The following questions drive our understanding:

• Is the workload primarily read-driven, write-driven, or balanced?

• What sort of access pattern is generated? Is it primarily random or primarily sequential?

• What sort of resiliency to failure is required? How many disk failures must be tolerated? How sensitive is the environment to degraded performance in the event of a disk failure?

Think carefully about these issues as you design complex disk subsystems.

6.4.1 Choosing a RAID Level

The hallmark of RAID design is simple and important. Even though we've already said it, it bears restating.

Fast, cheap, safe.

Of these three attributes, you may pick only two.

One fairly common approach to picking a RAID level is to pick the two most important attributes in order. Then we can build the selection map shown in Figure 6-7.

Figure 6-7. RAID roadmap