Chapter NINETEEN. Availability

Chapter Syllabus

19.1 Critical Application Classes

19.2 Percentages of Availability

19.3 Availability-Level Server Classifications

19.4 Increasing Availability Through Power Protection

19.5 Improving Server Availability with Memory Technologies

After reading this chapter, you will be able to do the following:

• Define availability and describe the critical application classes, percentages of availability, and server availability level classifications.

• Explain how to increase server availability by implementing power protection.

• Explain how to increase server availability by implementing memory technologies, such as the following:

  • Online spare memory

  • Single-board mirrored memory

  • Hot-plug mirrored memory

  • Hot Plug RAID Memory

Availability is a measure of the time that a server system or component is functioning normally and of its ability to maintain services despite hardware or software failures, planned or unplanned. Availability is expressed as a percentage and is measured against a period of one year. The best possible rating, 100%, indicates that the system is available all the time.

Availability can be defined in terms of

• Critical application classes.

• Percentages of availability.

• Availability server level classifications.

This chapter first explains availability in terms of the various levels of application classes and their availability requirements, the methods for measuring availability, and the availability-level server classifications.

The chapter then explains how to increase overall server availability by (1) implementing power protection, and (2) increasing memory availability.