1.1 General high availability and disaster recovery overview This section describes HA and DR topics as a high-level overview. Before providing details,  the terminology that is used throughout this publication is defined. These definitions might not  match the information that you find on the internet because sometimes different meanings  exist for the same phrase. Note:  In most cases when people talk about HA, they mean CA. Availability The ability of a service component to perform its  required function at a stated instant or over a stated  period. It is expressed as the availability ratio, for  example, the proportion of time that the service is  available for use by the customers within the agreed  service hours. Continuous availability (CA) The attribute of a system to deliver nondisruptive  service to the user 365 days a year, 24 hours a day  (assuming no planned or unplanned outages exist).  Figure 1-1 shows this relationship. Figure 1-1 Continuous availability, HA, and continuous operation relationship Continuous operations (CO) The ability of a system to continuously operate and  mask planned outages from users. It uses redundant  hardware and software components (often clustering)  along with nondisruptive maintenance and change  management procedures. 2  Implementing IBM VM Recovery Manager for IBM Power Systems