Availability, for the most part, is a post-operative factor. In other words, availability management covers redundancy, mirrored or duplexed systems, failovers, and so on. Note that failover is emphasized because the term itself denotes taking over from a system that has failed.
The clustering of systems, or load balancing, conversely, is also as much disaster prevention as it is a performance-level maintenance practice. Using performance management, you would take systems to a performance point that is nearing threshold or maximum level; then you switch additional requests for service to other resources. A failover is a machine or process that picks up the users and processes that were on a system that has just failed, and it is supposed to enable the workload to continue uninterrupted on the failover systems. A good example of failover is a mirrored disk, or a RAID-5 storage set: The failure of one disk does not interrupt the processing, which carries on oblivious to the failure on the remaining disks, giving management time to replace the defective components.
Note There are several other SL-related areas that IT spends time on and which impact SLM. These include change management and control, software distribution, and systems management. See Chapter 11 for an extensive discussion of change management.
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