6.3Facility management

However redundant a system is, it stops if power fails, air conditioning halts, or an earthquake topples the equipment. The facility is the physical foundation of service availability, and a service manager must judge how to choose the redundancy of power, cooling, seismic protection, and access control neither too much nor too little, trading off against cost, to meet the availability target implied by business impact. Making everything top-tier is safe but sends costs soaring—choosing a level that matches the target is the practice.

6.3.1Power: UPS, on-site generator, distribution

• UPS (uninterruptible power supply) supplies power from a built-in battery without interruption at the instant of an outage or a momentary voltage dip, keeping equipment running. But its battery runtime is short—minutes to a dozen or so minutes—and it alone cannot cover a long outage. The UPS's role is "to fill the momentary gap of an outage and bridge until the on-site generator starts, or until a safe shutdown completes."
• On-site generator generates power from fuel (such as diesel) and, during a long outage, takes over from the UPS to keep supplying the facility. It takes a dozen to several dozen seconds to start, and the UPS fills that gap. Combining a UPS and a generator with a division of roles (UPS for the momentary gap, generator for the long haul) is the standard practice; either alone is insufficient as outage protection. Fuel stock determines the continuous run time.
• Power distribution (distribution board / power feeds) distributes received power to racks and equipment. Dual feeds (A-side / B-side) supplying each device via separate paths keep operation going even if one feed or breaker fails. With a single feed, the distribution board or breaker becomes a single point of failure, and no matter how redundant the servers are, they all go down together.