What Is High Availability? - Definition From - TechTarget

How availability is measured

Availability can be measured relative to a system being 100% operational or never failing -- meaning it has no outages. For example, you can calculate the monthly availability rate based on the number of minutes in the month and the number of minutes of downtime during that month, as shown in the following formula:

Availability = (minutes in month - minutes of downtime) * 100/minutes in month

For example, the number of minutes for a 30-day month is 43,200. If the downtime is 10 minutes, you can use the following equation to calculate the availability rate:

Availability = (43,200 - 10) * 100/43,200 = 43,190 * 100/43,200 = 4,319,000/43,200 = 99.976852

This comes to an availability rate of about 99.98%. In comparison, the amount of downtime for the same month at an availability rate of 99.99% (four nines) would be only 4.32 minutes.

IT teams also use other metrics to measure the availability of their systems:

• Mean time between failures. MTBF is the expected time between two failures for the given system.
• Mean downtime. MDT is the average time that a system is nonoperational.
• Recovery time objective. RTO is the total time a planned outage or recovery from an unplanned outage will take. RTO is also referred to as estimated time of repair.
• Recovery point objective. RPO is the maximum amount of data loss that an organization can tolerate if failure occurs.

IT teams can use these metrics as guidelines when planning the levels of availability they're trying to achieve in their systems. Service providers can also use these metrics when guaranteeing a certain level of service to their customers, as stipulated in their service-level agreements (SLAs). An SLA is a contract that outlines the type and level of services to be provided, including the level of availability.

Availability metrics are subject to interpretation as to what constitutes the availability of the system or service to the end user. Even if systems continue to partially function, users might deem them unusable based on performance problems.

Despite this level of subjectivity, availability metrics should still be concretely defined in the SLAs that service providers offer their customers. For example, if an SLA promises 99.999% availability (five nines), customers can expect the service to be unavailable for the following amounts of time:

Time period	Time system is unavailable
Daily	0.9 seconds
Weekly	6.0 seconds
Monthly	26.3 seconds
Yearly	5 minutes and 15.6 seconds

To provide context, if a company adheres to the three-nines standard (99.9%), there will be about 8 hours and 45 minutes of system downtime in a year. Downtime with a two-nines standard (99%) is even more dramatic, equating to a little over three days of downtime a year. Cloud service providers generally promise at least 99.9% availability for their paid services, although they've more recently moved to 99.99% availability for some services.