High availability criteria
IBM® i high
availability offers a choice of different technologies for data resiliency
and application availability. Each of the different technologies
has different characteristics. These characteristics should be matched
with the unique requirements of each individual business application.
The following parameters should be understood and considered when
choosing which data resiliency technique is best for your business.
Each high availability solution has an associated cost. The cost for the solution must be compared to the benefit achieved for your business. When asked about a high availability solution, most customers will say that they want continuous availability with zero downtime. While this is technically possible, the cost of the protection offered by the solution may be too great.
Up-time requirements refers to the total amount of time that the system is available for end-use applications. The value is stated as a percent of total scheduled working hours.
What kind of outage is the business trying to protect against? Backup window reduction, planned maintenance, unplanned outages, or site disasters are events to consider when choosing a high availability solution.
Recovery time objective (RTO)
Recovery time objective (RTO) is the length of time that it takes to recover from an outage (scheduled, unscheduled, or disaster) and resume normal operations for an application or a set of applications.
Recovery point objective (RPO)
Recovery point objective (RPO) is the point in time relative to the failure to which you need preservation of data. Data changes preceding the failure or disaster by at least this time period are preserved by recovery processing. Zero is a valid value and is equivalent to a "zero data loss" requirement.
The business must identify what it is that needs to be protected when the system hosting the application experiences an outage. The resilience requirements are the set of applications, data and system environments required to be preserved across an outage of the production system. These entities remain available through a failover even when the system currently hosting them experiences an outage.
Automated failover and switchover
The business must define how much control is given up to automation during unplanned outages. IBM i high availability solutions have a customizable level of business interaction in failover processing. In case of a failure, the application can automatically failover to a backup system, including all application environment start.
Distance between systems, or geographic dispersion, has benefits but is gated by physical and practical limits. For a disaster recovery solution, there are always benefits in having geographic dispersion between the systems. Typically, the greater the distance between the systems, the greater the protection you will have from area wide disasters. However, this distance will come with application environment impacts.
Number of backup systems
Different data resilience technologies offer differing numbers of possible backup systems and copies of application data.
Access to a secondary copy of the data
Different data resilience technologies have different restrictions to the backup data set. Access to the backup data set requirements indicates the level of access that is required to secondary copies of the data for other work activity off-loaded from primary copies, such as saves and queries/reports. You should consider the frequency, duration, and what type of access is needed for the backup copy of the data.
Implementing high availability may have performance implications. The requirements of the business may determine what data resilience technology is required.