The recovery time objective (RTO) is the most important consideration when making a decision to invest in a hot site due to service criticality. The RTO is the maximum acceptable time that an IT service or process can be unavailable or disrupted before it causes significant damage to the business operations and objectives. A hot site is a fully equipped and operational backup facility that can be activated immediately in the event of a disaster or disruption. A hot site can help an organization achieve a very low RTO, as it can resume the service with minimal or no downtime. The maximum tolerable downtime (MTD) is the maximum acceptable time that an IT service or process can be unavailable or disrupted before it causes intolerable damage to the business operations and objectives. The MTD is usually longer than the RTO, as it represents the worst-case scenario.
The recovery point objective (RPO) is the maximum acceptable amount of data loss that an IT service or process can tolerate in the event of a disaster or disruption. The RPO is measured in terms of time, such as hours or minutes, and indicates how frequently the data should be backed up or replicated. The mean time to repair (MTTR) is the average time that it takes to restore an IT service or process after a failure or disruption.
The MTTR is a measure of the efficiency and effectiveness of the recovery process, but it does not reflect the service criticality or the business impact. References: IS Audit and Assurance Tools and Techniques, CISA Certification | Certified Information Systems Auditor | ISACA