Which metric should be less than the Maximum Tolerable Downtime (MTD)?

The metric that should be less than the Maximum Tolerable Downtime (MTD) is the Recovery Time Objective (RTO). RTO is the targeted duration of time and a service level within which a business process must be restored after a disruption to avoid unacceptable consequences. Therefore, the RTO must be shorter than the MTD, ensuring that recovery efforts can successfully restore normal operations before the maximum acceptable downtime is reached.

In contrast, Mean Time to Repair (MTTR) measures the average time taken to repair a failed component or system. While it is important for understanding operational efficiency, it is not directly tied to MTD in the same way that RTO is. Recovery Point Objective (RPO) refers to the maximum acceptable amount of data loss measured in time and does not directly relate to downtime. Business Impact Analysis (BIA) involves identifying the effects of business disruptions, which inherently includes considerations of MTD but is not a specific measure that needs to fall under it like RTO does.