Estimating the Hardware MTTR | ||
Where are hardware spares kept? | How is site manned? | Estimated MTTR |
Onsite | 24 hours a day | 30 minutes |
Onsite | Operator is on call 24 hours a day | 2 hours |
Onsite | Regular working hours on week days as well as weekends and holidays | 14 hours |
Onsite | Regular working hours on week days only | 3 days |
Offsite. Shipped by courier when fault condition is encountered. | Operator paged by system when a fault is detected. | 1 week |
Offsite. Maintained in an operator controlled warehouse | System is remotely located. Operator needs to be flown in to replace the hardware. | 2 week |
Estimating Software MTTR | ||
Software fault recovery mechanism | Software reboot mechanism on fault detection | Estimate MTTR |
Software failure is detected by watchdog and/or health messages | Processor automatically reboots from a ROM resident image. | 30 seconds |
Software failure is detected by watchdog and/or health messages | Processor automatically restarts the offending tasks, without needing an operating system reboot | 30 seconds |
Software failure is detected by watchdog and/or health messages | Processor automatically reboots and the operating system reboots from disk image and restarts applications | 3 minutes |
Software failure is detected by watchdog and/or health messages | Processor automatically reboots and the operating system and application images have to be download from another machine | 10 minutes |
Software failure detection is not supported. | Manually operator reboot is required. | 30 minutes to 2 weeks (software MTTR is same as hardware MTTR) |
Availability | Downtime |
90% (1-nine) | 36.5 days/year |
99% (2-nines) | 3.65 days/year |
99.9% (3-nines) | 8.76 hours/year |
99.99% (4-nines) | 52 minutes/year |
99.999% (5-nines) | 5 minutes/year |
99.9999% (6-nines) | 31 seconds/year ! |