🔁 RPO vs RTO
| Term | Stands For | Meaning | Example |
|---|---|---|---|
| RPO | Recovery Point Objective | Maximum acceptable data loss (in time) during a failure | If RPO = 4 hours → You can lose up to 4 hours of data |
| RTO | Recovery Time Objective | Maximum acceptable time to restore service after an incident | If RTO = 2 hours → You must restore service within 2 hours |
💡 RPO is about data loss.
💡 RTO is about downtime duration.
🧮 Uptime Calculation
Uptime is the percentage of time a system is operational over a specific period.
📌 Formula:
Uptime (%) = [(Total Time - Downtime) / Total Time] × 100
📅 Example:
- You want to calculate monthly uptime.
- Month length = 30 days → 30 × 24 = 720 hours
- If the system was down for 3 hours, then:
Uptime = [(720 - 3) / 720] × 100 = (717 / 720) × 100 ≈ 99.58%
💎 Common Uptime Targets
| Uptime % | Max Downtime/Month | Max Downtime/Year |
|---|---|---|
| 99% | ~7.2 hours | ~3.65 days |
| 99.9% | ~43.8 minutes | ~8.76 hours |
| 99.99% | ~4.38 minutes | ~52.56 minutes |
| 99.999% | ~26.3 seconds | ~5.26 minutes |
🧠 Key Metrics: MTBF, MTTR, and IA
🔧 MTBF (Mean Time Between Failures)
- Definition: Average time a system operates without failure.
- Formula:
MTBF = Total Uptime / Number of Failures
- Example: If a server runs for 1,000 hours and fails 4 times:
MTBF = 1,000 / 4 = 250 hours
🛠️ MTTR (Mean Time To Repair)
- Definition: Average time taken to restore a system after failure.
- Formula:
MTTR = Total Downtime / Number of Failures
- Example: If the total downtime across 4 failures is 8 hours:
MTTR = 8 / 4 = 2 hours
💡 IA (Inherent Availability)
- Definition: Ratio of uptime to total time (uptime + downtime).
- Formula:
IA = MTBF / (MTBF + MTTR)
= Uptime / (Uptime + Downtime)
- Example: Using values above (MTBF = 250, MTTR = 2):
IA = 250 / (250 + 2) ≈ 0.992 or 99.2%
🧮 Summary Table
| Metric | Meaning | Formula | Goal |
|---|---|---|---|
| MTBF | Mean time between failures | Uptime / Failures | Higher = Better |
| MTTR | Mean time to repair | Downtime / Failures | Lower = Better |
| IA | Inherent Availability | MTBF / (MTBF + MTTR) | Closer to 1 = Better |
- RPO tells you how much data you can afford to lose.
- RTO tells you how quickly systems must be recovered.
- Uptime reflects how reliable the system is.
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