Introduction

Understanding Capacity Planning for SRE

Before diving into specific methodologies, let’s establish what capacity planning means in the context of Site Reliability Engineering.

What is Capacity Planning?

Capacity planning is the process of determining the resources required to meet expected workloads while maintaining service level objectives (SLOs). For SRE teams, this involves:

1. Forecasting demand: Predicting future workload based on historical data and business projections
2. Resource modeling: Understanding how workload translates to resource requirements
3. Capacity allocation: Provisioning appropriate resources across services and regions
4. Performance analysis: Ensuring systems meet performance targets under expected load
5. Cost optimization: Balancing reliability requirements with infrastructure costs

Why Capacity Planning Matters for SRE

Effective capacity planning directly impacts several key aspects of reliability engineering:

1. Reliability: Ensuring sufficient capacity to handle expected and unexpected loads
2. Performance: Maintaining response times and throughput under varying conditions
3. Cost efficiency: Avoiding over-provisioning while maintaining reliability
4. Incident prevention: Proactively addressing capacity issues before they cause outages
5. Scalability: Supporting business growth without service degradation

The Capacity Planning Lifecycle

Capacity planning is not a one-time activity but a continuous process:

┌─────────────────┐
│                 │
│  Collect Data   │
│                 │
└────────┬────────┘
         │
         ▼
┌─────────────────┐
│                 │
│  Analyze Trends │
│                 │
└────────┬────────┘
         │
         ▼
┌─────────────────┐
│                 │
│  Forecast Demand│
│                 │
└────────┬────────┘
         │
         ▼
┌─────────────────┐
│                 │
│  Model Resource │
│  Requirements   │
│                 │
└────────┬────────┘
         │
         ▼
┌─────────────────┐
│                 │
│  Plan Capacity  │
│                 │
└────────┬────────┘
         │
         ▼
┌─────────────────┐
│                 │
│  Implement      │
│  Changes        │
│                 │
└────────┬────────┘
         │
         ▼
┌─────────────────┐
│                 │
│  Monitor and    │
│  Validate       │
│                 │
└────────┬────────┘
         │
         └─────────────► (Back to Collect Data)

Key Metrics for Capacity Planning

Effective capacity planning relies on tracking and analyzing the right metrics.

Resource Utilization Metrics

These metrics measure how much of your available resources are being used:

1. CPU Utilization: Percentage of CPU capacity being used

   • Target: Typically 60-80% for headroom
   • Formula: (CPU time used / CPU time available) * 100%

2. Memory Utilization: Percentage of memory being used

   • Target: Typically 70-85% for headroom
   • Formula: (Memory used / Total memory) * 100%

3. Disk Utilization: Percentage of storage capacity being used

   • Target: Typically <80% for performance reasons
   • Formula: (Disk space used / Total disk space) * 100%

4. Network Utilization: Percentage of network bandwidth being used

   • Target: Typically <70% to avoid congestion
   • Formula: (Network traffic / Network capacity) * 100%