容量規劃

依資料驅動之容量規劃，預測資源需求並防止飽和。

適用時機

• 季節性流量高峰前（節慶、促銷活動）
• 規劃新功能上線時
• 季度容量檢視期間
• 資源使用率呈上升趨勢時
• 預算規劃週期前

輸入

• 必要：歷史指標（CPU、記憶體、磁碟、網路、每秒請求數）
• 必要：趨勢分析之時間範圍（至少 4 週）
• 選擇性：業務成長預測（預期使用者成長、功能上線）
• 選擇性：預算限制

步驟

步驟一：蒐集歷史指標

向 Prometheus 查詢關鍵資源指標：

# CPU usage trend over 8 weeks
avg(rate(node_cpu_seconds_total{mode!="idle"}[5m])) by (instance)

# Memory usage trend
avg(node_memory_MemTotal_bytes - node_memory_MemAvailable_bytes) by (instance)

# Disk usage growth
avg(node_filesystem_size_bytes - node_filesystem_free_bytes) by (instance, device)

# Request rate growth
sum(rate(http_requests_total[5m])) by (service)

# Database connection pool usage
avg(db_connection_pool_used / db_connection_pool_max) by (instance)

匯出以分析：

# Export 8 weeks of CPU data
curl -G 'http://prometheus:9090/api/v1/query_range' \
  --data-urlencode 'query=avg(rate(node_cpu_seconds_total{mode!="idle"}[5m])) by (instance)' \
  --data-urlencode 'start=2024-12-15T00:00:00Z' \
  --data-urlencode 'end=2025-02-09T00:00:00Z' \
  --data-urlencode 'step=1h' | jq '.data.result' > cpu_8weeks.json

預期： 各資源之乾淨時間序列資料，無大幅缺漏。

失敗時： 資料缺漏將降低預測準確度。檢查指標保留期與抓取間隔。

步驟二：以 predict_linear 計算成長率

以 Prometheus 之 predict_linear() 預測飽和：

# Predict when CPU will hit 80% (4 weeks ahead)
predict_linear(
  avg(rate(node_cpu_seconds_total{mode!="idle"}[5m]))[8w:],
  4*7*24*3600  # 4 weeks in seconds
) > 0.80

# Predict disk full date (8 weeks ahead)
predict_linear(
  avg(node_filesystem_size_bytes - node_filesystem_free_bytes)[8w:],
  8*7*24*3600
) > 0.95 * avg(node_filesystem_size_bytes)

# Predict memory pressure (2 weeks ahead)
predict_linear(
  avg(node_memory_MemTotal_bytes - node_memory_MemAvailable_bytes)[8w:],
  2*7*24*3600
) / avg(node_memory_MemTotal_bytes) > 0.90

# Predict request rate capacity breach (4 weeks ahead)
predict_linear(
  sum(rate(http_requests_total[5m]))[8w:],
  4*7*24*3600
) > 10000  # known capacity limit

建立預測儀表板：

{
  "dashboard": {
    "title": "Capacity Forecast",
    "panels": [
      {
        "title": "CPU Saturation Forecast (4 weeks)",
        "targets": [
          {
            "expr": "predict_linear(avg(rate(node_cpu_seconds_total{mode!=\"idle\"}[5m]))[8w:], 4*7*24*3600)",
            "legendFormat": "Predicted CPU"
          },
          {
            "expr": "0.80",
            "legendFormat": "Target Threshold (80%)"
          }
        ]
      },
      {
        "title": "Disk Full Date",
        "targets": [
          {
            "expr": "(avg(node_filesystem_size_bytes) - predict_linear(avg(node_filesystem_free_bytes)[8w:], 8*7*24*3600)) / avg(node_filesystem_size_bytes)",
            "legendFormat": "Predicted Usage %"
          }
        ]
      }
    ]
  }
}

預期： 清晰之視覺化，顯示資源何時將觸及門檻。

失敗時： 若預測結果有異（負值、劇烈波動），檢查：

• 歷史不足（至少需 4 週）
• 階梯式尖峰（部署、遷移）扭曲趨勢
• 線性模型未捕捉之季節性

步驟三：計算當前餘裕

定飽和前之安全邊際：

# CPU headroom (percentage remaining before 80% threshold)
(0.80 - avg(rate(node_cpu_seconds_total{mode!="idle"}[5m]))) / 0.80 * 100

# Memory headroom (bytes remaining before 90% usage)
avg(node_memory_MemAvailable_bytes) - (avg(node_memory_MemTotal_bytes) * 0.10)

# Request rate headroom (requests/sec before saturation)
10000 - sum(rate(http_requests_total[5m]))

# Time until saturation (weeks until CPU hits 80%)
(0.80 - avg(rate(node_cpu_seconds_total{mode!="idle"}[5m]))) /
  deriv(avg(rate(node_cpu_seconds_total{mode!="idle"}[5m]))[8w:]) /
  (7*24*3600)

製作餘裕摘要報告：

cat > capacity_headroom.md <<'EOF'
# Capacity Headroom Report (2025-02-09)

## Current Utilization
- **CPU**: 45% average (target: <80%)
- **Memory**: 62% (target: <90%)
- **Disk**: 71% (target: <95%)
- **Request Rate**: 4,200 req/s (capacity: 10,000)

## Headroom Analysis
- **CPU**: 35% headroom → ~12 weeks until saturation
- **Memory**: 28% headroom → ~16 weeks until saturation
- **Disk**: 24% headroom → ~8 weeks until full
- **Request Rate**: 5,800 req/s headroom → ~20 weeks until capacity

## Priority Actions
1. **Disk**: Implement log rotation or expand volume within 4 weeks
2. **CPU**: Plan horizontal scaling in next quarter
3. **Memory**: Monitor but no immediate action needed
EOF

預期： 各資源之量化餘裕，附飽和時間估計。

失敗時： 若餘裕已為負，已陷被動模式。需立即擴容。

步驟四：建模成長情境

納入業務預測：

# Example Python script for scenario modeling
import pandas as pd
import numpy as np

# Load historical data
df = pd.read_json('cpu_8weeks.json')

# Calculate weekly growth rate
growth_rate_weekly = df['value'].pct_change(periods=7).mean()

# Scenario 1: Current trend
weeks_ahead = 12
current_trend = df['value'].iloc[-1] * (1 + growth_rate_weekly) ** weeks_ahead

# Scenario 2: 2x user growth (marketing campaign)
accelerated_trend = df['value'].iloc[-1] * (1 + growth_rate_weekly * 2) ** weeks_ahead

# Scenario 3: New feature launch (+30% baseline)
feature_launch = (df['value'].iloc[-1] * 1.30) * (1 + growth_rate_weekly) ** weeks_ahead

print(f"Current Trend (12 weeks): {current_trend:.1%} CPU")
print(f"2x Growth Scenario: {accelerated_trend:.1%} CPU")
print(f"Feature Launch Scenario: {feature_launch:.1%} CPU")
print(f"Threshold: 80%")

預期： 多種情境，呈現業務變動對容量之影響。

失敗時： 若情境超出容量，於事件前優先擴容。

步驟五：產生擴容建議

擬定可執行之建議：

## Capacity Scaling Plan

### Immediate Actions (Next 4 Weeks)
1. **Disk Expansion** [Priority: HIGH]
   - Current: 500GB, 71% used
   - Projected full date: 2025-04-01 (8 weeks)
   - Action: Expand to 1TB by 2025-03-15
   - Cost: $50/month additional
   - Justification: 5 weeks lead time needed

2. **Log Rotation Policy** [Priority: MEDIUM]
   - Current: Logs retained 90 days
   - Action: Reduce to 30 days, archive to S3
   - Savings: ~150GB disk space
   - Cost: $5/month S3 storage

### Near-Term Actions (Next Quarter)
3. **Horizontal Scaling - API Tier** [Priority: MEDIUM]
   - Current: 4 instances, 45% CPU
   - Projected: 65% CPU by 2025-05-01
   - Action: Add 2 instances (to 6 total)
   - Cost: $400/month
   - Trigger: When CPU avg exceeds 60% for 7 days

4. **Database Connection Pool** [Priority: LOW]
   - Current: 50 max connections, 40% used
   - Projected: 55% by Q3
   - Action: Increase to 75 in Q2
   - Cost: None (configuration change)

### Long-Term Planning (Next 6 Months)
5. **Migration to Auto-Scaling** [Priority: MEDIUM]
   - Current: Manual scaling
   - Action: Implement Kubernetes HPA (Horizontal Pod Autoscaler)
   - Timeline: Q3 2025
   - Benefit: Automatic response to load spikes

預期： 已排序之清單，附成本、時程與觸發條件。

失敗時： 若建議因成本被否，重新檢視門檻或接受風險。

步驟六：設置容量告警

對低餘裕設置告警：

# capacity_alerts.yml
groups:
  - name: capacity
    interval: 1h
    rules:
      - alert: CPUCapacityLow
        expr: |
          (0.80 - avg(rate(node_cpu_seconds_total{mode!="idle"}[5m]))) / 0.80 < 0.20
        for: 24h
        labels:
          severity: warning
        annotations:
          summary: "CPU headroom below 20%"
          description: "Current CPU headroom: {{ $value | humanizePercentage }}. Scaling needed within 4 weeks."

      - alert: DiskFillForecast
        expr: |
          predict_linear(avg(node_filesystem_free_bytes)[8w:], 4*7*24*3600) < 0.10 * avg(node_filesystem_size_bytes)
        for: 1h
        labels:
          severity: warning
        annotations:
          summary: "Disk projected to fill within 4 weeks"
          description: "Expand disk volume soon."

      - alert: MemoryCapacityLow
        expr: |
          avg(node_memory_MemAvailable_bytes) < 0.15 * avg(node_memory_MemTotal_bytes)
        for: 6h
        labels:
          severity: warning
        annotations:
          summary: "Memory headroom below 15%"

預期： 告警於飽和前觸發，留下主動擴容之時間。

失敗時： 若告警過頻（告警疲勞）或過遲（被動應急），調整門檻。

驗證

• 歷史指標至少涵蓋 8 週
• predict_linear() 查詢回傳合理預測（無負值）
• 已對所有關鍵資源計算餘裕
• 成長情境已納入業務預測
• 擴容建議附成本與時程
• 容量告警已設置並測試
• 報告已與工程領導及財務檢視

常見陷阱

• 歷史不足：線性預測需至少 4 週資料；不足則預測不可靠。
• 忽略階梯變動：部署、遷移或功能上線造成之尖峰將扭曲趨勢，宜過濾或標註。
• 線性假設：並非所有成長皆線性；指數成長（病毒式產品）需另行建模。
• 遺忘前置時間：雲端配置雖快，採購、預算與遷移需數週。早作規劃。
• 未對齊預算：未取得預算共識之容量規劃將導致最後關頭之倉促；應及早納入財務。

相關技能

• setup-prometheus-monitoring —— 蒐集容量規劃所用之指標
• build-grafana-dashboards —— 視覺化預測與餘裕
• optimize-cloud-costs —— 在容量規劃與成本最佳化間取得平衡