218 lines
6.5 KiB
Markdown
218 lines
6.5 KiB
Markdown
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---
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tags: [kubernetes, hpa, autoscaling, statefulset, horizontal-pod-autoscaler]
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create time: 2026-05-18 00:30
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---
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# K8s 扩缩容与有状态应用 — HPA & StatefulSet
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## 概述
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K8s 提供多种扩缩容机制。**HPA** 基于 CPU/Memory 或其他自定义指标实现 Pod 级别的弹性伸缩，适用于无状态服务。**StatefulSet** 则是数据库、消息队列等有状态组件的理想选择。两者各有适用场景。
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## HPA 弹性伸缩
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### 基础示例
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```yaml
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apiVersion: autoscaling/v2
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kind: HorizontalPodAutoscaler
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metadata:
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name: order-service-hpa
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spec:
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scaleTargetRef:
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apiVersion: apps/v1
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kind: Deployment
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name: order-service
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minReplicas: 3
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maxReplicas: 20
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metrics:
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- type: Resource
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resource:
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name: cpu
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target:
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type: Utilization
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averageUtilization: 70 # CPU > 70% 时扩容
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- type: Resource
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resource:
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name: memory
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target:
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type: Utilization
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averageUtilization: 80
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behavior:
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scaleUp:
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stabilizationWindowSeconds: 60 # 扩容稳定期
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policies:
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- type: Pods
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value: 2
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periodSeconds: 60 # 每分钟最多扩 2 个
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scaleDown:
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stabilizationWindowSeconds: 300 # 缩容稳定期 5min（防抖动）
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```
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### 扩展指标：基于自定义 Metric 扩缩容
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```yaml
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metrics:
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- type: Pods
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pods:
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metric:
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name: http_requests_per_second
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target:
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type: AverageValue
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averageValue: "100" # 每个 Pod 平均 100 QPS 时扩容
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- type: Object
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object:
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metric:
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name: active_users
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describedObject:
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apiVersion: apps/v1
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kind: Deployment
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name: web-app
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target:
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type: Value
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value: "500" # 每 500 活跃用户触发一次扩容
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```
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> [!info] HPA 的工作机制
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>
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> HPA 控制器每 15 秒查询 Metrics Server（CPU/Memory）或自定义 Metrics Adapter（QPS 等），计算所需 Replica 数。公式：`ceil(当前指标值 / 目标指标值 × 当前副本数)`。为了防止抖动，scaleDown 的稳定窗口通常比 scaleUp 长很多。
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### HPA 调优最佳实践
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| 参数 | 推荐值 | 说明 |
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|------|--------|------|
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| `averageUtilization` (CPU) | 60~70% | 低于 60% 浪费资源，高于 80% 延迟风险大 |
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| `scaleUp.stabilizationWindow` | 0~60s | 突发流量快速响应 |
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| `scaleDown.stabilizationWindow` | 300~600s | 防止抖动导致的频繁扩缩 |
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| `pods[].periodSeconds` | 60s | 限制扩容频率，避免雪崩 |
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## StatefulSet — 有状态应用
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Deployment 适合无状态服务，而有状态服务（数据库、分布式协调器等）需要 **稳定的网络标识 + 有序的启停**。StatefulSet 就是为此而生的控制器。
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### 完整示例
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```yaml
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apiVersion: apps/v1
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kind: StatefulSet
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metadata:
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name: mysql-cluster
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spec:
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serviceName: mysql # 绑定 Headless Service
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replicas: 3
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podManagementPolicy: Parallel # 并行启动（生产建议 OrderedReady 逐步启动）
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updateStrategy:
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type: RollingUpdate
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rollingUpdate:
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partition: 0 # 从最后一个 Pod 开始逐步滚动
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selector:
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matchLabels:
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app: mysql
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template:
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metadata:
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labels:
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app: mysql
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spec:
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containers:
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- name: mysql
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image: mysql:8.0
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ports:
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- containerPort: 3306
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volumeMounts:
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- name: data
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mountPath: /var/lib/mysql
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volumeClaimTemplates:
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- metadata:
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name: data
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spec:
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accessModes: ["ReadWriteOnce"]
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resources:
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requests:
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storage: 20Gi
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```
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### StatefulSet 的核心特性
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| 特性 | 说明 |
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|------|------|
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| **稳定名称** | Pod 名为 `{statefulset-name}-{ordinal}`，如 `mysql-cluster-0` |
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| **有序部署** | 默认依次启动：0 → 1 → 2（设置 `Parallel` 可并行） |
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| **有序删除** | 反向删除：2 → 1 → 0（保证主从切换安全） |
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| **持久化存储** | `volumeClaimTemplates` 为每个 Pod 独立创建 PVC，Pod 重建后数据不丢失 |
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| **Headless Service** | 需搭配 `ClusterIP: None` 的 Service，每个 Pod 有独立的 DNS 记录 |
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### StatefulSet vs Deployment 对比
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| 维度 | Deployment | StatefulSet |
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|------|-----------|-------------|
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| **Pod 命名** | 随机后缀 | `{name}-{0,1,2,...}` |
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| **存储** | 共享 PVC 或不持久 | 每个 Pod 独立 PVC |
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| **启动顺序** | 并发 | 串行（默认） |
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| **删除顺序** | 随机 | 逆序 |
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| **DNS 稳定性** | 每次重建 IP 变 | 域名不变 |
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| **适用场景** | Web/API 无状态服务 | DB、ZooKeeper、Kafka |
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> [!note] 为什么有状态应用不适合 Deployment？
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> Deployment 会随机销毁和重建 Pod——这意味着 PVC 会被重新绑定到不同的 Pod，存储卷中的数据和节点 ID 完全错位。StatefulSet 保证同一个 Pod 始终使用同一片存储，这对数据库、Kafka、ZooKeeper 等组件至关重要。
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### Headless Service — StatefulSet 的灵魂搭档
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```yaml
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apiVersion: v1
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kind: Service
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metadata:
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name: mysql-headless
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spec:
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clusterIP: None # 关键：不分配 ClusterIP
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selector:
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app: mysql
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ports:
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- port: 3306
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```
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通过 DNS 可以直接访问单个 Pod：
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- `mysql-cluster-0.mysql-headless.production.svc.cluster.local:3306`
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- `mysql-cluster-1.mysql-headless.production.svc.cluster.local:3306`
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## 补充：Vertical Pod Autoscaler (VPA)
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HPA 只管 Pod 数量，不管单 Pod 的资源配置。如果你经常遇到 OOMKill 或 CPU Throttling，可以尝试 VPA：
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```yaml
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apiVersion: autoscaling.k8s.io/v1
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kind: VerticalPodAutoscaler
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metadata:
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name: order-service-vpa
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spec:
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targetRef:
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apiVersion: apps/v1
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kind: Deployment
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name: order-service
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updatePolicy:
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updateMode: "Auto" # Auto / Initial / Off
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resourcePolicy:
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containerPolicy:
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minAllowed:
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cpu: 100m
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memory: 128Mi
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maxAllowed:
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cpu: "2"
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memory: 4Gi
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```
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> [!warning] VPA 的限制
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>
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> VPA 在调整资源时会**驱逐并重建 Pod**。与 HPA 不同，VPA 不能无缝运行——建议在开发/测试环境中先用 `Initial` 模式观察推荐值，确定合理后再切到 `Auto`。生产环境通常结合 HPA（管数量）+ 手动调优 Resources（管质量）。
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## 关联笔记
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- [[../01-核心概念与Deployment]] — Deployment 基础，HPA 的目标控制器
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- [[../02-配置管理]] — StatefulSet 的数据库凭据通过 Secret 注入
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- [[../03-网络与服务发现]] — Headless Service 为 StatefulSet 提供 DNS 解析
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- [[../hhs/MS/05-部署运维/04-SRE实践]] — SLO/Error Budget 决定扩容阈值
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