> For the complete documentation index, see [llms.txt](https://pshizhsysu.gitbook.io/prometheus/llms.txt). Markdown versions of documentation pages are available by appending `.md` to page URLs; this page is available as [Markdown](https://pshizhsysu.gitbook.io/prometheus/ff08-san-ff09-prometheus-gao-jing-chu-li/shi-li-dui-zhu-ji-jin-xing-jian-kong-gao-jing.md).

# 示例 - 对主机进行监控告警

## 准备条件

在主机上安装好NodeExporter

## 定义告警规则

修改prometheus.yml文件，配置告警规则文件host.yml：

```
rule_files:
  - /etc/prometheus/rule_files/host.yml
```

创建文件`/usr/local/prometheus/rule_files/host.yml`，内容如下

```
groups:
- name: Host
  rules:
  - alert: HostCPU
    expr: 100 * (1 - avg(irate(node_cpu_seconds_total{mode="idle"}[2m])) by(instance)) > 80
    for: 5m
    labels:
      serverity: high
    annotations:
      summary: "{{$labels.instance}}: High CPU Usage Detected"
      description: "{{$labels.instance}}: CPU usage is {{$value}}, above 80%"

  - alert: HostMemory
    expr: (node_memory_MemTotal_bytes - node_memory_MemAvailable_bytes) / node_memory_MemTotal_bytes * 100 > 80
    for: 5m
    labels:
      serverity: middle
    annotations:
      summary: "{{$labels.instance}}: High Memory Usage Detected"
      description: "{{$labels.instance}}: Memory Usage i{{ $value }}, above 80%"

  - alert: HostDisk
    expr: 100 * (node_filesystem_size_bytes{fstype=~"xfs|ext4"} - node_filesystem_available_bytes) / node_filesystem_size_bytes > 80
    for: 5m
    labels:
      serverity: low
    annotations:
      summary: "{{$labels.instance}}: High Disk Usage Detected"
      description: "{{$labels.instance}}, mountpoint {{$labels.mountpoint}}: Disk Usage is {{ $value }}, above 80%"
```

然后，重启Prometheus。

访问UI，查看当前告警的活动状态，如下，都为Inactive

![](/files/-MA1FfBj6wa5uQTQTYhE)

此时，我们在主机上可以手动拉高系统的CPU使用率，验证Prometheus的告警流程，在主机上运行以下命令（可以执行命令`pkill -9 dd`停止下面的进程）：

```
$ for i in `seq 1 $(cat /proc/cpuinfo |grep "physical id" |wc -l)`; do dd if=/dev/zero of=/dev/null & done
```

15秒后（因为我们设置的prometheus采集周期为15s），我们就可以从UI上看到cpu使用率为100%

![](/files/-MA1FfBk-2IM-wFn8ago)

同时我们也可以在`Alerts`页面可以看到`Pending`状态中已经有了`HostCPU`，

![](/files/-MA1FfBlTziF6pqZvr8c)

由于我们在告警规则中设置的持续时间为5分钟，所以`HostCPU`的状态还不会变成`Firing`。此时，如果我们执行命令`pkill -9 dd`让CPU的使用率降下去，再去看一下，`HostCPU`的状态又会变成了`Inactive`。

五分钟后，我们再看，`HostCPU`的状态就由`Pending`变成了`Firing`

![](/files/-MA1FfBmxvx9AjaF_3qe)

下一节，我们将介绍如果通过AlertManager将告警发送出去


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