<p>I followed this post <a href="https://stackoverflow.com/questions/44325048/kubernetes-configmap-only-one-file">Kubernetes configMap - only one file</a> to pass a config file to a deployment, but got an error. Why?</p>

<p>The config file <code>config-prom-prometheus.yml</code>:</p>

<pre><code>scrape_configs:
- job_name: job-leo-prometheus
  kubernetes_sd_configs:
  - role: endpoints
</code></pre>

<p>The .yaml file <code>prom-prometheus.yaml</code>:</p>

<pre><code>apiVersion: apps/v1
kind: Deployment
metadata:
  name: prom-prometheus-deployment
spec:
  selector:
    matchLabels:
      app: prom-prometheus
  replicas: 1
  template:
    metadata:
      labels:
        app: prom-prometheus
    spec:
      containers:
      - name: prom-prometheus
        image: 127.0.0.1:30400/prom/prometheus
        ports:
        - name: port9090
          containerPort: 9090
        volumeMounts:
        - name: volume-prometheus
          mountPath: /etc/prometheus/prometheus.yml
          subPath: prometheus.yml
      volumes:
      - name: volume-prometheus
        configMap:
          name: config-prom

---
apiVersion: v1
kind: Service
metadata:
  name: prom-prometheus
spec:
  type: NodePort
  ports:
  - name: port9090
    protocol: TCP
    port: 9090
    targetPort: 9090
    nodePort: 30090
  selector:
    app: prom-prometheus
</code></pre>

<p>Commands:</p>

<pre><code>kubectl create configmap config-prom --from-file=config-prom-prometheus.yml
kubectl -f prom-prometheus.yaml apply
</code></pre>

<p>Results:</p>

<pre><code>Events:
  Type     Reason                 Age               From               Message
  ----     ------                 ----              ----               -------
  Normal   Scheduled              17s               default-scheduler  Successfully assigned prom-prometheus-deployment-66887dcdbf-bfqd4 to minikube
  Normal   SuccessfulMountVolume  17s               kubelet, minikube  MountVolume.SetUp succeeded for volume "default-token-ml6w5"
  Normal   SuccessfulMountVolume  17s               kubelet, minikube  MountVolume.SetUp succeeded for volume "volume-prometheus"
  Warning  Failed                 9s                kubelet, minikube  Error: failed to start container "prom-prometheus": Error response from daemon: OCI runtime create failed: container_linux.go:348: starting container process caused "process_linux.go:402: container init caused \"rootfs_linux.go:58: mounting \\\"/var/lib/kubelet/pods/ec99da92-e994-11e8-a578-08002742f2a3/volume-subpaths/volume-prometheus/prom-prometheus/0\\\" to rootfs \\\"/var/lib/docker/overlay2/12c7da1c07c55fe2ec5dff61e5c457fa8aeaa32d47232c28a1d7e127c4f81bf0/merged\\\" at \\\"/var/lib/docker/overlay2/12c7da1c07c55fe2ec5dff61e5c457fa8aeaa32d47232c28a1d7e127c4f81bf0/merged/etc/prometheus/prometheus.yml\\\" caused \\\"not a directory\\\"\"": unknown: Are you trying to mount a directory onto a file (or vice-versa)? Check if the specified host path exists and is the expected type
  Normal   Pulling                7s (x2 over 13s)  kubelet, minikube  pulling image "127.0.0.1:30400/prom/prometheus"
  Normal   Pulled                 7s (x2 over 13s)  kubelet, minikube  Successfully pulled image "127.0.0.1:30400/prom/prometheus"
  Normal   Created                6s (x2 over 10s)  kubelet, minikube  Created container
  Warning  Failed                 4s                kubelet, minikube  Error: failed to start container "prom-prometheus": Error response from daemon: OCI runtime create failed: container_linux.go:348: starting container process caused "process_linux.go:402: container init caused \"rootfs_linux.go:58: mounting \\\"/var/lib/kubelet/pods/ec99da92-e994-11e8-a578-08002742f2a3/volume-subpaths/volume-prometheus/prom-prometheus/0\\\" to rootfs \\\"/var/lib/docker/overlay2/7b07728ae4439e4d479386eab6b042948e2cb586c54171941f24d03352a7c8b4/merged\\\" at \\\"/var/lib/docker/overlay2/7b07728ae4439e4d479386eab6b042948e2cb586c54171941f24d03352a7c8b4/merged/etc/prometheus/prometheus.yml\\\" caused \\\"not a directory\\\"\"": unknown: Are you trying to mount a directory onto a file (or vice-versa)? Check if the specified host path exists and is the expected type
</code></pre>

<p>This is not well documented but as per my experience name of configmap yaml (config-prom-prometheus.yml in your case) should be the same as <code>mountPath</code> and <code>subPath</code> in <code>Deployment</code>.</p>

<p>If you use <code>subPath: prometheus.yml</code> - rename <code>config-prom-prometheus.yml</code> to <code>prometheus.yml</code> and try again.</p>
