OPen shitf PV PVC to mount to multi‐replica deployment - unix1998/technical_notes GitHub Wiki

When using Persistent Volumes (PVs) and Persistent Volume Claims (PVCs) with a Jenkins deployment in OpenShift, you need to be careful about how the storage is mounted and accessed. By default, most types of PVs are designed to be mounted as ReadWriteOnce (RWO), which means they can only be mounted by a single pod at a time. Here's how you can handle this scenario:

Single Writer Scenario (ReadWriteOnce)

If your PV is of type ReadWriteOnce (RWO), only one Jenkins pod can write to it at any given time. This can be problematic when you have multiple replicas. However, Jenkins typically uses its storage for configurations, plugins, and job workspace, which often needs to be accessible by all replicas. Here are the possible solutions:

1. NFS or Shared Storage: Use a storage solution that supports ReadWriteMany (RWX) access mode. For example, NFS, GlusterFS, or other shared file systems can be used to allow multiple Jenkins pods to access the same storage simultaneously.

2. Helm Chart for Jenkins: The official Jenkins Helm chart provides a way to deploy Jenkins with persistent storage and handles this correctly. It often uses an RWX volume for Jenkins home directory.

3. Distributed Filesystem: Use a distributed filesystem like CephFS, which allows multiple pods to read and write to the same volume.

Example Setup with NFS (ReadWriteMany)

1. Provision an NFS Server: Ensure you have an NFS server running and export a directory for Jenkins.

2. Create a Persistent Volume:

   apiVersion: v1
   kind: PersistentVolume
   metadata:
     name: jenkins-pv
   spec:
     capacity:
       storage: 10Gi
     accessModes:
       - ReadWriteMany
     nfs:
       path: /path/to/nfs/export
       server: nfs-server.example.com
   

3. Create a Persistent Volume Claim:

   apiVersion: v1
   kind: PersistentVolumeClaim
   metadata:
     name: jenkins-pvc
   spec:
     accessModes:
       - ReadWriteMany
     resources:
       requests:
         storage: 10Gi
   

4. Mount the PVC in the Jenkins Deployment:

   apiVersion: apps/v1
   kind: Deployment
   metadata:
     name: jenkins
   spec:
     replicas: 3
     selector:
       matchLabels:
         app: jenkins
     template:
       metadata:
         labels:
           app: jenkins
       spec:
         containers:
         - name: jenkins
           image: jenkins/jenkins:lts
           ports:
           - containerPort: 8080
           volumeMounts:
           - mountPath: /var/jenkins_home
             name: jenkins-storage
         volumes:
         - name: jenkins-storage
           persistentVolumeClaim:
             claimName: jenkins-pvc
   

Considerations

1. Concurrency: Ensure your Jenkins setup can handle concurrent access. Some configurations or plugins might not work well with concurrent writes, so test your setup thoroughly.
2. Data Consistency: Consider how data consistency is managed. Using shared storage can introduce complexities in ensuring data consistency.
3. Performance: Shared storage might introduce performance bottlenecks, especially if Jenkins performs I/O-intensive tasks.

Conclusion

Using PVs and PVCs with Jenkins in OpenShift is feasible with the right storage backend. For multiple replicas to access the same storage, ensure you use a PV type that supports ReadWriteMany (RWX). NFS, GlusterFS, CephFS, or similar shared filesystems are good options. Make sure your Jenkins setup and plugins can handle concurrent access to avoid data corruption or other issues.