Kubernetes Pv Access Modes FAQ & Answers

ReadWriteOnce (RWO) allows volume mounted read-write by single node only. Multiple pods on SAME node can mount simultaneously (node-level restriction, not pod-level). Most common mode for stateful workloads. Use cases: Single-instance databases (PostgreSQL, MySQL primary), file-based locks, stateful apps requiring exclusive write access. Supported by: AWS EBS (gp3, io2), GCP Persistent Disk, Azure Disk, Ceph RBD, local volumes. Behavior: PVC bound to PV, first pod on node-A mounts successfully, second pod on node-A also mounts (shares node), third pod on node-B fails with Multi-Attach error. StatefulSet pattern: each pod gets separate PVC/PV (web-0 → pvc-0, web-1 → pvc-1) ensuring no conflicts.

ReadOnlyMany (ROX) allows volume mounted read-only by multiple nodes simultaneously. All pods across cluster can read, no pod can write. Use cases: Shared configuration files (app configs, ML models, static assets), read replicas consuming immutable datasets, content delivery. Supported by: NFS, GCP Persistent Disk (read-only mode), Azure File, CephFS, GlusterFS. Pattern: Single PVC with ROX, multiple pods across nodes mount for reading. Initiate writes externally via separate RWO/RWX volume, then expose via ROX for safe multi-reader access. Example: ML inference pods reading shared model.pkl from NFS - training job writes via RWX, inference pods read via ROX ensuring no accidental modifications.

ReadWriteMany (RWX) allows volume mounted read-write by multiple nodes simultaneously. All pods across cluster can read AND write concurrently. Use cases: Shared storage for distributed apps (shared log aggregation, multi-writer filesystems), content management systems (WordPress multi-instance with shared uploads), distributed caches. Supported by: NFS (most common), Azure File (SMB), CephFS, GlusterFS, AWS EFS (via CSI), GCP Filestore. NOT supported by block storage (EBS, Azure Disk - cannot multi-attach for writes). Performance: Network-based filesystems have higher latency (NFS: 2-5ms vs EBS: 0.5-1ms). Consistency: Concurrent writes require application-level coordination (file locking) - filesystem doesn't guarantee write ordering across nodes. Example: WordPress Deployment with 3 replicas, PVC with RWX storageClassName: nfs-client, all pods mount shared /var/www/html for uploaded media.

ReadWriteOncePod (RWOP, Kubernetes 1.29 stable) allows volume mounted read-write by single pod only across entire cluster (strictest isolation). Ensures exclusive access at pod level, not just node level (stronger than RWO). Use cases: Databases requiring absolute exclusivity (prevents split-brain during failover), license-restricted software (single-instance constraint), compliance requirements (audit logs with single-writer guarantee). Supported by: CSI drivers with SINGLE_NODE_SINGLE_WRITER capability - AWS EBS CSI 1.13+, GCP PD CSI 1.8+, Azure Disk CSI 1.23+. Behavior: First pod mounts successfully, second pod anywhere in cluster (even same node) fails with "volume already in use" error. Example: PostgreSQL primary with RWOP PVC ensures no accidental multi-master scenario during pod rescheduling.

Common pitfalls: (1) RWO doesn't mean single pod: Multiple pods on same node can mount RWO volume. For single-pod guarantee, use RWOP. (2) RWX not available on EBS/Azure Disk: Attempting RWX PVC with gp3 storage class fails with ProvisioningFailed (block storage limitation). Solution: Switch to NFS/EFS storage class or redesign for pod-local storage. (3) Data corruption with RWX: Concurrent writes without locking cause file corruption. Two pods writing same log file simultaneously → interleaved corrupted entries. Solution: Application-level file locking (flock), database-backed storage, or message queue pattern. (4) Zone affinity with RWO: PV in us-east-1a, pod in us-east-1b fails to mount. Solution: Use volumeBindingMode: WaitForFirstConsumer in StorageClass (delays provisioning until pod scheduled, ensures same zone).