Access Modes and Volume Modes
In Kubernetes, PersistentVolumeClaims (PVCs) and StorageClasses work together to manage how storage is provisioned and accessed by workloads. Two essential concepts in this domain are Access Modes and Volume Modes. This article explores these concepts and highlights how different storage systems support them.
Access Modes in Kubernetes
Access Modes define how a volume can be mounted and used by pods. The key access modes are:
- ReadWriteOnce (RWO): The volume can be mounted as read-write by a single node.
- ReadOnlyMany (ROX): The volume can be mounted as read-only by multiple nodes.
- ReadWriteMany (RWX): The volume can be mounted as read-write by multiple nodes.
Access Modes by Storage Class
| Storage Class | RWO Supported | ROX Supported | RWX Supported |
|---|---|---|---|
| CephFS File Storage | Yes | No | Yes |
| CephRBD Block Storage | Yes | No | No |
| TopoLVM | Yes | No | No |
| NFS Shared Storage | Yes | No | Yes |
As shown above, file-based storage systems like CephFS and NFS support multiple concurrent write or read operations, making them suitable for shared-access scenarios. On the other hand, block storage systems like CephRBD and TopoLVM provide exclusive access to a single node at a time.
Volume Modes in Kubernetes
Volume Modes define how the data is exposed to the pod:
- Filesystem: The volume is mounted into the pod as a filesystem.
- Block: The volume is presented as a raw block device.
Volume Modes by Storage Class
| Storage Class | Type | Supported Volume Modes |
|---|---|---|
| CephFS File Storage | File Storage | Filesystem |
| CephRBD Block Storage | Block Storage | Filesystem, Block |
| TopoLVM | Block Storage | Filesystem, Block |
| NFS Shared Storage | File Storage | Filesystem |
Block storage systems like CephRBD and TopoLVM offer both filesystem and raw block access, providing flexibility for different application needs. File storage systems such as CephFS and NFS, in contrast, only support the filesystem mode.
Storage Features: Snapshots and Expansion
Kubernetes also supports advanced features like volume snapshots and dynamic expansion of PVCs, depending on the storage class used.
| Storage Class | Volume Snapshot | Expansion |
|---|---|---|
| CephFS File Storage | Supported | Supported |
| CephRBD Block Storage | Supported | Supported |
| TopoLVM | Supported | Supported |
| NFS Shared Storage | Not Supported | Not Supported |
Only dynamically provisioned PVCs using a StorageClass support volume snapshots. This feature is useful for backups and cloning environments.
Conclusion
When configuring storage in Kubernetes, understanding the Access Modes and Volume Modes of PVCs and their backing StorageClasses is critical for choosing the right solution for your workload. File storage solutions such as CephFS and NFS are ideal for shared access scenarios, while block storage like CephRBD and TopoLVM excel in high-performance, single-node deployments. Furthermore, support for features like snapshots and expansion can greatly enhance storage flexibility and data management strategies.