CSI (Container Storage Interface)
Excellent question! Let's break down your 5 volume categories and classify them as either:
- ✅ Handled directly by Kubernetes (in-tree)
- ⚠️ Deprecated / moved to CSI
- 🔌 CSI-based (needs external driver)
✅ 1. Ephemeral Volumes¶
These are short-lived volumes tied to the lifecycle of a Pod.
Common Types:¶
| Volume Type | CSI? | Explanation |
|---|---|---|
emptyDir | ❌ No | In-tree, for scratch space during Pod life |
configMap | ❌ No | In-tree, for injecting config |
secret | ❌ No | In-tree, for injecting sensitive data |
downwardAPI | ❌ No | In-tree, for pod metadata |
ephemeral (CSI Ephemeral Volumes) | ✅ Yes | CSI-based, dynamic on-the-fly volumes via CSI drivers |
🔸 Conclusion:
→ Most are in-tree, except ephemeral (CSI variant) which is CSI-based.
💾 2. Persistent Volumes¶
These survive pod restarts and require external storage (EBS, GCE, etc.)
Common Types:¶
| Volume Type | CSI? | Explanation |
|---|---|---|
persistentVolumeClaim | ✅ Yes | Claims CSI-backed PersistentVolumes |
awsElasticBlockStore | ⚠️ Deprecated → CSI | Replaced by ebs.csi.aws.com |
gcePersistentDisk | ⚠️ Deprecated → CSI | Replaced by pd.csi.storage.gke.io |
azureDisk | ⚠️ Deprecated → CSI | Replaced by disk.csi.azure.com |
nfs | ❌ No | Still in-tree (but may use CSI too if needed) |
cephfs, rbd | ⚠️ Deprecated → CSI | Replaced by Ceph CSI drivers |
🔸 Conclusion:
→ Persistent volumes mostly need CSI now.
→ persistentVolumeClaim is CSI entry point, not CSI itself.
🖥 3. Host-Based Volumes¶
These directly access the host filesystem.
Common Types:¶
| Volume Type | CSI? | Explanation |
|---|---|---|
hostPath | ❌ No | In-tree, but not safe for general use |
emptyDir (memory-backed) | ❌ No | Also in-tree |
🔸 Conclusion:
→ These are Kubernetes-native, no CSI needed.
🔗 4. fc (Fibre Channel)¶
Direct access to SAN storage using Fibre Channel protocol.
| Volume Type | CSI? | Explanation |
|---|---|---|
fc | ❌ No | Still in-tree, but requires SAN configuration manually |
| CSI-FC | ✅ Yes | You can use CSI-based FC drivers, but native fc exists |
🔸 Conclusion:
→ Can be used both ways, but CSI is encouraged for automation.
📦 5. projected¶
Used to combine secret, configMap, downwardAPI into one volume.
| Volume Type | CSI? | Explanation |
|---|---|---|
projected | ❌ No | In-tree only, for mixing ephemeral config volumes |
🔸 Conclusion:
→ Purely Kubernetes-native, not CSI.
🔍 Final Classification Table¶
| Category | Volume Types | CSI Required? |
|---|---|---|
| Ephemeral | emptyDir, configMap, secret, downwardAPI, ephemeral | ❌ (except ephemeral = ✅) |
| Persistent | persistentVolumeClaim, awsElasticBlockStore, gcePersistentDisk, etc. | ✅ Mostly CSI now |
| Host-Based | hostPath, emptyDir | ❌ In-tree only |
| fc | fc | ❌ (native) or ✅ (optional CSI) |
| projected | projected | ❌ In-tree only |