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Proposal for adding PVC info to VolumeStats
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Flushes out details for part 1 of the changes described in
kubernetes/community#855

Feature: kubernetes/enhancements#363
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Vaibhav Kamra committed Dec 1, 2021
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# Add PVC reference in Volume Stats

## Background
Pod volume stats tracked by kubelet do not currently include any information about the PVC (if the pod volume was referenced via a PVC)

This prevents exposing (and querying) volume metrics labeled by PVC name which is preferable for users, given that PVC is a top-level API object.

## Proposal

Modify ```VolumeStats``` tracked in Kubelet and populate with PVC info:

```
// VolumeStats contains data about Volume filesystem usage.
type VolumeStats struct {
// Embedded FsStats
FsStats
// Name is the name given to the Volume
// +optional
Name string `json:"name,omitempty"`
+ // PVCRef is a reference to the measured PVC.
+ // +optional
+ PVCRef PVCReference `json:"pvcRef"`
}
+// PVCReference contains enough information to describe the referenced PVC.
+type PVCReference struct {
+ Name string `json:"name"`
+ Namespace string `json:"namespace"`
+}
```

## Implementation
2 options are described below. Option 1 supports current requirements/requested use cases. Option 2 supports an additional use case that was being discussed and is called out for completeness/discussion/feedback.

### Option 1
- Modify ```kubelet::server::stats::calcAndStoreStats()```
- If the pod volume is referenced via a PVC, populate ```PVCRef``` in VolumeStats using the Pod spec

- The Pod spec is already available in this method, so the changes are contained to this function.

- The limitation of this approach is that we're limited to reporting only what is available in the pod spec (Pod namespace and PVC claimname)

### Option 2
- Modify the ```volumemanager::GetMountedVolumesForPod()``` (or add a new function) to return additional volume information from the ASOW/DSOW caches
- Use this to populate PVCRef in VolumeStats

- This allows us to get information not available in the Pod spec such as the PV name/UID which can be used to label metrics - enables exposing/querying volume metrics by PV name
- It's unclear whether this is a use case we need to/should support:
* Volume metrics are only refreshed for mounted volumes which implies a bound/available PVC
* We expect most user-storage interactions to be via the PVC




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