Proposal to add metadata about owning PVC in RBD Image

Author: Mayank Kumar

contributors/design-proposals/storage/pv-to-rbd-mapping.md

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+# RBD Volume to PV Mapping 
+
+Authors: krmayankk@
+
+### Problem
+
+The RBD Dynamic Provisioner currently generates rbd volume names which are random. 
+The current implementation generates a UUID and the rbd image name becomes 
+image := fmt.Sprintf("kubernetes-dynamic-pvc-%s", uuid.NewUUID()). This RBD image 
+name is stored in the PV. The PV also has a reference to the PVC to which it binds. 
+The problem with this approach is that if there is a catastrophic etcd data loss 
+and all PV's are gone, there is no way to recover the mapping from RBD to PVC. The
+RBD volumes for the customer still exist, but we have no way to tell which rbd 
+volumes belong to which customer.
+
+## Goal
+We want to store some information about the PVC in RBD image name/metadata, so that 
+in catastrophic situations, we can derive the PVC name from rbd image name/metadata
+and allow customer the following options:
+- Backup RBD volume data for specific customers and hand them their copy before deleting 
+  the RBD volume. Without knowing from rbd image name/metadata, which customers they 
+  belong to we cannot hand those customers their data.
+- Create PV with the given RBD name and pre-bind it to the desired PVC so that customer
+  can get its data back.
+
+## Non Goals
+This proposal doesnt attempt to undermine the importance of etcd backups to restore
+data in catastrophic situations. This is one additional line of defense in case our
+backups are not working.
+
+## Motivation
+
+We recently had an etcd data loss which resulted in loss of this rbd to pv mapping 
+and there was no way to restore customer data. This proposal aims to store pvc name 
+as metadata in the RBD image so that in catostrophic scenarios, the mapping can be 
+restored by just looking at the RBD's.
+
+## Current Implementation
+
+```go
+func (r *rbdVolumeProvisioner) Provision() (*v1.PersistentVolume, error) {
+...
+
+ // create random image name
+ image := fmt.Sprintf("kubernetes-dynamic-pvc-%s", uuid.NewUUID())
+ r.rbdMounter.Image = image
+```
+## Finalized Proposal
+Use `rbd image-meta set` command to store additional metadata in the RBD image about the PVC which owns
+the RBD image. 
+
+`rbd image-meta set  --pool hdd kubernetes-dynamic-pvc-fabd715f-0d24-11e8-91fa-1418774b3e9d pvcname <pvcname>`
+`rbd image-meta set  --pool hdd kubernetes-dynamic-pvc-fabd715f-0d24-11e8-91fa-1418774b3e9d pvcnamespace <pvcnamespace>`
+
+### Pros
+- Simple to implement
+- Does not cause regression in RBD image names, which remains same as earlier.
+- The metada information is not immediately visible to RBD admins
+
+### Cons
+- NA
+
+Since this Proposal does not change the RBD image name and is able to store additional metadata about 
+the PVC to which it belongs, this is preferred over other two proposals. Also it does a better job 
+of hiding the PVC name in the metadata rather than making it more obvious in the RBD image name. The
+metadata can only be seen by admins with appropriate permissions to run the rbd image-meta command. In
+addition, this Proposal , doesnt impose any limitations on the length of metadata that can be stored 
+and hence can accomodate any pvc names and namespaces which are stored as arbitrary key value pairs.
+It also leaves room for storing any other metadata about the PVC.
+
+
+## Proposal 1
+
+Make the RBD Image name as base64 encoded PVC name(namespace+name)
+
+```go
+import b64 "encoding/base64"
+...
+
+
+func (r *rbdVolumeProvisioner) Provision() (*v1.PersistentVolume, error) {
+...
+
+ // Create a base64 encoding of the PVC Namespace and Name
+ rbdImageName := b64.StdEncoding.EncodeToString([]byte(r.options.PVC.Name+"/"+r.options.PVC.Namespace))
+
+ // Append the base64 encoding to the string `kubernetes-dynamic-pvc-`
+ rbdImageName = fmt.Sprintf("kubernetes-dynamic-pvc-%s", rbdImageName)
+ r.rbdMounter.Image = rbdImageName
+
+```
+
+### Pros
+- Simple scheme which encodes the fully qualified PVC name in the RBD image name
+
+### Cons
+- Causes regression since RBD image names will change from one version of K8s to another.
+- Some older versions of librbd/krbd start having issues with names longer than 95 characters.
+
+
+## Proposal 2 
+
+Make the RBD Image name as the stringified PVC namespace plus PVC name.
+
+### Pros
+- Simple to implement. 
+
+### Cons
+- Causes regression since RBD image names will change from one version of K8s to another.
+- This exposes the customer name directly to Ceph Admins. Earlier it was hidden as base64 encoding
+
+
+## Misc
+- Document how Pre-Binding of PV to PVC works in dynamic provisioning 
+- Document/Test if there are other issues with restoring PVC/PV after a 
+  etcd backup is restored