docs: add guide doc for flex migration

Adding guide doc for flex migration steps.

Signed-off-by: subhamkrai <srai@redhat.com>

Documentation/flex-to-csi-migration.md

@@ -0,0 +1,68 @@
+---
+title: Flex Migration
+weight: 11900
+indent: true
+---
+
+# Flex to CSI Migration
+
+In Rook v1.8, the Flex driver has been deprecated. Before updating to v1.8, any Flex volumes created in previous versions of Rook will need to be converted to Ceph-CSI volumes.
+
+The tool [persistent-volume-migrator](https://github.com/ceph/persistent-volume-migrator) will help automate migration of Flex rbd volumes to Ceph-CSI volumes.
+
+> **Note** Migration of CephFS FlexVolumes is not supported for now.
+
+## Migration Preparation
+
+1. Rook v1.7.x is required. If you have a previous version of Rook running, follow the [upgrade guide](https://rook.io/docs/rook/v1.7/ceph-upgrade.html) to upgrade from previous releases until on v1.7.x.
+2. Enable the CSI driver if not already enabled. See the [operator settings](https://github.com/rook/rook/blob/release-1.7/cluster/examples/kubernetes/ceph/operator.yaml#L29-L32) such as `ROOK_CSI_ENABLE_RBD`.
+3. Confirm the Rook-Ceph cluster is healthy (`ceph status` shows `health: OK`)
+4. Create the CSI storage class to which you want to migrate
+5. Create the migrator pod
+   1) `kubectl create -f https://raw.githubusercontent.com/ceph/persistent-volume-migrator/main/manifests/migrator.yaml`
+
+**NOTE**: The Migration procedure will come with a downtime as we need to scale down the applications using the volumes before migration.
+
+## Migrate a PVC
+
+1. Stop the application pods that are consuming the flex volume(s) that need to be converted.
+2. Connect to migration pod
+   1. `migration_pod=$(kubectl -n rook-ceph get pod -l app=rook-ceph-migrator -o jsonpath='{.items[*].metadata.name}')`
+
+   2. `kubectl -n rook-ceph exec -it "$migration_pod" -- sh`
+3. Run the tool to migrate the PVC. See the [section below](#migration-tool-options) for more details on the options and the sample output.
+   1. `pv-migrator --pvc=<pvc-name> --pvc-ns=<pvc-namespace> --destination-sc=<storageclass-name-to-migrate-in> --rook-ns=<rook-ceph> --ceph-cluster-ns=<ceph-cluster-namespace>`
+4. Start the application pods which were stopped in step 1.
+
+## Migration Tool Options
+
+These are the options for converting a single PVC. For more options, see the [tool documentation](https://github.com/ceph/persistent-volume-migrator), for example to convert all PVCs automatically that belong to the same storage class.
+
+1. `--pvc`: **required**: reads name of the pvc to migrate
+2. `--pvc-ns`: **required**: reads the namespace of the PVC which is going to migrate
+3. `--destination-sc`: **required**: read the name of the storageclass in which you want mirgrate.
+4. `--rook-ns`: **optional** namespace where the rook operator is running. **default: rook-ceph**.
+5. `--ceph-cluster-ns`: **optional** namespace where the ceph cluster is running. **default: rook-ceph**.
+
+```console
+pv-migrator --pvc=rbd-pvc --pvc-ns=default --destination-sc=csi-rook-ceph-block
+```
+
+```console
+I1125 07:56:22.247311      63 log.go:34] Create Kubernetes Client
+I1125 07:56:22.259115      63 log.go:34] List all the PVC from the source storageclass
+I1125 07:56:22.261205      63 log.go:34] 1 PVCs found with source StorageClass
+I1125 07:56:22.261221      63 log.go:34] Start Migration of PVCs to CSI
+I1125 07:56:22.261226      63 log.go:34] migrating PVC "rbd-pvc" from namespace "default"
+I1125 07:56:22.261229      63 log.go:34] Fetch PV information from PVC rbd-pvc
+I1125 07:56:22.266734      63 log.go:34] PV found "pvc-30a01887-8821-4baf-835c-16a7e55ba7f0"
+---
+I1125 07:56:26.483172      63 log.go:34] successfully renamed volume csi-vol-2f8de58f-4dc5-11ec-a130-0242ac110005 -> pvc-30a01887-8821-4baf-835c-16a7e55ba7f0
+I1125 07:56:26.483194      63 log.go:34] Delete old PV object: pvc-30a01887-8821-4baf-835c-16a7e55ba7f0
+I1125 07:56:26.504578      63 log.go:34] waiting for PV pvc-30a01887-8821-4baf-835c-16a7e55ba7f0 in state &PersistentVolumeStatus{Phase:Bound,Message:,Reason:,} to be deleted (0 seconds elapsed)
+I1125 07:56:26.702921      63 log.go:34] deleted persistent volume pvc-30a01887-8821-4baf-835c-16a7e55ba7f0
+I1125 07:56:26.702944      63 log.go:34] successfully migrated pvc rbd-pvc
+I1125 07:56:26.703335      63 log.go:34] Successfully migrated all the PVCs to CSI
+```
+
+After running above command you should see something similar to this output