doc: add documentation regarding read affinity

This commit adds documentation about read affinity supported
for rbd volumes.

Signed-off-by: Rakshith R <rar@redhat.com>

deploy/rbd/kubernetes/csi-rbdplugin.yaml

@@ -68,6 +68,15 @@ spec:
             # and pass the label names below, for CSI to consume and advertise
             # its equivalent topology domain
             # - "--domainlabels=failure-domain/region,failure-domain/zone"
+            #
+            # Options to enable read affinity.
+            # If enabled Ceph CSI will fetch labels from kubernetes node
+            # pass `read_from_replica=localize,crush_location=type:value` during
+            # rbd map command. refer:
+            # https://docs.ceph.com/en/latest/man/8/rbd/#kernel-rbd-krbd-options
+            # for more details.
+            # - "--enable-read-affinity=true"
+            # - "--crush-location-labels=topology.io/zone,topology.io/rack"
           env:
             - name: POD_IP
               valueFrom:

docs/deploy-rbd.md

@@ -48,6 +48,8 @@ make image-cephcsi
 | `--skipforceflatten`     | `false`                       | skip image flattening on kernel < 5.2 which support mapping of rbd images which has the deep-flatten feature                                                                                                                                                                         |
 | `--maxsnapshotsonimage`  | `450`                         | Maximum number of snapshots allowed on rbd image without flattening                                                                                                                                                                                                                  |
 | `--setmetadata`          | `false`                       | Set metadata on volume                                                                                                                                                                                                                                                               |
+| `--enable-read-affinity` | `false`                       | enable read affinity                                                                                                                                                                                                                                                                 |
+| `--crush-location-labels`| _empty_                       | Kubernetes node labels that determines the crush location the node belongs to, separated by ','                                                                                                                                                                                      |
 
 **Available volume parameters:**
 
@@ -196,6 +198,24 @@ The Helm chart is located in `charts/ceph-csi-rbd`.
 
 [See the Helm chart readme for installation instructions.](../charts/ceph-csi-rbd/README.md)
 
+## Read Affinity using crush locations for RBD volumes
+
+Ceph CSI supports mapping RBD volumes with krbd options
+`"read_from_replica=localize,crush_location=type1:value1|type2:value2"` to achieve
+reads from the most local serving OSD.
+Refer [krbd-options](https://docs.ceph.com/en/latest/man/8/rbd/#kernel-rbd-krbd-options)
+for more details.
+
+This can be enabled by adding labels to Kubernetes nodes in the format `"topology.io/zone=east"`
+and passing cmdline arguments `"--enable-read-affinity=true"` and
+`"--crush-location-labels=topology.io/zone,topology.io/region"`
+to Ceph CSI RBD daemonset pod "csi-rbdplugin" container, resulting in Ceph CSI adding
+`"--options read_from_replica=localize,crush_location=zone:east"` krbd options during
+rbd map operation.
+If enabled, this option will be added to all RBD volumes mapped by Ceph CSI.
+Well known labels can be found
+[here](https://kubernetes.io/docs/reference/labels-annotations-taints/).
+
 ## Encryption for RBD volumes
 
 > Enabling encryption on volumes created without encryption is **not supported**