add info on default PVC size

Author: kaitlynmichael

content/operate/kubernetes/re-clusters/expand-pvc.md

@@ -20,6 +20,14 @@ This process involves deleting and recreating the REC StatefulSet with a larger
 
 {{<note>}}Shrinking (reducing the size) of your PVC is not allowed. This process only allows you to expand (size up) your PVC.{{</note>}}
 
+### Default PVC size
+
+By default, if you omit [`spec.persistentSpec.volumeSize`]({{< relref "/operate/kubernetes/reference/api/redis_enterprise_cluster_api#specpersistentspec" >}}), the operator allocates a persistent volume that is five times (5x) the Redis Enterprise node memory request defined in [`spec.redisEnterpriseNodeResources.requests.memory`]({{< relref "/operate/kubernetes/reference/api/redis_enterprise_cluster_api#specredisenterprisenoderesources" >}}) (per node). This 5x ratio is the recommended minimum capacity.
+
+- If you set [`spec.persistentSpec.volumeSize`]({{< relref "/operate/kubernetes/reference/api/redis_enterprise_cluster_api#specpersistentspec" >}}) explicitly, that exact size is used and the 5x default does not apply.
+- Changing node memory requests does not automatically resize existing PVCs. Use the procedure below to expand the PVC if you want to maintain the 5x ratio after changing memory.
+- See the [volume size recommendations]({{< relref "/operate/kubernetes/recommendations/persistent-volumes#volume-size" >}}) and [hardware requirements]({{< relref "/operate/rs/installing-upgrading/install/plan-deployment/hardware-requirements" >}}) for guidance.
+
 ## Prerequisites
 
 {{<warning>}}Do not change any other REC fields related to the StatefulSet while resizing is in progress.