server/rook/builtin-mgr.yaml

# https://github.com/rook/rook/issues/11368
apiVersion: ceph.rook.io/v1
kind: CephBlockPool
metadata:
  # If the built-in Ceph pool used by the Ceph mgr needs to be configured with alternate
  # settings, create this pool with any of the pool properties. Create this pool immediately
  # with the cluster CR, or else some properties may not be applied when Ceph creates the
  # pool by default.
  name: builtin-mgr
  namespace: rook-ceph
spec:
  # The required pool name with underscores cannot be specified as a K8s resource name, thus we override
  # the pool name created in Ceph with this name property.
  name: .mgr
  failureDomain: host
  replicated:
    size: 3
    requireSafeReplicaSize: true
  deviceClass: ssd
  parameters:
    compression_mode: none
  mirroring:
    enabled: false