diff --git a/controllers/machineset_controller.go b/controllers/machineset_controller.go
index dbeba3f..befc96f 100644
--- a/controllers/machineset_controller.go
+++ b/controllers/machineset_controller.go
@@ -31,6 +31,7 @@ const (
 	memoryKey = "machine.openshift.io/memoryMb"
 	gpuKey    = "machine.openshift.io/GPU"
 	labelsKey = "capacity.cluster-autoscaler.kubernetes.io/labels"
+	diskKey   = "capacity.cluster-autoscaler.kubernetes.io/ephemeral-disk"
 
 	gpuKeyValue = "0"
 	arch        = "kubernetes.io/arch=amd64"
@@ -67,9 +68,15 @@ func (r *MachineSetReconciler) Reconcile(ctx context.Context, req ctrl.Request)
 		return ctrl.Result{}, fmt.Errorf("failed to parse flavor %q: %w", spec.Flavor, err)
 	}
 
+	if spec.RootVolumeSizeGB == 0 {
+		return ctrl.Result{}, fmt.Errorf("root volume size is not set")
+	}
+
 	machineSet.Annotations[cpuKey] = strconv.Itoa(flavor.CPU)
 	machineSet.Annotations[memoryKey] = strconv.Itoa(flavor.MemGB * 1024)
 	machineSet.Annotations[gpuKey] = gpuKeyValue
+	// According to https://www.cloudscale.ch/en/api/v1#create-a-server GB here means GiB
+	machineSet.Annotations[diskKey] = fmt.Sprintf("%dGi", spec.RootVolumeSizeGB)
 
 	// We guarantee that any existing labels provided via the capacity annotations are preserved.
 	// See https://github.com/kubernetes/autoscaler/pull/5382 and https://github.com/kubernetes/autoscaler/pull/5697
diff --git a/controllers/machineset_controller_test.go b/controllers/machineset_controller_test.go
index 3b3d42d..f6a6bd0 100644
--- a/controllers/machineset_controller_test.go
+++ b/controllers/machineset_controller_test.go
@@ -14,6 +14,8 @@ import (
 	ctrl "sigs.k8s.io/controller-runtime"
 	"sigs.k8s.io/controller-runtime/pkg/client"
 	"sigs.k8s.io/controller-runtime/pkg/client/fake"
+
+	csv1beta1 "github.com/appuio/machine-api-provider-cloudscale/api/cloudscale/provider/v1beta1"
 )
 
 func Test_MachineSetReconciler_Reconcile(t *testing.T) {
@@ -37,7 +39,12 @@ func Test_MachineSetReconciler_Reconcile(t *testing.T) {
 		Spec: machinev1beta1.MachineSetSpec{},
 	}
 
-	setFlavorOnMachineSet(ms, "plus-4-2")
+	providerData := csv1beta1.CloudscaleMachineProviderSpec{
+		Flavor:           "plus-4-2",
+		RootVolumeSizeGB: 50,
+	}
+
+	setMachineSetProviderData(ms, &providerData)
 
 	c := fake.NewClientBuilder().
 		WithScheme(scheme).
@@ -57,10 +64,11 @@ func Test_MachineSetReconciler_Reconcile(t *testing.T) {
 	assert.Equal(t, "4096", updated.Annotations[memoryKey])
 	assert.Equal(t, "0", updated.Annotations[gpuKey])
 	assert.Equal(t, "a=a,b=b,kubernetes.io/arch=amd64", updated.Annotations[labelsKey])
+	assert.Equal(t, "50Gi", updated.Annotations[diskKey])
 }
 
-func setFlavorOnMachineSet(machine *machinev1beta1.MachineSet, flavor string) {
+func setMachineSetProviderData(machine *machinev1beta1.MachineSet, providerData *csv1beta1.CloudscaleMachineProviderSpec) {
 	machine.Spec.Template.Spec.ProviderSpec.Value = &runtime.RawExtension{
-		Raw: []byte(fmt.Sprintf(`{"flavor": "%s"}`, flavor)),
+		Raw: []byte(fmt.Sprintf(`{"flavor": "%s", "rootVolumeSizeGB": %d}`, providerData.Flavor, providerData.RootVolumeSizeGB)),
 	}
 }