diff --git a/config/internal.go b/config/internal.go
index 2e71ac16eb7..318bb339232 100644
--- a/config/internal.go
+++ b/config/internal.go
@@ -1,7 +1,8 @@
 package config
 
 type Internal struct {
-	Bitswap *InternalBitswap `json:",omitempty"` // This is omitempty since we are expecting to make changes to all subcomponents of Internal
+	Bitswap                     *InternalBitswap `json:",omitempty"` // This is omitempty since we are expecting to make changes to all subcomponents of Internal
+	UnixFSShardingSizeThreshold *OptionalString  `json:",omitempty"`
 }
 
 type InternalBitswap struct {
diff --git a/config/types.go b/config/types.go
index ac90fa9b82a..baa073ab460 100644
--- a/config/types.go
+++ b/config/types.go
@@ -313,3 +313,55 @@ func (p OptionalInteger) String() string {
 
 var _ json.Unmarshaler = (*OptionalInteger)(nil)
 var _ json.Marshaler = (*OptionalInteger)(nil)
+
+// OptionalString represents a string that has a default value
+//
+// When encoded in json, Default is encoded as "null"
+type OptionalString struct {
+	value *string
+}
+
+// WithDefault resolves the integer with the given default.
+func (p *OptionalString) WithDefault(defaultValue string) (value string) {
+	if p == nil || p.value == nil {
+		return defaultValue
+	}
+	return *p.value
+}
+
+// IsDefault returns if this is a default optional integer
+func (p *OptionalString) IsDefault() bool {
+	return p == nil || p.value == nil
+}
+
+func (p OptionalString) MarshalJSON() ([]byte, error) {
+	if p.value != nil {
+		return json.Marshal(p.value)
+	}
+	return json.Marshal(nil)
+}
+
+func (p *OptionalString) UnmarshalJSON(input []byte) error {
+	switch string(input) {
+	case "null", "undefined":
+		*p = OptionalString{}
+	default:
+		var value string
+		err := json.Unmarshal(input, &value)
+		if err != nil {
+			return err
+		}
+		*p = OptionalString{value: &value}
+	}
+	return nil
+}
+
+func (p OptionalString) String() string {
+	if p.value == nil {
+		return "default"
+	}
+	return fmt.Sprintf("%d", p.value)
+}
+
+var _ json.Unmarshaler = (*OptionalInteger)(nil)
+var _ json.Marshaler = (*OptionalInteger)(nil)
diff --git a/config/types_test.go b/config/types_test.go
index 06ea73a260c..2477f8bcf9a 100644
--- a/config/types_test.go
+++ b/config/types_test.go
@@ -389,10 +389,106 @@ func TestOptionalInteger(t *testing.T) {
 	for _, invalid := range []string{
 		"foo", "-1.1", "1.1", "0.0", "[]",
 	} {
-		var p Priority
+		var p OptionalInteger
 		err := json.Unmarshal([]byte(invalid), &p)
 		if err == nil {
 			t.Errorf("expected to fail to decode %s as a priority", invalid)
 		}
 	}
 }
+
+func TestOptionalString(t *testing.T) {
+	makeStringPointer := func(v string) *string {
+		return &v
+	}
+
+	var defaultOptionalString OptionalString
+	if !defaultOptionalString.IsDefault() {
+		t.Fatal("should be the default")
+	}
+	if val := defaultOptionalString.WithDefault(""); val != "" {
+		t.Errorf("optional integer should have been empty, got %s", val)
+	}
+
+	if val := defaultOptionalString.WithDefault("foo"); val != "foo" {
+		t.Errorf("optional integer should have been foo, got %s", val)
+	}
+
+	var filledStr OptionalString
+	filledStr = OptionalString{value: makeStringPointer("foo")}
+	if filledStr.IsDefault() {
+		t.Fatal("should not be the default")
+	}
+	if val := filledStr.WithDefault("bar"); val != "foo" {
+		t.Errorf("optional integer should have been foo, got %s", val)
+	}
+
+	filledStr = OptionalString{value: makeStringPointer("")}
+	if val := filledStr.WithDefault("foo"); val != "" {
+		t.Errorf("optional integer should have been 0, got %s", val)
+	}
+
+	for jsonStr, goValue := range map[string]OptionalString{
+		"null":     {},
+		"\"0\"":    {value: makeStringPointer("0")},
+		`"1"`:      {value: makeStringPointer("1")},
+		`"-1"`:     {value: makeStringPointer("-1")},
+		`"qwerty"`: {value: makeStringPointer("qwerty")},
+	} {
+		var d OptionalString
+		err := json.Unmarshal([]byte(jsonStr), &d)
+		if err != nil {
+			t.Fatal(err)
+		}
+
+		if goValue.value == nil && d.value == nil {
+		} else if goValue.value == nil && d.value != nil {
+			t.Errorf("expected default, got %s", d)
+		} else if *d.value != *goValue.value {
+			t.Fatalf("expected %s, got %s", goValue, d)
+		}
+
+		// Reverse
+		out, err := json.Marshal(goValue)
+		if err != nil {
+			t.Fatal(err)
+		}
+		if string(out) != jsonStr {
+			t.Fatalf("expected %s, got %s", jsonStr, string(out))
+		}
+	}
+
+	// marshal with omitempty
+	type Foo struct {
+		S *OptionalString `json:",omitempty"`
+	}
+	out, err := json.Marshal(new(Foo))
+	if err != nil {
+		t.Fatal(err)
+	}
+	expected := "{}"
+	if string(out) != expected {
+		t.Fatal("expected omitempty to omit the optional integer")
+	}
+	// unmarshal from omitempty output and get default value
+	var foo2 Foo
+	if err := json.Unmarshal(out, &foo2); err != nil {
+		t.Fatalf("%s failed to unmarshall with %s", string(out), err)
+	}
+	if s := foo2.S.WithDefault("foo"); s != "foo" {
+		t.Fatalf("expected default value to be used, got %s", s)
+	}
+	if !foo2.S.IsDefault() {
+		t.Fatal("expected value to be the default")
+	}
+
+	for _, invalid := range []string{
+		"[]", "{}", "0", "a", "'b'",
+	} {
+		var p OptionalString
+		err := json.Unmarshal([]byte(invalid), &p)
+		if err == nil {
+			t.Errorf("expected to fail to decode %s as an optional string", invalid)
+		}
+	}
+}