diff --git a/repo/fsrepo/config_test.go b/repo/fsrepo/config_test.go
index 1c62241a2..79fda4c85 100644
--- a/repo/fsrepo/config_test.go
+++ b/repo/fsrepo/config_test.go
@@ -87,9 +87,9 @@ func TestDefaultDatastoreConfig(t *testing.T) {
 		t.Fatal(err)
 	}
 
-	expected := "/blocks:{flatfs;blocks;/repo/flatfs/shard/v1/next-to-last/2};/:{levelds;datastore};"
-	if dsc.DiskId() != expected {
-		t.Errorf("expected '%s' got '%s' as DiskId", expected, dsc.DiskId())
+	expected := `{"mounts":[{"mountpoint":{"string":"/blocks"},"path":"blocks","shardFunc":"/repo/flatfs/shard/v1/next-to-last/2","type":"flatfs"},{"mountpoint":{"string":"/"},"path":"datastore","type":"levelds"}],"type":"mount"}`
+	if dsc.DiskSpec().String() != expected {
+		t.Errorf("expected '%s' got '%s' as DiskId", expected, dsc.DiskSpec().String())
 	}
 
 	ds, err := dsc.Create(dir)
@@ -125,9 +125,9 @@ func TestLevelDbConfig(t *testing.T) {
 		t.Fatal(err)
 	}
 
-	expected := "levelds;datastore"
-	if dsc.DiskId() != expected {
-		t.Errorf("expected '%s' got '%s' as DiskId", expected, dsc.DiskId())
+	expected := `{"path":"datastore","type":"levelds"}`
+	if dsc.DiskSpec().String() != expected {
+		t.Errorf("expected '%s' got '%s' as DiskId", expected, dsc.DiskSpec().String())
 	}
 
 	ds, err := dsc.Create(dir)
@@ -163,9 +163,9 @@ func TestFlatfsConfig(t *testing.T) {
 		t.Fatal(err)
 	}
 
-	expected := "flatfs;blocks;/repo/flatfs/shard/v1/next-to-last/2"
-	if dsc.DiskId() != expected {
-		t.Errorf("expected '%s' got '%s' as DiskId", expected, dsc.DiskId())
+	expected := `{"path":"blocks","shardFunc":"/repo/flatfs/shard/v1/next-to-last/2","type":"flatfs"}`
+	if dsc.DiskSpec().String() != expected {
+		t.Errorf("expected '%s' got '%s' as DiskId", expected, dsc.DiskSpec().String())
 	}
 
 	ds, err := dsc.Create(dir)
@@ -201,9 +201,9 @@ func TestMeasureConfig(t *testing.T) {
 		t.Fatal(err)
 	}
 
-	expected := "flatfs;blocks;/repo/flatfs/shard/v1/next-to-last/2"
-	if dsc.DiskId() != expected {
-		t.Errorf("expected '%s' got '%s' as DiskId", expected, dsc.DiskId())
+	expected := `{"path":"blocks","shardFunc":"/repo/flatfs/shard/v1/next-to-last/2","type":"flatfs"}`
+	if dsc.DiskSpec().String() != expected {
+		t.Errorf("expected '%s' got '%s' as DiskId", expected, dsc.DiskSpec().String())
 	}
 
 	ds, err := dsc.Create(dir)
diff --git a/repo/fsrepo/datastores.go b/repo/fsrepo/datastores.go
index 807097ac3..c5b81ce15 100644
--- a/repo/fsrepo/datastores.go
+++ b/repo/fsrepo/datastores.go
@@ -2,6 +2,7 @@ package fsrepo
 
 import (
 	"bytes"
+	"encoding/json"
 	"fmt"
 	"path/filepath"
 
@@ -18,17 +19,28 @@ import (
 // ConfigFromMap creates a new datastore config from a map
 type ConfigFromMap func(map[string]interface{}) (DatastoreConfig, error)
 
+type DiskSpec map[string]interface{}
+
 type DatastoreConfig interface {
-	// DiskId is a unique id representing the Datastore config as
-	// stored on disk, runtime config values are not part of this Id.
-	// Returns an empty string if the datastore does not have an on
-	// disk representation. No length limit.
-	DiskId() string
+	// DiskSpec returns a minimal configuration of the datastore
+	// represting what is stored on disk.  Run time values are
+	// excluded.
+	DiskSpec() DiskSpec
 
 	// Create instantiate a new datastore from this config
 	Create(path string) (repo.Datastore, error)
 }
 
+func (spec DiskSpec) String() string {
+	b, err := json.Marshal(spec)
+	if err != nil {
+		// should not happen
+		panic(err)
+	}
+	b = bytes.TrimSpace(b)
+	return string(b)
+}
+
 var datastores map[string]ConfigFromMap
 
 func init() {
@@ -94,12 +106,19 @@ func MountDatastoreConfig(params map[string]interface{}) (DatastoreConfig, error
 	return &res, nil
 }
 
-func (c *mountDatastoreConfig) DiskId() string {
-	buf := new(bytes.Buffer)
-	for _, m := range c.mounts {
-		fmt.Fprintf(buf, "%s:{%s};", m.prefix.String(), m.ds.DiskId())
+func (c *mountDatastoreConfig) DiskSpec() DiskSpec {
+	cfg := map[string]interface{}{"type": "mount"}
+	mounts := make([]interface{}, len(c.mounts))
+	for i, m := range c.mounts {
+		c := m.ds.DiskSpec()
+		if c == nil {
+			c = make(map[string]interface{})
+		}
+		c["mountpoint"] = m.prefix
+		mounts[i] = c
 	}
-	return buf.String()
+	cfg["mounts"] = mounts
+	return cfg
 }
 
 func (c *mountDatastoreConfig) Create(path string) (repo.Datastore, error) {
@@ -147,8 +166,12 @@ func FlatfsDatastoreConfig(params map[string]interface{}) (DatastoreConfig, erro
 	return &c, nil
 }
 
-func (c *flatfsDatastoreConfig) DiskId() string {
-	return fmt.Sprintf("flatfs;%s;%s", c.path, c.shardFun.String())
+func (c *flatfsDatastoreConfig) DiskSpec() DiskSpec {
+	return map[string]interface{}{
+		"type":      "flatfs",
+		"path":      c.path,
+		"shardFunc": c.shardFun.String(),
+	}
 }
 
 func (c *flatfsDatastoreConfig) Create(path string) (repo.Datastore, error) {
@@ -188,8 +211,11 @@ func LeveldsDatastoreConfig(params map[string]interface{}) (DatastoreConfig, err
 	return &c, nil
 }
 
-func (c *leveldsDatastoreConfig) DiskId() string {
-	return fmt.Sprintf("levelds;%s", c.path)
+func (c *leveldsDatastoreConfig) DiskSpec() DiskSpec {
+	return map[string]interface{}{
+		"type": "levelds",
+		"path": c.path,
+	}
 }
 
 func (c *leveldsDatastoreConfig) Create(path string) (repo.Datastore, error) {
@@ -211,8 +237,8 @@ func MemDatastoreConfig(params map[string]interface{}) (DatastoreConfig, error)
 	return &memDatastoreConfig{params}, nil
 }
 
-func (c *memDatastoreConfig) DiskId() string {
-	return ""
+func (c *memDatastoreConfig) DiskSpec() DiskSpec {
+	return nil
 }
 
 func (c *memDatastoreConfig) Create(string) (repo.Datastore, error) {
@@ -249,8 +275,8 @@ func (c *logDatastoreConfig) Create(path string) (repo.Datastore, error) {
 	return ds.NewLogDatastore(child, c.name), nil
 }
 
-func (c *logDatastoreConfig) DiskId() string {
-	return c.child.DiskId()
+func (c *logDatastoreConfig) DiskSpec() DiskSpec {
+	return c.child.DiskSpec()
 }
 
 type measureDatastoreConfig struct {
@@ -274,8 +300,8 @@ func MeasureDatastoreConfig(params map[string]interface{}) (DatastoreConfig, err
 	return &measureDatastoreConfig{child, prefix}, nil
 }
 
-func (c *measureDatastoreConfig) DiskId() string {
-	return c.child.DiskId()
+func (c *measureDatastoreConfig) DiskSpec() DiskSpec {
+	return c.child.DiskSpec()
 }
 
 func (c measureDatastoreConfig) Create(path string) (repo.Datastore, error) {
diff --git a/repo/fsrepo/fsrepo.go b/repo/fsrepo/fsrepo.go
index 526464776..44ef42382 100644
--- a/repo/fsrepo/fsrepo.go
+++ b/repo/fsrepo/fsrepo.go
@@ -1,7 +1,6 @@
 package fsrepo
 
 import (
-	"encoding/json"
 	"errors"
 	"fmt"
 	"io"
@@ -368,16 +367,16 @@ func (r *FSRepo) openDatastore() error {
 	if err != nil {
 		return err
 	}
-	diskId := dsc.DiskId()
+	spec := dsc.DiskSpec()
 
-	oldId, _, err := r.readSpec()
+	oldSpec, err := r.readSpec()
 	if err == nil {
-		if oldId != diskId {
+		if oldSpec != spec.String() {
 			return fmt.Errorf("Datastore configuration of '%s' does not match what is on disk '%s'",
-				oldId, diskId)
+				oldSpec, spec.String())
 		}
 	} else if os.IsNotExist(err) {
-		err := r.writeSpec(diskId, r.config.Datastore.Spec)
+		err := r.writeSpec(spec.String())
 		if err != nil {
 			return err
 		}
@@ -398,55 +397,27 @@ func (r *FSRepo) openDatastore() error {
 	return nil
 }
 
-var SpecFn = "spec"
+var SpecFn = "datastore_spec"
 
-func (r *FSRepo) readSpec() (string, map[string]interface{}, error) {
+func (r *FSRepo) readSpec() (string, error) {
 	fn, err := config.Path(r.path, SpecFn)
 	if err != nil {
-		return "", nil, err
+		return "", err
 	}
 	b, err := ioutil.ReadFile(fn)
 	if err != nil {
-		return "", nil, err
+		return "", err
 	}
-	idspec := make(map[string]interface{})
-	err = json.Unmarshal(b, &idspec)
-	if err != nil {
-		return "", nil, err
-	}
-	id, ok := idspec["id"].(string)
-	if !ok {
-		return "", nil, fmt.Errorf("could not retrieve 'id' field from spec file")
-	}
-	spec, ok := idspec["spec"].(map[string]interface{})
-	if !ok {
-		return "", nil, fmt.Errorf("could not retrieve 'spec' field from spec file")
-	}
-
-	dsc, err := AnyDatastoreConfig(spec)
-	if err != nil {
-		return "", nil, err
-	}
-	computedId := dsc.DiskId()
-	if computedId != id {
-		return "", nil, fmt.Errorf("bad spec file, computed id (%s) does not match given (%s)",
-			computedId, id)
-	}
-
-	return id, spec, nil
+	return strings.TrimSpace(string(b)), nil
 }
 
-func (r *FSRepo) writeSpec(id string, spec map[string]interface{}) error {
+func (r *FSRepo) writeSpec(spec string) error {
 	fn, err := config.Path(r.path, SpecFn)
 	if err != nil {
 		return err
 	}
-	idspec := map[string]interface{}{
-		"id":   id,
-		"spec": spec,
-	}
-	b, err := json.Marshal(idspec)
-	err = ioutil.WriteFile(fn, b, 0666)
+	b := []byte(spec)
+	err = ioutil.WriteFile(fn, b, 0600)
 	if err != nil {
 		return err
 	}