perf(storage): use a lightweight method to get the data size of the s…

…torage

The previous method to get the data size of storage belonging to a log stream was costly. Concretely
it used too many heap objects to call `filepath.Walk` API. To avoid this problem, this patch changes
the implementation of `internal/storage.(*Storage).DiskUsage` to use the built-in method of pebble
DB - `DiskSpaceUsage`.

Resolves #210

internal/storage/storage.go

@@ -8,7 +8,6 @@ import (
 	"go.uber.org/multierr"
 
 	"github.com/kakao/varlog/pkg/types"
-	"github.com/kakao/varlog/pkg/util/fputil"
 	"github.com/kakao/varlog/proto/varlogpb"
 )
 
@@ -285,8 +284,8 @@ func (s *Storage) Path() string {
 	return s.path
 }
 
-func (s *Storage) DiskUsage() int64 {
-	return fputil.DirectorySize(s.path)
+func (s *Storage) DiskUsage() uint64 {
+	return s.db.Metrics().DiskSpaceUsage()
 }
 
 // Close closes the storage.

proto/snpb/metadata.proto

@@ -25,8 +25,7 @@ message StorageNodeMetadataDescriptor {
 
   // ClusterID is the identifier of the cluster.
   uint32 cluster_id = 1 [
-    (gogoproto.casttype) =
-        "github.com/kakao/varlog/pkg/types.ClusterID",
+    (gogoproto.casttype) = "github.com/kakao/varlog/pkg/types.ClusterID",
     (gogoproto.customname) = "ClusterID",
     (gogoproto.jsontag) = "clusterId"
   ];
@@ -74,8 +73,7 @@ message LogStreamReplicaMetadataDescriptor {
   // Version is the latest version of the commit received from the metadata
   // repository.
   uint64 version = 3
-      [(gogoproto.casttype) =
-           "github.com/kakao/varlog/pkg/types.Version"];
+      [(gogoproto.casttype) = "github.com/kakao/varlog/pkg/types.Version"];
 
   // GlobalHighWatermark is the latest high watermark received from the metadata
   // repository.
@@ -100,7 +98,7 @@ message LogStreamReplicaMetadataDescriptor {
 
   // Path is the directory where the data for the log stream replica is stored.
   string path = 7;
-  int64 storage_size_bytes = 8;
+  uint64 storage_size_bytes = 8;
 
   // CreatedTime
   //