Merge pull request #4006 from MichaelEischer/deviceID-only-for-hardlinks

archiver: only store deviceID for hardlinks

changelog/unreleased/pull-4006

@@ -0,0 +1,16 @@
+Enhancement: (alpha) Store deviceID only for hardlinks
+
+Set `RESTIC_FEATURES=device-id-for-hardlinks` to enable this alpha feature.
+The feature flag will be removed after repository format version 3 becomes
+available or be replaced with a different solution.
+
+When creating backups from a filesystem snapshot, for example created using
+btrfs subvolumes, the deviceID of the filesystem changes compared to previous
+snapshots. This prevented restic from deduplicating the directory metadata of
+a snapshot.
+
+When this alpha feature is enabled, then the deviceID is only stored for
+hardlinks. This significantly reduces the metadata duplication for most
+backups.
+
+https://github.com/restic/restic/pull/4006

changelog/unreleased/pull-4503

@@ -4,4 +4,5 @@ If files on different devices had the same inode id, then the `stats` command
 did not correctly calculate the snapshot size. This has been fixed.
 
 https://github.com/restic/restic/pull/4503
+https://github.com/restic/restic/pull/4006
 https://forum.restic.net/t/possible-bug-in-stats/6461/8

cmd/restic/cmd_stats.go

@@ -270,13 +270,16 @@ func statsWalkTree(repo restic.Loader, opts StatsOptions, stats *statsContainer,
 			// will still be restored
 			stats.TotalFileCount++
 
-			// if inodes are present, only count each inode once
+			if node.Links == 1 || node.Type == "dir" {
-			// (hard links do not increase restore size)
+				stats.TotalSize += node.Size
+			} else {
+				// if hardlinks are present only count each deviceID+inode once
 				if !hardLinkIndex.Has(node.Inode, node.DeviceID) || node.Inode == 0 {
 					hardLinkIndex.Add(node.Inode, node.DeviceID, struct{}{})
 					stats.TotalSize += node.Size
 				}
 			}
+		}
 
 		return nil
 	}

internal/archiver/archiver.go

@@ -12,6 +12,7 @@ import (
 
 	"github.com/restic/restic/internal/debug"
 	"github.com/restic/restic/internal/errors"
+	"github.com/restic/restic/internal/feature"
 	"github.com/restic/restic/internal/fs"
 	"github.com/restic/restic/internal/restic"
 	"golang.org/x/sync/errgroup"
@@ -188,6 +189,14 @@ func (arch *Archiver) nodeFromFileInfo(snPath, filename string, fi os.FileInfo)
 	if !arch.WithAtime {
 		node.AccessTime = node.ModTime
 	}
+	if feature.Flag.Enabled(feature.DeviceIDForHardlinks) {
+		if node.Links == 1 || node.Type == "dir" {
+			// the DeviceID is only necessary for hardlinked files
+			// when using subvolumes or snapshots their deviceIDs tend to change which causes
+			// restic to upload new tree blobs
+			node.DeviceID = 0
+		}
+	}
 	// overwrite name to match that within the snapshot
 	node.Name = path.Base(snPath)
 	if err != nil {

internal/archiver/archiver_test.go

@@ -19,6 +19,7 @@ import (
 	"github.com/restic/restic/internal/backend/mem"
 	"github.com/restic/restic/internal/checker"
 	"github.com/restic/restic/internal/errors"
+	"github.com/restic/restic/internal/feature"
 	"github.com/restic/restic/internal/fs"
 	"github.com/restic/restic/internal/repository"
 	"github.com/restic/restic/internal/restic"
@@ -2125,6 +2126,8 @@ const (
 )
 
 func TestMetadataChanged(t *testing.T) {
+	defer feature.TestSetFlag(t, feature.Flag, feature.DeviceIDForHardlinks, true)()
+
 	files := TestDir{
 		"testfile": TestFile{
 			Content: "foo bar test file",
@@ -2153,6 +2156,7 @@ func TestMetadataChanged(t *testing.T) {
 	sn, node2 := snapshot(t, repo, fs, nil, "testfile")
 
 	// set some values so we can then compare the nodes
+	want.DeviceID = 0
 	want.Content = node2.Content
 	want.Path = ""
 	if len(want.ExtendedAttributes) == 0 {

internal/archiver/archiver_unix_test.go

@@ -6,6 +6,12 @@ package archiver
 import (
 	"os"
 	"syscall"
+	"testing"
+
+	"github.com/restic/restic/internal/feature"
+	"github.com/restic/restic/internal/fs"
+	"github.com/restic/restic/internal/restic"
+	restictest "github.com/restic/restic/internal/test"
 )
 
 type wrappedFileInfo struct {
@@ -39,3 +45,45 @@ func wrapFileInfo(fi os.FileInfo) os.FileInfo {
 
 	return res
 }
+
+func statAndSnapshot(t *testing.T, repo restic.Repository, name string) (*restic.Node, *restic.Node) {
+	fi := lstat(t, name)
+	want, err := restic.NodeFromFileInfo(name, fi)
+	restictest.OK(t, err)
+
+	_, node := snapshot(t, repo, fs.Local{}, nil, name)
+	return want, node
+}
+
+func TestHardlinkMetadata(t *testing.T) {
+	defer feature.TestSetFlag(t, feature.Flag, feature.DeviceIDForHardlinks, true)()
+
+	files := TestDir{
+		"testfile": TestFile{
+			Content: "foo bar test file",
+		},
+		"linktarget": TestFile{
+			Content: "test file",
+		},
+		"testlink": TestHardlink{
+			Target: "./linktarget",
+		},
+		"testdir": TestDir{},
+	}
+
+	tempdir, repo := prepareTempdirRepoSrc(t, files)
+
+	back := restictest.Chdir(t, tempdir)
+	defer back()
+
+	want, node := statAndSnapshot(t, repo, "testlink")
+	restictest.Assert(t, node.DeviceID == want.DeviceID, "device id mismatch expected %v got %v", want.DeviceID, node.DeviceID)
+	restictest.Assert(t, node.Links == want.Links, "link count mismatch expected %v got %v", want.Links, node.Links)
+	restictest.Assert(t, node.Inode == want.Inode, "inode mismatch expected %v got %v", want.Inode, node.Inode)
+
+	_, node = statAndSnapshot(t, repo, "testfile")
+	restictest.Assert(t, node.DeviceID == 0, "device id mismatch for testfile expected %v got %v", 0, node.DeviceID)
+
+	_, node = statAndSnapshot(t, repo, "testdir")
+	restictest.Assert(t, node.DeviceID == 0, "device id mismatch for testdir expected %v got %v", 0, node.DeviceID)
+}

internal/archiver/testing.go

@@ -6,6 +6,7 @@ import (
 	"path"
 	"path/filepath"
 	"runtime"
+	"sort"
 	"strings"
 	"testing"
 	"time"
@@ -63,11 +64,29 @@ func (s TestSymlink) String() string {
 	return "<Symlink>"
 }
 
+// TestHardlink describes a hardlink created for a test.
+type TestHardlink struct {
+	Target string
+}
+
+func (s TestHardlink) String() string {
+	return "<Hardlink>"
+}
+
 // TestCreateFiles creates a directory structure described by dir at target,
 // which must already exist. On Windows, symlinks aren't created.
 func TestCreateFiles(t testing.TB, target string, dir TestDir) {
 	t.Helper()
-	for name, item := range dir {
+
+	// ensure a stable order such that it can be guaranteed that a hardlink target already exists
+	var names []string
+	for name := range dir {
+		names = append(names, name)
+	}
+	sort.Strings(names)
+
+	for _, name := range names {
+		item := dir[name]
 		targetPath := filepath.Join(target, name)
 
 		switch it := item.(type) {
@@ -81,6 +100,11 @@ func TestCreateFiles(t testing.TB, target string, dir TestDir) {
 			if err != nil {
 				t.Fatal(err)
 			}
+		case TestHardlink:
+			err := fs.Link(filepath.Join(target, filepath.FromSlash(it.Target)), targetPath)
+			if err != nil {
+				t.Fatal(err)
+			}
 		case TestDir:
 			err := fs.Mkdir(targetPath, 0755)
 			if err != nil {

internal/feature/registry.go

@@ -5,13 +5,13 @@ var Flag = New()
 
 // flag names are written in kebab-case
 const (
-	ExampleFeature       FlagName = "example-feature"
 	DeprecateLegacyIndex FlagName = "deprecate-legacy-index"
+	DeviceIDForHardlinks FlagName = "device-id-for-hardlinks"
 )
 
 func init() {
 	Flag.SetFlags(map[FlagName]FlagDesc{
-		ExampleFeature:       {Type: Alpha, Description: "just for testing"},
 		DeprecateLegacyIndex: {Type: Beta, Description: "disable support for index format used by restic 0.1.0. Use `restic repair index` to update the index if necessary."},
+		DeviceIDForHardlinks: {Type: Alpha, Description: "store deviceID only for hardlinks to reduce metadata changes for example when using btrfs subvolumes. Will be removed in a future restic version after repository format 3 is available"},
 	})
 }

internal/restic/node.go

@@ -82,7 +82,7 @@ type Node struct {
 	User       string      `json:"user,omitempty"`
 	Group      string      `json:"group,omitempty"`
 	Inode      uint64      `json:"inode,omitempty"`
-	DeviceID   uint64      `json:"device_id,omitempty"` // device id of the file, stat.st_dev
+	DeviceID   uint64      `json:"device_id,omitempty"` // device id of the file, stat.st_dev, only stored for hardlinks
 	Size       uint64      `json:"size,omitempty"`
 	Links      uint64      `json:"links,omitempty"`
 	LinkTarget string      `json:"linktarget,omitempty"`