Merge pull request #3854 from MichaelEischer/sparsefiles

restore: Add support for sparse files

changelog/unreleased/issue-79

@@ -0,0 +1,17 @@
+Enhancement: Restore files with many zeros as sparse files
+
+When using `restore --sparse`, the restorer may now write files containing long
+runs of zeros as sparse files (also called files with holes): the zeros are not
+actually written to disk.
+
+How much space is saved by writing sparse files depends on the operating
+system, file system and the distribution of zeros in the file.
+
+During backup restic still reads the whole file including sparse regions. We
+have optimized the processing speed of sparse regions.
+
+https://github.com/restic/restic/issues/79
+https://github.com/restic/restic/issues/3903
+https://github.com/restic/restic/pull/2601
+https://github.com/restic/restic/pull/3854
+https://forum.restic.net/t/sparse-file-support/1264

cmd/restic/cmd_restore.go

@@ -42,6 +42,7 @@ type RestoreOptions struct {
 	InsensitiveInclude []string
 	Target             string
 	snapshotFilterOptions
+	Sparse bool
 	Verify bool
 }
 
@@ -58,6 +59,7 @@ func init() {
 	flags.StringVarP(&restoreOptions.Target, "target", "t", "", "directory to extract data to")
 
 	initSingleSnapshotFilterOptions(flags, &restoreOptions.snapshotFilterOptions)
+	flags.BoolVar(&restoreOptions.Sparse, "sparse", false, "restore files as sparse")
 	flags.BoolVar(&restoreOptions.Verify, "verify", false, "verify restored files content")
 }
 
@@ -147,7 +149,7 @@ func runRestore(opts RestoreOptions, gopts GlobalOptions, args []string) error {
 		return err
 	}
 
-	res, err := restorer.NewRestorer(ctx, repo, id)
+	res, err := restorer.NewRestorer(ctx, repo, id, opts.Sparse)
 	if err != nil {
 		Exitf(2, "creating restorer failed: %v\n", err)
 	}

internal/repository/repository.go

@@ -818,7 +818,14 @@ func (r *Repository) SaveBlob(ctx context.Context, t restic.BlobType, buf []byte
 
 	// compute plaintext hash if not already set
 	if id.IsNull() {
+		// Special case the hash calculation for all zero chunks. This is especially
+		// useful for sparse files containing large all zero regions. For these we can
+		// process chunks as fast as we can read the from disk.
+		if len(buf) == chunker.MinSize && restic.ZeroPrefixLen(buf) == chunker.MinSize {
+			newID = ZeroChunk()
+		} else {
 			newID = restic.Hash(buf)
+		}
 	} else {
 		newID = id
 	}
@@ -972,3 +979,14 @@ func streamPackPart(ctx context.Context, beLoad BackendLoadFn, key *crypto.Key,
 	})
 	return errors.Wrap(err, "StreamPack")
 }
+
+var zeroChunkOnce sync.Once
+var zeroChunkID restic.ID
+
+// ZeroChunk computes and returns (cached) the ID of an all-zero chunk with size chunker.MinSize
+func ZeroChunk() restic.ID {
+	zeroChunkOnce.Do(func() {
+		zeroChunkID = restic.Hash(make([]byte, chunker.MinSize))
+	})
+	return zeroChunkID
+}

internal/restic/zeroprefix.go

@@ -0,0 +1,21 @@
+package restic
+
+import "bytes"
+
+// ZeroPrefixLen returns the length of the longest all-zero prefix of p.
+func ZeroPrefixLen(p []byte) (n int) {
+	// First skip 1kB-sized blocks, for speed.
+	var zeros [1024]byte
+
+	for len(p) >= len(zeros) && bytes.Equal(p[:len(zeros)], zeros[:]) {
+		p = p[len(zeros):]
+		n += len(zeros)
+	}
+
+	for len(p) > 0 && p[0] == 0 {
+		p = p[1:]
+		n++
+	}
+
+	return n
+}

internal/restic/zeroprefix_test.go

@@ -0,0 +1,52 @@
+package restic_test
+
+import (
+	"math/rand"
+	"testing"
+
+	"github.com/restic/restic/internal/restic"
+	"github.com/restic/restic/internal/test"
+)
+
+func TestZeroPrefixLen(t *testing.T) {
+	var buf [2048]byte
+
+	// test zero prefixes of various lengths
+	for i := len(buf) - 1; i >= 0; i-- {
+		buf[i] = 42
+		skipped := restic.ZeroPrefixLen(buf[:])
+		test.Equals(t, i, skipped)
+	}
+	// test buffers of various sizes
+	for i := 0; i < len(buf); i++ {
+		skipped := restic.ZeroPrefixLen(buf[i:])
+		test.Equals(t, 0, skipped)
+	}
+}
+
+func BenchmarkZeroPrefixLen(b *testing.B) {
+	var (
+		buf        [4<<20 + 37]byte
+		r          = rand.New(rand.NewSource(0x618732))
+		sumSkipped int64
+	)
+
+	b.ReportAllocs()
+	b.SetBytes(int64(len(buf)))
+	b.ResetTimer()
+
+	for i := 0; i < b.N; i++ {
+		j := r.Intn(len(buf))
+		buf[j] = 0xff
+
+		skipped := restic.ZeroPrefixLen(buf[:])
+		sumSkipped += int64(skipped)
+
+		buf[j] = 0
+	}
+
+	// The closer this is to .5, the better. If it's far off, give the
+	// benchmark more time to run with -benchtime.
+	b.Logf("average number of zeros skipped: %.3f",
+		float64(sumSkipped)/(float64(b.N*len(buf))))
+}

internal/restorer/filerestorer.go

@@ -27,6 +27,7 @@ const (
 type fileInfo struct {
 	lock       sync.Mutex
 	inProgress bool
+	sparse     bool
 	size       int64
 	location   string      // file on local filesystem relative to restorer basedir
 	blobs      interface{} // blobs of the file
@@ -51,6 +52,8 @@ type fileRestorer struct {
 
 	workerCount int
 	filesWriter *filesWriter
+	zeroChunk   restic.ID
+	sparse      bool
 
 	dst   string
 	files []*fileInfo
@@ -61,7 +64,8 @@ func newFileRestorer(dst string,
 	packLoader repository.BackendLoadFn,
 	key *crypto.Key,
 	idx func(restic.BlobHandle) []restic.PackedBlob,
-	connections uint) *fileRestorer {
+	connections uint,
+	sparse bool) *fileRestorer {
 
 	// as packs are streamed the concurrency is limited by IO
 	workerCount := int(connections)
@@ -71,6 +75,8 @@ func newFileRestorer(dst string,
 		idx:         idx,
 		packLoader:  packLoader,
 		filesWriter: newFilesWriter(workerCount),
+		zeroChunk:   repository.ZeroChunk(),
+		sparse:      sparse,
 		workerCount: workerCount,
 		dst:         dst,
 		Error:       restorerAbortOnAllErrors,
@@ -133,7 +139,16 @@ func (r *fileRestorer) restoreFiles(ctx context.Context) error {
 				packOrder = append(packOrder, packID)
 			}
 			pack.files[file] = struct{}{}
+			if blob.ID.Equal(r.zeroChunk) {
+				file.sparse = r.sparse
+			}
 		})
+		if len(fileBlobs) == 1 {
+			// no need to preallocate files with a single block, thus we can always consider them to be sparse
+			// in addition, a short chunk will never match r.zeroChunk which would prevent sparseness for short files
+			file.sparse = r.sparse
+		}
+
 		if err != nil {
 			// repository index is messed up, can't do anything
 			return err
@@ -253,7 +268,7 @@ func (r *fileRestorer) downloadPack(ctx context.Context, pack *packInfo) error {
 						file.inProgress = true
 						createSize = file.size
 					}
-					return r.filesWriter.writeToFile(r.targetPath(file.location), blobData, offset, createSize)
+					return r.filesWriter.writeToFile(r.targetPath(file.location), blobData, offset, createSize, file.sparse)
 				}
 				err := sanitizeError(file, writeToFile())
 				if err != nil {

internal/restorer/filerestorer_test.go

@@ -147,10 +147,10 @@ func newTestRepo(content []TestFile) *TestRepo {
 	return repo
 }
 
-func restoreAndVerify(t *testing.T, tempdir string, content []TestFile, files map[string]bool) {
+func restoreAndVerify(t *testing.T, tempdir string, content []TestFile, files map[string]bool, sparse bool) {
 	repo := newTestRepo(content)
 
-	r := newFileRestorer(tempdir, repo.loader, repo.key, repo.Lookup, 2)
+	r := newFileRestorer(tempdir, repo.loader, repo.key, repo.Lookup, 2, sparse)
 
 	if files == nil {
 		r.files = repo.files
@@ -188,6 +188,7 @@ func TestFileRestorerBasic(t *testing.T) {
 	tempdir, cleanup := rtest.TempDir(t)
 	defer cleanup()
 
+	for _, sparse := range []bool{false, true} {
 		restoreAndVerify(t, tempdir, []TestFile{
 			{
 				name: "file1",
@@ -211,7 +212,8 @@ func TestFileRestorerBasic(t *testing.T) {
 					{"data3-1", "pack3-1"},
 				},
 			},
-	}, nil)
+		}, nil, sparse)
+	}
 }
 
 func TestFileRestorerPackSkip(t *testing.T) {
@@ -221,6 +223,7 @@ func TestFileRestorerPackSkip(t *testing.T) {
 	files := make(map[string]bool)
 	files["file2"] = true
 
+	for _, sparse := range []bool{false, true} {
 		restoreAndVerify(t, tempdir, []TestFile{
 			{
 				name: "file1",
@@ -242,7 +245,8 @@ func TestFileRestorerPackSkip(t *testing.T) {
 					{"data1-6", "pack1"},
 				},
 			},
-	}, files)
+		}, files, sparse)
+	}
 }
 
 func TestErrorRestoreFiles(t *testing.T) {
@@ -264,7 +268,7 @@ func TestErrorRestoreFiles(t *testing.T) {
 		return loadError
 	}
 
-	r := newFileRestorer(tempdir, repo.loader, repo.key, repo.Lookup, 2)
+	r := newFileRestorer(tempdir, repo.loader, repo.key, repo.Lookup, 2, false)
 	r.files = repo.files
 
 	err := r.restoreFiles(context.TODO())
@@ -304,7 +308,7 @@ func testPartialDownloadError(t *testing.T, part int) {
 		return loader(ctx, h, length, offset, fn)
 	}
 
-	r := newFileRestorer(tempdir, repo.loader, repo.key, repo.Lookup, 2)
+	r := newFileRestorer(tempdir, repo.loader, repo.key, repo.Lookup, 2, false)
 	r.files = repo.files
 	r.Error = func(s string, e error) error {
 		// ignore errors as in the `restore` command

internal/restorer/fileswriter.go

@@ -19,30 +19,34 @@ type filesWriter struct {
 
 type filesWriterBucket struct {
 	lock  sync.Mutex
-	files map[string]*os.File
-	users map[string]int
+	files map[string]*partialFile
+}
+
+type partialFile struct {
+	*os.File
+	users  int // Reference count.
+	sparse bool
 }
 
 func newFilesWriter(count int) *filesWriter {
 	buckets := make([]filesWriterBucket, count)
 	for b := 0; b < count; b++ {
-		buckets[b].files = make(map[string]*os.File)
-		buckets[b].users = make(map[string]int)
+		buckets[b].files = make(map[string]*partialFile)
 	}
 	return &filesWriter{
 		buckets: buckets,
 	}
 }
 
-func (w *filesWriter) writeToFile(path string, blob []byte, offset int64, createSize int64) error {
+func (w *filesWriter) writeToFile(path string, blob []byte, offset int64, createSize int64, sparse bool) error {
 	bucket := &w.buckets[uint(xxhash.Sum64String(path))%uint(len(w.buckets))]
 
-	acquireWriter := func() (*os.File, error) {
+	acquireWriter := func() (*partialFile, error) {
 		bucket.lock.Lock()
 		defer bucket.lock.Unlock()
 
 		if wr, ok := bucket.files[path]; ok {
-			bucket.users[path]++
+			bucket.files[path].users++
 			return wr, nil
 		}
 
@@ -53,16 +57,22 @@ func (w *filesWriter) writeToFile(path string, blob []byte, offset int64, create
 			flags = os.O_WRONLY
 		}
 
-		wr, err := os.OpenFile(path, flags, 0600)
+		f, err := os.OpenFile(path, flags, 0600)
 		if err != nil {
 			return nil, err
 		}
 
+		wr := &partialFile{File: f, users: 1, sparse: sparse}
 		bucket.files[path] = wr
-		bucket.users[path] = 1
 
 		if createSize >= 0 {
-			err := preallocateFile(wr, createSize)
+			if sparse {
+				err = truncateSparse(f, createSize)
+				if err != nil {
+					return nil, err
+				}
+			} else {
+				err := preallocateFile(wr.File, createSize)
 				if err != nil {
 					// Just log the preallocate error but don't let it cause the restore process to fail.
 					// Preallocate might return an error if the filesystem (implementation) does not
@@ -72,20 +82,20 @@ func (w *filesWriter) writeToFile(path string, blob []byte, offset int64, create
 					debug.Log("Failed to preallocate %v with size %v: %v", path, createSize, err)
 				}
 			}
+		}
 
 		return wr, nil
 	}
 
-	releaseWriter := func(wr *os.File) error {
+	releaseWriter := func(wr *partialFile) error {
 		bucket.lock.Lock()
 		defer bucket.lock.Unlock()
 
-		if bucket.users[path] == 1 {
+		if bucket.files[path].users == 1 {
 			delete(bucket.files, path)
-			delete(bucket.users, path)
 			return wr.Close()
 		}
-		bucket.users[path]--
+		bucket.files[path].users--
 		return nil
 	}
 

internal/restorer/fileswriter_test.go

@@ -16,21 +16,17 @@ func TestFilesWriterBasic(t *testing.T) {
 	f1 := dir + "/f1"
 	f2 := dir + "/f2"
 
-	rtest.OK(t, w.writeToFile(f1, []byte{1}, 0, 2))
+	rtest.OK(t, w.writeToFile(f1, []byte{1}, 0, 2, false))
 	rtest.Equals(t, 0, len(w.buckets[0].files))
-	rtest.Equals(t, 0, len(w.buckets[0].users))
 
-	rtest.OK(t, w.writeToFile(f2, []byte{2}, 0, 2))
+	rtest.OK(t, w.writeToFile(f2, []byte{2}, 0, 2, false))
 	rtest.Equals(t, 0, len(w.buckets[0].files))
-	rtest.Equals(t, 0, len(w.buckets[0].users))
 
-	rtest.OK(t, w.writeToFile(f1, []byte{1}, 1, -1))
+	rtest.OK(t, w.writeToFile(f1, []byte{1}, 1, -1, false))
 	rtest.Equals(t, 0, len(w.buckets[0].files))
-	rtest.Equals(t, 0, len(w.buckets[0].users))
 
-	rtest.OK(t, w.writeToFile(f2, []byte{2}, 1, -1))
+	rtest.OK(t, w.writeToFile(f2, []byte{2}, 1, -1, false))
 	rtest.Equals(t, 0, len(w.buckets[0].files))
-	rtest.Equals(t, 0, len(w.buckets[0].users))
 
 	buf, err := ioutil.ReadFile(f1)
 	rtest.OK(t, err)

internal/restorer/restorer.go

@@ -18,6 +18,7 @@ import (
 type Restorer struct {
 	repo   restic.Repository
 	sn     *restic.Snapshot
+	sparse bool
 
 	Error        func(location string, err error) error
 	SelectFilter func(item string, dstpath string, node *restic.Node) (selectedForRestore bool, childMayBeSelected bool)
@@ -26,9 +27,10 @@ type Restorer struct {
 var restorerAbortOnAllErrors = func(location string, err error) error { return err }
 
 // NewRestorer creates a restorer preloaded with the content from the snapshot id.
-func NewRestorer(ctx context.Context, repo restic.Repository, id restic.ID) (*Restorer, error) {
+func NewRestorer(ctx context.Context, repo restic.Repository, id restic.ID, sparse bool) (*Restorer, error) {
 	r := &Restorer{
 		repo:         repo,
+		sparse:       sparse,
 		Error:        restorerAbortOnAllErrors,
 		SelectFilter: func(string, string, *restic.Node) (bool, bool) { return true, true },
 	}
@@ -219,7 +221,7 @@ func (res *Restorer) RestoreTo(ctx context.Context, dst string) error {
 	}
 
 	idx := NewHardlinkIndex()
-	filerestorer := newFileRestorer(dst, res.repo.Backend().Load, res.repo.Key(), res.repo.Index().Lookup, res.repo.Connections())
+	filerestorer := newFileRestorer(dst, res.repo.Backend().Load, res.repo.Key(), res.repo.Index().Lookup, res.repo.Connections(), res.sparse)
 	filerestorer.Error = res.Error
 
 	debug.Log("first pass for %q", dst)

internal/restorer/restorer_test.go

@@ -4,6 +4,7 @@ import (
 	"bytes"
 	"context"
 	"io/ioutil"
+	"math"
 	"os"
 	"path/filepath"
 	"runtime"
@@ -11,6 +12,7 @@ import (
 	"testing"
 	"time"
 
+	"github.com/restic/restic/internal/archiver"
 	"github.com/restic/restic/internal/fs"
 	"github.com/restic/restic/internal/repository"
 	"github.com/restic/restic/internal/restic"
@@ -324,7 +326,7 @@ func TestRestorer(t *testing.T) {
 			_, id := saveSnapshot(t, repo, test.Snapshot)
 			t.Logf("snapshot saved as %v", id.Str())
 
-			res, err := NewRestorer(context.TODO(), repo, id)
+			res, err := NewRestorer(context.TODO(), repo, id, false)
 			if err != nil {
 				t.Fatal(err)
 			}
@@ -447,7 +449,7 @@ func TestRestorerRelative(t *testing.T) {
 			_, id := saveSnapshot(t, repo, test.Snapshot)
 			t.Logf("snapshot saved as %v", id.Str())
 
-			res, err := NewRestorer(context.TODO(), repo, id)
+			res, err := NewRestorer(context.TODO(), repo, id, false)
 			if err != nil {
 				t.Fatal(err)
 			}
@@ -682,7 +684,7 @@ func TestRestorerTraverseTree(t *testing.T) {
 			defer cleanup()
 			sn, id := saveSnapshot(t, repo, test.Snapshot)
 
-			res, err := NewRestorer(context.TODO(), repo, id)
+			res, err := NewRestorer(context.TODO(), repo, id, false)
 			if err != nil {
 				t.Fatal(err)
 			}
@@ -764,7 +766,7 @@ func TestRestorerConsistentTimestampsAndPermissions(t *testing.T) {
 		},
 	})
 
-	res, err := NewRestorer(context.TODO(), repo, id)
+	res, err := NewRestorer(context.TODO(), repo, id, false)
 	rtest.OK(t, err)
 
 	res.SelectFilter = func(item string, dstpath string, node *restic.Node) (selectedForRestore bool, childMayBeSelected bool) {
@@ -824,7 +826,7 @@ func TestVerifyCancel(t *testing.T) {
 
 	_, id := saveSnapshot(t, repo, snapshot)
 
-	res, err := NewRestorer(context.TODO(), repo, id)
+	res, err := NewRestorer(context.TODO(), repo, id, false)
 	rtest.OK(t, err)
 
 	tempdir, cleanup := rtest.TempDir(t)
@@ -849,3 +851,58 @@ func TestVerifyCancel(t *testing.T) {
 	rtest.Equals(t, 1, len(errs))
 	rtest.Assert(t, strings.Contains(errs[0].Error(), "Invalid file size for"), "wrong error %q", errs[0].Error())
 }
+
+func TestRestorerSparseFiles(t *testing.T) {
+	repo, cleanup := repository.TestRepository(t)
+	defer cleanup()
+
+	var zeros [1<<20 + 13]byte
+
+	target := &fs.Reader{
+		Mode:       0600,
+		Name:       "/zeros",
+		ReadCloser: ioutil.NopCloser(bytes.NewReader(zeros[:])),
+	}
+	sc := archiver.NewScanner(target)
+	err := sc.Scan(context.TODO(), []string{"/zeros"})
+	rtest.OK(t, err)
+
+	arch := archiver.New(repo, target, archiver.Options{})
+	_, id, err := arch.Snapshot(context.Background(), []string{"/zeros"},
+		archiver.SnapshotOptions{})
+	rtest.OK(t, err)
+
+	res, err := NewRestorer(context.TODO(), repo, id, true)
+	rtest.OK(t, err)
+
+	tempdir, cleanup := rtest.TempDir(t)
+	defer cleanup()
+
+	ctx, cancel := context.WithCancel(context.Background())
+	defer cancel()
+
+	err = res.RestoreTo(ctx, tempdir)
+	rtest.OK(t, err)
+
+	filename := filepath.Join(tempdir, "zeros")
+	content, err := ioutil.ReadFile(filename)
+	rtest.OK(t, err)
+
+	rtest.Equals(t, len(zeros[:]), len(content))
+	rtest.Equals(t, zeros[:], content)
+
+	blocks := getBlockCount(t, filename)
+	if blocks < 0 {
+		return
+	}
+
+	// st.Blocks is the size in 512-byte blocks.
+	denseBlocks := math.Ceil(float64(len(zeros)) / 512)
+	sparsity := 1 - float64(blocks)/denseBlocks
+
+	// This should report 100% sparse. We don't assert that,
+	// as the behavior of sparse writes depends on the underlying
+	// file system as well as the OS.
+	t.Logf("wrote %d zeros as %d blocks, %.1f%% sparse",
+		len(zeros), blocks, 100*sparsity)
+}

internal/restorer/restorer_unix_test.go

@@ -30,7 +30,7 @@ func TestRestorerRestoreEmptyHardlinkedFileds(t *testing.T) {
 		},
 	})
 
-	res, err := NewRestorer(context.TODO(), repo, id)
+	res, err := NewRestorer(context.TODO(), repo, id, false)
 	rtest.OK(t, err)
 
 	res.SelectFilter = func(item string, dstpath string, node *restic.Node) (selectedForRestore bool, childMayBeSelected bool) {
@@ -60,3 +60,13 @@ func TestRestorerRestoreEmptyHardlinkedFileds(t *testing.T) {
 		rtest.Equals(t, s1.Ino, s2.Ino)
 	}
 }
+
+func getBlockCount(t *testing.T, filename string) int64 {
+	fi, err := os.Stat(filename)
+	rtest.OK(t, err)
+	st := fi.Sys().(*syscall.Stat_t)
+	if st == nil {
+		return -1
+	}
+	return st.Blocks
+}

internal/restorer/restorer_windows_test.go

@@ -0,0 +1,35 @@
+//go:build windows
+// +build windows
+
+package restorer
+
+import (
+	"math"
+	"syscall"
+	"testing"
+	"unsafe"
+
+	rtest "github.com/restic/restic/internal/test"
+	"golang.org/x/sys/windows"
+)
+
+func getBlockCount(t *testing.T, filename string) int64 {
+	libkernel32 := windows.NewLazySystemDLL("kernel32.dll")
+	err := libkernel32.Load()
+	rtest.OK(t, err)
+	proc := libkernel32.NewProc("GetCompressedFileSizeW")
+	err = proc.Find()
+	rtest.OK(t, err)
+
+	namePtr, err := syscall.UTF16PtrFromString(filename)
+	rtest.OK(t, err)
+
+	result, _, _ := proc.Call(uintptr(unsafe.Pointer(namePtr)), 0)
+
+	const invalidFileSize = uintptr(4294967295)
+	if result == invalidFileSize {
+		return -1
+	}
+
+	return int64(math.Ceil(float64(result) / 512))
+}

internal/restorer/sparsewrite.go

@@ -0,0 +1,37 @@
+//go:build !windows
+// +build !windows
+
+package restorer
+
+import (
+	"github.com/restic/restic/internal/restic"
+)
+
+// WriteAt writes p to f.File at offset. It tries to do a sparse write
+// and updates f.size.
+func (f *partialFile) WriteAt(p []byte, offset int64) (n int, err error) {
+	if !f.sparse {
+		return f.File.WriteAt(p, offset)
+	}
+
+	n = len(p)
+
+	// Skip the longest all-zero prefix of p.
+	// If it's long enough, we can punch a hole in the file.
+	skipped := restic.ZeroPrefixLen(p)
+	p = p[skipped:]
+	offset += int64(skipped)
+
+	switch {
+	case len(p) == 0:
+		// All zeros, file already big enough. A previous WriteAt or
+		// Truncate will have produced the zeros in f.File.
+
+	default:
+		var n2 int
+		n2, err = f.File.WriteAt(p, offset)
+		n = skipped + n2
+	}
+
+	return n, err
+}

internal/restorer/truncate_other.go

@@ -0,0 +1,10 @@
+//go:build !windows
+// +build !windows
+
+package restorer
+
+import "os"
+
+func truncateSparse(f *os.File, size int64) error {
+	return f.Truncate(size)
+}

internal/restorer/truncate_windows.go

@@ -0,0 +1,19 @@
+package restorer
+
+import (
+	"os"
+
+	"github.com/restic/restic/internal/debug"
+	"golang.org/x/sys/windows"
+)
+
+func truncateSparse(f *os.File, size int64) error {
+	// try setting the sparse file attribute, but ignore the error if it fails
+	var t uint32
+	err := windows.DeviceIoControl(windows.Handle(f.Fd()), windows.FSCTL_SET_SPARSE, nil, 0, nil, 0, &t, nil)
+	if err != nil {
+		debug.Log("failed to set sparse attribute for %v: %v", f.Name(), err)
+	}
+
+	return f.Truncate(size)
+}