diff --git a/Godeps/_workspace/src/github.com/pkg/sftp/debug.go b/Godeps/_workspace/src/github.com/pkg/sftp/debug.go
index 3e264abe3..87c1fe8e9 100644
--- a/Godeps/_workspace/src/github.com/pkg/sftp/debug.go
+++ b/Godeps/_workspace/src/github.com/pkg/sftp/debug.go
@@ -1,4 +1,4 @@
-// +build debug
+// +build debug_sftp
 
 package sftp
 
diff --git a/Godeps/_workspace/src/github.com/pkg/sftp/release.go b/Godeps/_workspace/src/github.com/pkg/sftp/release.go
index b695528fd..aa8262cda 100644
--- a/Godeps/_workspace/src/github.com/pkg/sftp/release.go
+++ b/Godeps/_workspace/src/github.com/pkg/sftp/release.go
@@ -1,4 +1,4 @@
-// +build !debug
+// +build !debug_sftp
 
 package sftp
 
diff --git a/README.md b/README.md
index bb5c694f2..b7ddde025 100644
--- a/README.md
+++ b/README.md
@@ -62,16 +62,18 @@ afterwards you'll find the binary in the current directory:
       backup     save file/directory
       cache      manage cache
       cat        dump something
+      check      check the repository
       find       find a file/directory
-      fsck       check the repository
       init       create repository
       key        manage keys
       list       lists data
       ls         list files
       restore    restore a snapshot
       snapshots  show snapshots
+      unlock     remove locks
       version    display version
 
+
 Contribute and Documentation
 ============================
 
diff --git a/backend/id_set.go b/backend/id_set.go
deleted file mode 100644
index 253147605..000000000
--- a/backend/id_set.go
+++ /dev/null
@@ -1,75 +0,0 @@
-package backend
-
-import (
-	"errors"
-	"sort"
-	"sync"
-)
-
-type IDSet struct {
-	list IDs
-	m    sync.Mutex
-}
-
-func NewIDSet() *IDSet {
-	return &IDSet{
-		list: make(IDs, 0),
-	}
-}
-
-func (s *IDSet) find(id ID) (int, error) {
-	pos := sort.Search(len(s.list), func(i int) bool {
-		return id.Compare(s.list[i]) >= 0
-	})
-
-	if pos < len(s.list) {
-		candID := s.list[pos]
-		if id.Compare(candID) == 0 {
-			return pos, nil
-		}
-	}
-
-	return pos, errors.New("ID not found")
-}
-
-func (s *IDSet) insert(id ID) {
-	pos, err := s.find(id)
-	if err == nil {
-		// already present
-		return
-	}
-
-	// insert blob
-	// https://code.google.com/p/go-wiki/wiki/SliceTricks
-	s.list = append(s.list, ID{})
-	copy(s.list[pos+1:], s.list[pos:])
-	s.list[pos] = id
-
-	return
-}
-
-func (s *IDSet) Insert(id ID) {
-	s.m.Lock()
-	defer s.m.Unlock()
-
-	s.insert(id)
-}
-
-func (s *IDSet) Find(id ID) error {
-	s.m.Lock()
-	defer s.m.Unlock()
-
-	_, err := s.find(id)
-	if err != nil {
-		return err
-	}
-
-	return nil
-}
-
-func (s *IDSet) Len() int {
-	s.m.Lock()
-	defer s.m.Unlock()
-
-	return len(s.list)
-}
diff --git a/backend/id_set_test.go b/backend/id_set_test.go
deleted file mode 100644
index 4b8139a36..000000000
--- a/backend/id_set_test.go
+++ /dev/null
@@ -1,41 +0,0 @@
-package backend_test
-
-import (
-	"crypto/rand"
-	"io"
-	"testing"
-
-	"github.com/restic/restic/backend"
-	. "github.com/restic/restic/test"
-)
-
-func randomID() []byte {
-	buf := make([]byte, backend.IDSize)
-	_, err := io.ReadFull(rand.Reader, buf)
-	if err != nil {
-		panic(err)
-	}
-	return buf
-}
-
-func TestSet(t *testing.T) {
-	s := backend.NewIDSet()
-
-	testID := randomID()
-	err := s.Find(testID)
-	Assert(t, err != nil, "found test ID in IDSet before insertion")
-
-	for i := 0; i < 238; i++ {
-		s.Insert(randomID())
-	}
-
-	s.Insert(testID)
-	OK(t, s.Find(testID))
-
-	for i := 0; i < 80; i++ {
-		s.Insert(randomID())
-	}
-
-	s.Insert(testID)
-	OK(t, s.Find(testID))
-}
diff --git a/checker/checker.go b/checker/checker.go
new file mode 100644
index 000000000..852c85a43
--- /dev/null
+++ b/checker/checker.go
@@ -0,0 +1,601 @@
+package checker
+
+import (
+	"encoding/hex"
+	"errors"
+	"fmt"
+	"sync"
+
+	"github.com/restic/restic"
+	"github.com/restic/restic/backend"
+	"github.com/restic/restic/debug"
+	"github.com/restic/restic/repository"
+)
+
+type mapID [backend.IDSize]byte
+
+func id2map(id backend.ID) (mid mapID) {
+	copy(mid[:], id)
+	return
+}
+
+func str2map(s string) (mid mapID, err error) {
+	data, err := hex.DecodeString(s)
+	if err != nil {
+		return mid, err
+	}
+
+	return id2map(data), nil
+}
+
+func map2str(id mapID) string {
+	return hex.EncodeToString(id[:])
+}
+
+func map2id(id mapID) backend.ID {
+	return backend.ID(id[:])
+}
+
+// Checker runs various checks on a repository. It is advisable to create an
+// exclusive Lock in the repository before running any checks.
+//
+// A Checker only tests for internal errors within the data structures of the
+// repository (e.g. missing blobs), and needs a valid Repository to work on.
+type Checker struct {
+	packs    map[mapID]struct{}
+	blobs    map[mapID]struct{}
+	blobRefs struct {
+		sync.Mutex
+		M map[mapID]uint
+	}
+	indexes       map[mapID]*repository.Index
+	orphanedPacks backend.IDs
+
+	masterIndex *repository.Index
+
+	repo *repository.Repository
+}
+
+// New returns a new checker which runs on repo.
+func New(repo *repository.Repository) *Checker {
+	c := &Checker{
+		packs:       make(map[mapID]struct{}),
+		blobs:       make(map[mapID]struct{}),
+		masterIndex: repository.NewIndex(),
+		indexes:     make(map[mapID]*repository.Index),
+		repo:        repo,
+	}
+
+	c.blobRefs.M = make(map[mapID]uint)
+
+	return c
+}
+
+const defaultParallelism = 40
+
+// LoadIndex loads all index files.
+func (c *Checker) LoadIndex() error {
+	debug.Log("LoadIndex", "Start")
+	type indexRes struct {
+		Index *repository.Index
+		ID    string
+	}
+
+	indexCh := make(chan indexRes)
+
+	worker := func(id string, done <-chan struct{}) error {
+		debug.Log("LoadIndex", "worker got index %v", id)
+		idx, err := repository.LoadIndex(c.repo, id)
+		if err != nil {
+			return err
+		}
+
+		select {
+		case indexCh <- indexRes{Index: idx, ID: id}:
+		case <-done:
+		}
+
+		return nil
+	}
+
+	var perr error
+	go func() {
+		defer close(indexCh)
+		debug.Log("LoadIndex", "start loading indexes in parallel")
+		perr = repository.FilesInParallel(c.repo.Backend(), backend.Index, defaultParallelism, worker)
+		debug.Log("LoadIndex", "loading indexes finished, error: %v", perr)
+	}()
+
+	done := make(chan struct{})
+	defer close(done)
+
+	for res := range indexCh {
+		debug.Log("LoadIndex", "process index %v", res.ID)
+		id, err := str2map(res.ID)
+		if err != nil {
+			return err
+		}
+
+		c.indexes[id] = res.Index
+		c.masterIndex.Merge(res.Index)
+
+		debug.Log("LoadIndex", "process blobs")
+		cnt := 0
+		for blob := range res.Index.Each(done) {
+			c.packs[id2map(blob.PackID)] = struct{}{}
+			c.blobs[id2map(blob.ID)] = struct{}{}
+			c.blobRefs.M[id2map(blob.ID)] = 0
+			cnt++
+		}
+
+		debug.Log("LoadIndex", "%d blobs processed", cnt)
+	}
+
+	debug.Log("LoadIndex", "done, error %v", perr)
+
+	c.repo.SetIndex(c.masterIndex)
+
+	return perr
+}
+
+// PackError describes an error with a specific pack.
+type PackError struct {
+	ID       backend.ID
+	Orphaned bool
+	Err      error
+}
+
+func (e PackError) Error() string {
+	return "pack " + e.ID.String() + ": " + e.Err.Error()
+}
+
+func packIDTester(repo *repository.Repository, inChan <-chan mapID, errChan chan<- error, wg *sync.WaitGroup, done <-chan struct{}) {
+	debug.Log("Checker.testPackID", "worker start")
+	defer debug.Log("Checker.testPackID", "worker done")
+
+	defer wg.Done()
+
+	for id := range inChan {
+		ok, err := repo.Backend().Test(backend.Data, map2str(id))
+		if err != nil {
+			err = PackError{ID: map2id(id), Err: err}
+		} else {
+			if !ok {
+				err = PackError{ID: map2id(id), Err: errors.New("does not exist")}
+			}
+		}
+
+		if err != nil {
+			debug.Log("Checker.testPackID", "error checking for pack %s: %v", map2id(id).Str(), err)
+			select {
+			case <-done:
+				return
+			case errChan <- err:
+			}
+
+			continue
+		}
+
+		debug.Log("Checker.testPackID", "pack %s exists", map2id(id).Str())
+	}
+}
+
+func collectErrors(in <-chan error, out chan<- []error, done <-chan struct{}) {
+	var errs []error
+
+outer:
+	for {
+		select {
+		case err, ok := <-in:
+			if !ok {
+				break outer
+			}
+			errs = append(errs, err)
+		case <-done:
+			break outer
+		}
+	}
+
+	out <- errs
+}
+
+// Packs checks that all packs referenced in the index are still available and
+// there are no packs that aren't in an index. errChan is closed after all
+// packs have been checked.
+func (c *Checker) Packs(errChan chan<- error, done <-chan struct{}) {
+	defer close(errChan)
+
+	debug.Log("Checker.Packs", "checking for %d packs", len(c.packs))
+	seenPacks := make(map[mapID]struct{})
+
+	var workerWG sync.WaitGroup
+
+	IDChan := make(chan mapID)
+	for i := 0; i < defaultParallelism; i++ {
+		workerWG.Add(1)
+		go packIDTester(c.repo, IDChan, errChan, &workerWG, done)
+	}
+
+	for id := range c.packs {
+		seenPacks[id] = struct{}{}
+		IDChan <- id
+	}
+	close(IDChan)
+
+	debug.Log("Checker.Packs", "waiting for %d workers to terminate", defaultParallelism)
+	workerWG.Wait()
+	debug.Log("Checker.Packs", "workers terminated")
+
+	for id := range c.repo.List(backend.Data, done) {
+		debug.Log("Checker.Packs", "check data blob %v", id.Str())
+		if _, ok := seenPacks[id2map(id)]; !ok {
+			c.orphanedPacks = append(c.orphanedPacks, id)
+			select {
+			case <-done:
+				return
+			case errChan <- PackError{ID: id, Orphaned: true, Err: errors.New("not referenced in any index")}:
+			}
+		}
+	}
+}
+
+// Error is an error that occurred while checking a repository.
+type Error struct {
+	TreeID backend.ID
+	BlobID backend.ID
+	Err    error
+}
+
+func (e Error) Error() string {
+	if e.BlobID != nil && e.TreeID != nil {
+		msg := "tree " + e.TreeID.String()
+		msg += ", blob " + e.BlobID.String()
+		msg += ": " + e.Err.Error()
+		return msg
+	}
+
+	if e.TreeID != nil {
+		return "tree " + e.TreeID.String() + ": " + e.Err.Error()
+	}
+
+	return e.Err.Error()
+}
+
+func loadTreeFromSnapshot(repo *repository.Repository, id backend.ID) (backend.ID, error) {
+	sn, err := restic.LoadSnapshot(repo, id)
+	if err != nil {
+		debug.Log("Checker.loadTreeFromSnapshot", "error loading snapshot %v: %v", id.Str(), err)
+		return nil, err
+	}
+
+	if sn.Tree == nil {
+		debug.Log("Checker.loadTreeFromSnapshot", "snapshot %v has no tree", id.Str())
+		return nil, fmt.Errorf("snapshot %v has no tree", id)
+	}
+
+	return sn.Tree, nil
+}
+
+// loadSnapshotTreeIDs loads all snapshots from backend and returns the tree IDs.
+func loadSnapshotTreeIDs(repo *repository.Repository) (backend.IDs, []error) {
+	var trees struct {
+		IDs backend.IDs
+		sync.Mutex
+	}
+
+	var errs struct {
+		errs []error
+		sync.Mutex
+	}
+
+	snapshotWorker := func(strID string, done <-chan struct{}) error {
+		id, err := backend.ParseID(strID)
+		if err != nil {
+			return err
+		}
+
+		debug.Log("Checker.Snaphots", "load snapshot %v", id.Str())
+
+		treeID, err := loadTreeFromSnapshot(repo, id)
+		if err != nil {
+			errs.Lock()
+			errs.errs = append(errs.errs, err)
+			errs.Unlock()
+			return nil
+		}
+
+		debug.Log("Checker.Snaphots", "snapshot %v has tree %v", id.Str(), treeID.Str())
+		trees.Lock()
+		trees.IDs = append(trees.IDs, treeID)
+		trees.Unlock()
+
+		return nil
+	}
+
+	err := repository.FilesInParallel(repo.Backend(), backend.Snapshot, defaultParallelism, snapshotWorker)
+	if err != nil {
+		errs.errs = append(errs.errs, err)
+	}
+
+	return trees.IDs, errs.errs
+}
+
+// TreeError is returned when loading a tree from the repository failed.
+type TreeError struct {
+	ID     backend.ID
+	Errors []error
+}
+
+func (e TreeError) Error() string {
+	return fmt.Sprintf("%v: %d errors", e.ID.String(), len(e.Errors))
+}
+
+type treeJob struct {
+	backend.ID
+	error
+	*restic.Tree
+}
+
+// loadTreeWorker loads trees from repo and sends them to out.
+func loadTreeWorker(repo *repository.Repository,
+	in <-chan backend.ID, out chan<- treeJob,
+	done <-chan struct{}, wg *sync.WaitGroup) {
+
+	defer func() {
+		debug.Log("checker.loadTreeWorker", "exiting")
+		wg.Done()
+	}()
+
+	var (
+		inCh  = in
+		outCh = out
+		job   treeJob
+	)
+
+	outCh = nil
+	for {
+		select {
+		case <-done:
+			return
+
+		case treeID, ok := <-inCh:
+			if !ok {
+				return
+			}
+			debug.Log("checker.loadTreeWorker", "load tree %v", treeID.Str())
+
+			tree, err := restic.LoadTree(repo, treeID)
+			debug.Log("checker.loadTreeWorker", "load tree %v (%v) returned err %v", tree, treeID.Str(), err)
+			job = treeJob{ID: treeID, error: err, Tree: tree}
+			outCh = out
+			inCh = nil
+
+		case outCh <- job:
+			debug.Log("checker.loadTreeWorker", "sent tree %v", job.ID.Str())
+			outCh = nil
+			inCh = in
+		}
+	}
+}
+
+// checkTreeWorker checks the trees received and sends out errors to errChan.
+func (c *Checker) checkTreeWorker(in <-chan treeJob, out chan<- TreeError, done <-chan struct{}, wg *sync.WaitGroup) {
+	defer func() {
+		debug.Log("checker.checkTreeWorker", "exiting")
+		wg.Done()
+	}()
+
+	var (
+		inCh      = in
+		outCh     = out
+		treeError TreeError
+	)
+
+	outCh = nil
+	for {
+		select {
+		case <-done:
+			return
+
+		case job, ok := <-inCh:
+			if !ok {
+				return
+			}
+
+			id := id2map(job.ID)
+			alreadyChecked := false
+			c.blobRefs.Lock()
+			if c.blobRefs.M[id] > 0 {
+				alreadyChecked = true
+			}
+			c.blobRefs.M[id]++
+			debug.Log("checker.checkTreeWorker", "tree %v refcount %d", job.ID.Str(), c.blobRefs.M[id])
+			c.blobRefs.Unlock()
+
+			if alreadyChecked {
+				continue
+			}
+
+			debug.Log("checker.checkTreeWorker", "load tree %v", job.ID.Str())
+
+			errs := c.checkTree(job.ID, job.Tree)
+			if len(errs) > 0 {
+				debug.Log("checker.checkTreeWorker", "checked tree %v: %v errors", job.ID.Str(), len(errs))
+				treeError = TreeError{ID: job.ID, Errors: errs}
+				outCh = out
+				inCh = nil
+			}
+
+		case outCh <- treeError:
+			debug.Log("checker.checkTreeWorker", "tree %v: sent %d errors", treeError.ID, len(treeError.Errors))
+			outCh = nil
+			inCh = in
+		}
+	}
+}
+
+func filterTrees(backlog backend.IDs, loaderChan chan<- backend.ID, in <-chan treeJob, out chan<- treeJob, done <-chan struct{}) {
+	defer func() {
+		debug.Log("checker.filterTrees", "closing output channels")
+		close(loaderChan)
+		close(out)
+	}()
+
+	var (
+		inCh                    = in
+		outCh                   = out
+		loadCh                  = loaderChan
+		job                     treeJob
+		nextTreeID              backend.ID
+		outstandingLoadTreeJobs = 0
+	)
+
+	outCh = nil
+	loadCh = nil
+
+	for {
+		if loadCh == nil && len(backlog) > 0 {
+			loadCh = loaderChan
+			nextTreeID, backlog = backlog[0], backlog[1:]
+		}
+
+		if loadCh == nil && outCh == nil && outstandingLoadTreeJobs == 0 {
+			debug.Log("checker.filterTrees", "backlog is empty, all channels nil, exiting")
+			return
+		}
+
+		select {
+		case <-done:
+			return
+
+		case loadCh <- nextTreeID:
+			outstandingLoadTreeJobs++
+			loadCh = nil
+
+		case j, ok := <-inCh:
+			if !ok {
+				debug.Log("checker.filterTrees", "input channel closed")
+				inCh = nil
+				in = nil
+				continue
+			}
+
+			outstandingLoadTreeJobs--
+			debug.Log("checker.filterTrees", "input job tree %v", j.ID.Str())
+
+			backlog = append(backlog, j.Tree.Subtrees()...)
+
+			job = j
+			outCh = out
+			inCh = nil
+
+		case outCh <- job:
+			outCh = nil
+			inCh = in
+		}
+	}
+}
+
+// Structure checks that for all snapshots all referenced data blobs and
+// subtrees are available in the index. errChan is closed after all trees have
+// been traversed.
+func (c *Checker) Structure(errChan chan<- error, done <-chan struct{}) {
+	defer close(errChan)
+
+	trees, errs := loadSnapshotTreeIDs(c.repo)
+	debug.Log("checker.Structure", "need to check %d trees from snapshots, %d errs returned", len(trees), len(errs))
+
+	for _, err := range errs {
+		select {
+		case <-done:
+			return
+		case errChan <- err:
+		}
+	}
+
+	treeIDChan := make(chan backend.ID)
+	treeJobChan1 := make(chan treeJob)
+	treeJobChan2 := make(chan treeJob)
+	treeErrChan := make(chan TreeError)
+
+	var wg sync.WaitGroup
+	for i := 0; i < defaultParallelism; i++ {
+		wg.Add(2)
+		go loadTreeWorker(c.repo, treeIDChan, treeJobChan1, done, &wg)
+		go c.checkTreeWorker(treeJobChan2, treeErrChan, done, &wg)
+	}
+
+	filterTrees(trees, treeIDChan, treeJobChan1, treeJobChan2, done)
+
+	wg.Wait()
+}
+
+func (c *Checker) checkTree(id backend.ID, tree *restic.Tree) (errs []error) {
+	debug.Log("Checker.checkTree", "checking tree %v", id.Str())
+
+	// if _, ok := c.blobs[id2map(id)]; !ok {
+	// 	errs = append(errs, Error{TreeID: id, Err: errors.New("not found in index")})
+	// }
+
+	// blobs, subtrees, treeErrors := c.tree(id)
+	// if treeErrors != nil {
+	// 	debug.Log("Checker.trees", "error checking tree %v: %v", id.Str(), treeErrors)
+	// 	errs = append(errs, treeErrors...)
+	// 	continue
+	// }
+
+	// treeIDs = append(treeIDs, subtrees...)
+
+	// treesChecked[id2map(id)] = struct{}{}
+
+	var blobs []backend.ID
+
+	for i, node := range tree.Nodes {
+		switch node.Type {
+		case "file":
+			blobs = append(blobs, node.Content...)
+		case "dir":
+			if node.Subtree == nil {
+				errs = append(errs, Error{TreeID: id, Err: fmt.Errorf("node %d is dir but has no subtree", i)})
+				continue
+			}
+		}
+	}
+
+	for _, blobID := range blobs {
+		mid := id2map(blobID)
+		c.blobRefs.Lock()
+		c.blobRefs.M[mid]++
+		debug.Log("Checker.checkTree", "blob %v refcount %d", blobID.Str(), c.blobRefs.M[mid])
+		c.blobRefs.Unlock()
+
+		if _, ok := c.blobs[id2map(blobID)]; !ok {
+			debug.Log("Checker.trees", "tree %v references blob %v which isn't contained in index", id.Str(), blobID.Str())
+
+			errs = append(errs, Error{TreeID: id, BlobID: blobID, Err: errors.New("not found in index")})
+		}
+	}
+
+	return errs
+}
+
+// UnusedBlobs returns all blobs that have never been referenced.
+func (c *Checker) UnusedBlobs() (blobs backend.IDs) {
+	c.blobRefs.Lock()
+	defer c.blobRefs.Unlock()
+
+	debug.Log("Checker.UnusedBlobs", "checking %d blobs", len(c.blobs))
+	for id := range c.blobs {
+		if c.blobRefs.M[id] == 0 {
+			debug.Log("Checker.UnusedBlobs", "blob %v not not referenced", map2id(id).Str())
+			blobs = append(blobs, map2id(id))
+		}
+	}
+
+	return blobs
+}
+
+// OrphanedPacks returns a slice of unused packs (only available after Packs() was run).
+func (c *Checker) OrphanedPacks() backend.IDs {
+	return c.orphanedPacks
+}
diff --git a/checker/checker_test.go b/checker/checker_test.go
new file mode 100644
index 000000000..eadb71181
--- /dev/null
+++ b/checker/checker_test.go
@@ -0,0 +1,142 @@
+package checker_test
+
+import (
+	"path/filepath"
+	"sort"
+	"testing"
+
+	"github.com/restic/restic/backend"
+	"github.com/restic/restic/checker"
+	"github.com/restic/restic/repository"
+	. "github.com/restic/restic/test"
+)
+
+var checkerTestData = filepath.Join("testdata", "checker-test-repo.tar.gz")
+
+func list(repo *repository.Repository, t backend.Type) (IDs []string) {
+	done := make(chan struct{})
+	defer close(done)
+
+	for id := range repo.List(t, done) {
+		IDs = append(IDs, id.String())
+	}
+
+	return IDs
+}
+
+func checkPacks(chkr *checker.Checker) (errs []error) {
+	done := make(chan struct{})
+	defer close(done)
+
+	errChan := make(chan error)
+
+	go chkr.Packs(errChan, done)
+
+	for err := range errChan {
+		errs = append(errs, err)
+	}
+
+	return errs
+}
+
+func checkStruct(chkr *checker.Checker) (errs []error) {
+	done := make(chan struct{})
+	defer close(done)
+
+	errChan := make(chan error)
+
+	go chkr.Structure(errChan, done)
+
+	for err := range errChan {
+		errs = append(errs, err)
+	}
+
+	return errs
+}
+
+func TestCheckRepo(t *testing.T) {
+	WithTestEnvironment(t, checkerTestData, func(repodir string) {
+		repo := OpenLocalRepo(t, repodir)
+
+		chkr := checker.New(repo)
+		OK(t, chkr.LoadIndex())
+		OKs(t, checkPacks(chkr))
+		OKs(t, checkStruct(chkr))
+	})
+}
+
+func TestMissingPack(t *testing.T) {
+	WithTestEnvironment(t, checkerTestData, func(repodir string) {
+		repo := OpenLocalRepo(t, repodir)
+
+		packID := "657f7fb64f6a854fff6fe9279998ee09034901eded4e6db9bcee0e59745bbce6"
+		OK(t, repo.Backend().Remove(backend.Data, packID))
+
+		chkr := checker.New(repo)
+		OK(t, chkr.LoadIndex())
+		errs := checkPacks(chkr)
+
+		Assert(t, len(errs) == 1,
+			"expected exactly one error, got %v", len(errs))
+
+		if err, ok := errs[0].(checker.PackError); ok {
+			Equals(t, packID, err.ID.String())
+		} else {
+			t.Errorf("expected error returned by checker.Packs() to be PackError, got %v", err)
+		}
+	})
+}
+
+func TestUnreferencedPack(t *testing.T) {
+	WithTestEnvironment(t, checkerTestData, func(repodir string) {
+		repo := OpenLocalRepo(t, repodir)
+
+		// index 8eb5 only references pack 60e0
+		indexID := "8eb5b61062bf8e959f244fba0c971108bc8d4d2a4b236f71a704998e28cc5cf6"
+		packID := "60e0438dcb978ec6860cc1f8c43da648170ee9129af8f650f876bad19f8f788e"
+		OK(t, repo.Backend().Remove(backend.Index, indexID))
+
+		chkr := checker.New(repo)
+		OK(t, chkr.LoadIndex())
+		errs := checkPacks(chkr)
+
+		Assert(t, len(errs) == 1,
+			"expected exactly one error, got %v", len(errs))
+
+		if err, ok := errs[0].(checker.PackError); ok {
+			Equals(t, packID, err.ID.String())
+		} else {
+			t.Errorf("expected error returned by checker.Packs() to be PackError, got %v", err)
+		}
+	})
+}
+
+func TestUnreferencedBlobs(t *testing.T) {
+	WithTestEnvironment(t, checkerTestData, func(repodir string) {
+		repo := OpenLocalRepo(t, repodir)
+
+		snID := "51d249d28815200d59e4be7b3f21a157b864dc343353df9d8e498220c2499b02"
+		OK(t, repo.Backend().Remove(backend.Snapshot, snID))
+
+		unusedBlobsBySnapshot := backend.IDs{
+			ParseID("58c748bbe2929fdf30c73262bd8313fe828f8925b05d1d4a87fe109082acb849"),
+			ParseID("988a272ab9768182abfd1fe7d7a7b68967825f0b861d3b36156795832c772235"),
+			ParseID("c01952de4d91da1b1b80bc6e06eaa4ec21523f4853b69dc8231708b9b7ec62d8"),
+			ParseID("bec3a53d7dc737f9a9bee68b107ec9e8ad722019f649b34d474b9982c3a3fec7"),
+			ParseID("2a6f01e5e92d8343c4c6b78b51c5a4dc9c39d42c04e26088c7614b13d8d0559d"),
+			ParseID("18b51b327df9391732ba7aaf841a4885f350d8a557b2da8352c9acf8898e3f10"),
+		}
+
+		sort.Sort(unusedBlobsBySnapshot)
+
+		chkr := checker.New(repo)
+		OK(t, chkr.LoadIndex())
+		OKs(t, checkPacks(chkr))
+		OKs(t, checkStruct(chkr))
+
+		blobs := chkr.UnusedBlobs()
+		sort.Sort(blobs)
+
+		Equals(t, unusedBlobsBySnapshot, blobs)
+	})
+}
diff --git a/checker/testdata/checker-test-repo.tar.gz b/checker/testdata/checker-test-repo.tar.gz
new file mode 100644
index 000000000..9cfc38573
Binary files /dev/null and b/checker/testdata/checker-test-repo.tar.gz differ
diff --git a/cmd/restic/cmd_check.go b/cmd/restic/cmd_check.go
new file mode 100644
index 000000000..d47de1c7b
--- /dev/null
+++ b/cmd/restic/cmd_check.go
@@ -0,0 +1,106 @@
+package main
+
+import (
+	"errors"
+	"fmt"
+	"os"
+
+	"github.com/restic/restic/backend"
+	"github.com/restic/restic/checker"
+)
+
+type CmdCheck struct {
+	ReadData       bool `long:"read-data"  description:"Read data blobs" default:"false"`
+	RemoveOrphaned bool `long:"remove"     description:"Remove data that isn't used" default:"false"`
+
+	global *GlobalOptions
+}
+
+func init() {
+	_, err := parser.AddCommand("check",
+		"check the repository",
+		"The check command check the integrity and consistency of the repository",
+		&CmdCheck{global: &globalOpts})
+	if err != nil {
+		panic(err)
+	}
+}
+
+func (cmd CmdCheck) Usage() string {
+	return "[check-options]"
+}
+
+func (cmd CmdCheck) Execute(args []string) error {
+	if len(args) != 0 {
+		return errors.New("check has no arguments")
+	}
+
+	repo, err := cmd.global.OpenRepository()
+	if err != nil {
+		return err
+	}
+
+	cmd.global.Verbosef("Create exclusive lock for repository\n")
+	lock, err := lockRepoExclusive(repo)
+	defer unlockRepo(lock)
+	if err != nil {
+		return err
+	}
+
+	chkr := checker.New(repo)
+
+	cmd.global.Verbosef("Load indexes\n")
+	if err = chkr.LoadIndex(); err != nil {
+		return err
+	}
+
+	done := make(chan struct{})
+	defer close(done)
+
+	errorsFound := false
+	errChan := make(chan error)
+
+	cmd.global.Verbosef("Check all packs\n")
+	go chkr.Packs(errChan, done)
+
+	foundOrphanedPacks := false
+	for err := range errChan {
+		errorsFound = true
+		fmt.Fprintf(os.Stderr, "%v\n", err)
+
+		if e, ok := err.(checker.PackError); ok && e.Orphaned {
+			foundOrphanedPacks = true
+		}
+	}
+
+	cmd.global.Verbosef("Check snapshots, trees and blobs\n")
+	errChan = make(chan error)
+	go chkr.Structure(errChan, done)
+
+	for err := range errChan {
+		errorsFound = true
+		fmt.Fprintf(os.Stderr, "error: %v\n", err)
+	}
+
+	for _, id := range chkr.UnusedBlobs() {
+		cmd.global.Verbosef("unused blob %v\n", id.Str())
+	}
+
+	if foundOrphanedPacks && cmd.RemoveOrphaned {
+		IDs := chkr.OrphanedPacks()
+		cmd.global.Verbosef("Remove %d orphaned packs... ", len(IDs))
+
+		for _, id := range IDs {
+			if err := repo.Backend().Remove(backend.Data, id.String()); err != nil {
+				fmt.Fprintf(os.Stderr, "%v\n", err)
+			}
+		}
+
+		cmd.global.Verbosef("done\n")
+	}
+
+	if errorsFound {
+		return errors.New("repository contains errors")
+	}
+	return nil
+}
diff --git a/cmd/restic/cmd_fsck.go b/cmd/restic/cmd_fsck.go
deleted file mode 100644
index b4e8d3e0d..000000000
--- a/cmd/restic/cmd_fsck.go
+++ /dev/null
@@ -1,272 +0,0 @@
-package main
-
-import (
-	"errors"
-	"fmt"
-	"os"
-
-	"github.com/restic/restic"
-	"github.com/restic/restic/backend"
-	"github.com/restic/restic/crypto"
-	"github.com/restic/restic/debug"
-	"github.com/restic/restic/pack"
-	"github.com/restic/restic/repository"
-)
-
-type CmdFsck struct {
-	CheckData      bool   `          long:"check-data"      description:"Read data blobs" default:"false"`
-	Snapshot       string `short:"s" long:"snapshot"        description:"Only check this snapshot"`
-	Orphaned       bool   `short:"o" long:"orphaned"        description:"Check for orphaned blobs"`
-	RemoveOrphaned bool   `short:"r" long:"remove-orphaned" description:"Remove orphaned blobs (implies -o)"`
-
-	global *GlobalOptions
-
-	// lists checking for orphaned blobs
-	o_data  *backend.IDSet
-	o_trees *backend.IDSet
-}
-
-func init() {
-	_, err := parser.AddCommand("fsck",
-		"check the repository",
-		"The fsck command check the integrity and consistency of the repository",
-		&CmdFsck{global: &globalOpts})
-	if err != nil {
-		panic(err)
-	}
-}
-
-func fsckFile(global CmdFsck, repo *repository.Repository, IDs []backend.ID) (uint64, error) {
-	debug.Log("restic.fsckFile", "checking file %v", IDs)
-	var bytes uint64
-
-	for _, id := range IDs {
-		debug.Log("restic.fsck", "  checking data blob %v\n", id)
-
-		// test if blob is in the index
-		packID, tpe, _, length, err := repo.Index().Lookup(id)
-		if err != nil {
-			return 0, fmt.Errorf("storage for blob %v (%v) not found", id, tpe)
-		}
-
-		bytes += uint64(length - crypto.Extension)
-		debug.Log("restic.fsck", "  blob found in pack %v\n", packID)
-
-		if global.CheckData {
-			// load content
-			_, err := repo.LoadBlob(pack.Data, id)
-			if err != nil {
-				return 0, err
-			}
-		} else {
-			// test if pack for data blob is there
-			ok, err := repo.Backend().Test(backend.Data, packID.String())
-			if err != nil {
-				return 0, err
-			}
-
-			if !ok {
-				return 0, fmt.Errorf("data blob %v not found", id)
-			}
-		}
-
-		// if orphan check is active, record storage id
-		if global.o_data != nil {
-			debug.Log("restic.fsck", "  recording blob %v as used\n", id)
-			global.o_data.Insert(id)
-		}
-	}
-
-	return bytes, nil
-}
-
-func fsckTree(global CmdFsck, repo *repository.Repository, id backend.ID) error {
-	debug.Log("restic.fsckTree", "checking tree %v", id.Str())
-
-	tree, err := restic.LoadTree(repo, id)
-	if err != nil {
-		return err
-	}
-
-	// if orphan check is active, record storage id
-	if global.o_trees != nil {
-		// add ID to list
-		global.o_trees.Insert(id)
-	}
-
-	var firstErr error
-
-	seenIDs := backend.NewIDSet()
-
-	for i, node := range tree.Nodes {
-		if node.Name == "" {
-			return fmt.Errorf("node %v of tree %v has no name", i, id.Str())
-		}
-
-		if node.Type == "" {
-			return fmt.Errorf("node %q of tree %v has no type", node.Name, id.Str())
-		}
-
-		switch node.Type {
-		case "file":
-			if node.Content == nil {
-				debug.Log("restic.fsckTree", "file node %q of tree %v has no content: %v", node.Name, id, node)
-				return fmt.Errorf("file node %q of tree %v has no content: %v", node.Name, id, node)
-			}
-
-			if node.Content == nil && node.Error == "" {
-				debug.Log("restic.fsckTree", "file node %q of tree %v has no content", node.Name, id)
-				return fmt.Errorf("file node %q of tree %v has no content", node.Name, id)
-			}
-
-			// record ids
-			for _, id := range node.Content {
-				seenIDs.Insert(id)
-			}
-
-			debug.Log("restic.fsckTree", "check file %v (%v)", node.Name, id.Str())
-			bytes, err := fsckFile(global, repo, node.Content)
-			if err != nil {
-				return err
-			}
-
-			if bytes != node.Size {
-				debug.Log("restic.fsckTree", "file node %q of tree %v has size %d, but only %d bytes could be found", node.Name, id, node.Size, bytes)
-				return fmt.Errorf("file node %q of tree %v has size %d, but only %d bytes could be found", node.Name, id, node.Size, bytes)
-			}
-		case "dir":
-			if node.Subtree == nil {
-				return fmt.Errorf("dir node %q of tree %v has no subtree", node.Name, id)
-			}
-
-			// record id
-			seenIDs.Insert(node.Subtree)
-
-			err = fsckTree(global, repo, node.Subtree)
-			if err != nil {
-				firstErr = err
-				fmt.Fprintf(os.Stderr, "%v\n", err)
-			}
-		}
-	}
-
-	// check map for unused ids
-	// for _, id := range tree.Map.IDs() {
-	// 	if seenIDs.Find(id) != nil {
-	// 		return fmt.Errorf("tree %v: map contains unused ID %v", id, id)
-	// 	}
-	// }
-
-	return firstErr
-}
-
-func fsckSnapshot(global CmdFsck, repo *repository.Repository, id backend.ID) error {
-	debug.Log("restic.fsck", "checking snapshot %v\n", id)
-
-	sn, err := restic.LoadSnapshot(repo, id)
-	if err != nil {
-		return fmt.Errorf("loading snapshot %v failed: %v", id, err)
-	}
-
-	err = fsckTree(global, repo, sn.Tree)
-	if err != nil {
-		debug.Log("restic.fsck", "  checking tree %v for snapshot %v\n", sn.Tree, id)
-		fmt.Fprintf(os.Stderr, "snapshot %v:\n  error for tree %v:\n    %v\n", id, sn.Tree, err)
-	}
-
-	return err
-}
-
-func (cmd CmdFsck) Usage() string {
-	return "[fsck-options]"
-}
-
-func (cmd CmdFsck) Execute(args []string) error {
-	if len(args) != 0 {
-		return errors.New("fsck has no arguments")
-	}
-
-	if cmd.RemoveOrphaned && !cmd.Orphaned {
-		cmd.Orphaned = true
-	}
-
-	repo, err := cmd.global.OpenRepository()
-	if err != nil {
-		return err
-	}
-
-	lock, err := lockRepoExclusive(repo)
-	defer unlockRepo(lock)
-	if err != nil {
-		return err
-	}
-
-	err = repo.LoadIndex()
-	if err != nil {
-		return err
-	}
-
-	if cmd.Snapshot != "" {
-		id, err := restic.FindSnapshot(repo, cmd.Snapshot)
-		if err != nil {
-			return fmt.Errorf("invalid id %q: %v", cmd.Snapshot, err)
-		}
-
-		err = fsckSnapshot(cmd, repo, id)
-		if err != nil {
-			fmt.Fprintf(os.Stderr, "check for snapshot %v failed\n", id)
-		}
-
-		return err
-	}
-
-	if cmd.Orphaned {
-		cmd.o_data = backend.NewIDSet()
-		cmd.o_trees = backend.NewIDSet()
-	}
-
-	done := make(chan struct{})
-	defer close(done)
-
-	var firstErr error
-	for id := range repo.List(backend.Snapshot, done) {
-		err = fsckSnapshot(cmd, repo, id)
-		if err != nil {
-			fmt.Fprintf(os.Stderr, "check for snapshot %v failed\n", id)
-			firstErr = err
-		}
-	}
-
-	if !cmd.Orphaned {
-		return firstErr
-	}
-
-	debug.Log("restic.fsck", "starting orphaned check\n")
-
-	cnt := make(map[pack.BlobType]*backend.IDSet)
-	cnt[pack.Data] = cmd.o_data
-	cnt[pack.Tree] = cmd.o_trees
-
-	for blob := range repo.Index().Each(done) {
-		debug.Log("restic.fsck", "checking %v blob %v\n", blob.Type, blob.ID)
-
-		err = cnt[blob.Type].Find(blob.ID)
-		if err != nil {
-			debug.Log("restic.fsck", "  blob %v is orphaned\n", blob.ID)
-
-			if !cmd.RemoveOrphaned {
-				fmt.Printf("orphaned %v blob %v\n", blob.Type, blob.ID)
-				continue
-			}
-
-			fmt.Printf("removing orphaned %v blob %v\n", blob.Type, blob.ID)
-			// err := s.Remove(d.tpe, name)
-			// if err != nil {
-			// 	return err
-			// }
-			return errors.New("not implemented")
-		}
-	}
-
-	return firstErr
-}
diff --git a/cmd/restic/integration_test.go b/cmd/restic/integration_test.go
index 101fa44eb..b06b85745 100644
--- a/cmd/restic/integration_test.go
+++ b/cmd/restic/integration_test.go
@@ -8,7 +8,6 @@ import (
 	"io"
 	"io/ioutil"
 	"os"
-	"os/exec"
 	"path/filepath"
 	"regexp"
 	"syscall"
@@ -20,20 +19,6 @@ import (
 	. "github.com/restic/restic/test"
 )
 
-func setupTarTestFixture(t testing.TB, outputDir, tarFile string) {
-	err := system("sh", "-c", `(cd "$1" && tar xz) < "$2"`,
-		"sh", outputDir, tarFile)
-	OK(t, err)
-}
-
-func system(command string, args ...string) error {
-	cmd := exec.Command(command, args...)
-	cmd.Stdout = os.Stdout
-	cmd.Stderr = os.Stderr
-
-	return cmd.Run()
-}
-
 func parseIDsFromReader(t testing.TB, rd io.Reader) backend.IDs {
 	IDs := backend.IDs{}
 	sc := bufio.NewScanner(rd)
@@ -83,8 +68,8 @@ func cmdRestore(t testing.TB, global GlobalOptions, dir string, snapshotID backe
 	cmd.Execute(append([]string{snapshotID.String(), dir}, args...))
 }
 
-func cmdFsck(t testing.TB, global GlobalOptions) {
-	cmd := &CmdFsck{global: &global, CheckData: true, Orphaned: true}
+func cmdCheck(t testing.TB, global GlobalOptions) {
+	cmd := &CmdCheck{global: &global, ReadData: true}
 	OK(t, cmd.Execute(nil))
 }
 
@@ -101,7 +86,7 @@ func TestBackup(t *testing.T) {
 
 		cmdInit(t, global)
 
-		setupTarTestFixture(t, env.testdata, datafile)
+		SetupTarTestFixture(t, env.testdata, datafile)
 
 		// first backup
 		cmdBackup(t, global, []string{env.testdata}, nil)
@@ -109,7 +94,7 @@ func TestBackup(t *testing.T) {
 		Assert(t, len(snapshotIDs) == 1,
 			"expected one snapshot, got %v", snapshotIDs)
 
-		cmdFsck(t, global)
+		cmdCheck(t, global)
 		stat1 := dirStats(env.repo)
 
 		// second backup, implicit incremental
@@ -124,7 +109,7 @@ func TestBackup(t *testing.T) {
 		}
 		t.Logf("repository grown by %d bytes", stat2.size-stat1.size)
 
-		cmdFsck(t, global)
+		cmdCheck(t, global)
 		// third backup, explicit incremental
 		cmdBackup(t, global, []string{env.testdata}, snapshotIDs[0])
 		snapshotIDs = cmdList(t, global, "snapshots")
@@ -146,7 +131,7 @@ func TestBackup(t *testing.T) {
 				"directories are not equal")
 		}
 
-		cmdFsck(t, global)
+		cmdCheck(t, global)
 	})
 }
 
@@ -161,7 +146,7 @@ func TestBackupNonExistingFile(t *testing.T) {
 		OK(t, err)
 		OK(t, fd.Close())
 
-		setupTarTestFixture(t, env.testdata, datafile)
+		SetupTarTestFixture(t, env.testdata, datafile)
 
 		cmdInit(t, global)
 
@@ -189,7 +174,7 @@ func TestBackupMissingFile1(t *testing.T) {
 		OK(t, err)
 		OK(t, fd.Close())
 
-		setupTarTestFixture(t, env.testdata, datafile)
+		SetupTarTestFixture(t, env.testdata, datafile)
 
 		cmdInit(t, global)
 
@@ -208,7 +193,7 @@ func TestBackupMissingFile1(t *testing.T) {
 		})
 
 		cmdBackup(t, global, []string{env.testdata}, nil)
-		cmdFsck(t, global)
+		cmdCheck(t, global)
 
 		Assert(t, ranHook, "hook did not run")
 		debug.RemoveHook("pipe.walk1")
@@ -226,7 +211,7 @@ func TestBackupMissingFile2(t *testing.T) {
 		OK(t, err)
 		OK(t, fd.Close())
 
-		setupTarTestFixture(t, env.testdata, datafile)
+		SetupTarTestFixture(t, env.testdata, datafile)
 
 		cmdInit(t, global)
 
@@ -245,7 +230,7 @@ func TestBackupMissingFile2(t *testing.T) {
 		})
 
 		cmdBackup(t, global, []string{env.testdata}, nil)
-		cmdFsck(t, global)
+		cmdCheck(t, global)
 
 		Assert(t, ranHook, "hook did not run")
 		debug.RemoveHook("pipe.walk2")
@@ -290,13 +275,13 @@ func TestIncrementalBackup(t *testing.T) {
 		OK(t, appendRandomData(testfile, incrementalFirstWrite))
 
 		cmdBackup(t, global, []string{datadir}, nil)
-		cmdFsck(t, global)
+		cmdCheck(t, global)
 		stat1 := dirStats(env.repo)
 
 		OK(t, appendRandomData(testfile, incrementalSecondWrite))
 
 		cmdBackup(t, global, []string{datadir}, nil)
-		cmdFsck(t, global)
+		cmdCheck(t, global)
 		stat2 := dirStats(env.repo)
 		if stat2.size-stat1.size > incrementalFirstWrite {
 			t.Errorf("repository size has grown by more than %d bytes", incrementalFirstWrite)
@@ -306,7 +291,7 @@ func TestIncrementalBackup(t *testing.T) {
 		OK(t, appendRandomData(testfile, incrementalThirdWrite))
 
 		cmdBackup(t, global, []string{datadir}, nil)
-		cmdFsck(t, global)
+		cmdCheck(t, global)
 		stat3 := dirStats(env.repo)
 		if stat3.size-stat2.size > incrementalFirstWrite {
 			t.Errorf("repository size has grown by more than %d bytes", incrementalFirstWrite)
@@ -387,7 +372,7 @@ func TestKeyAddRemove(t *testing.T) {
 		t.Logf("testing access with last password %q\n", global.password)
 		cmdKey(t, global, "list")
 
-		cmdFsck(t, global)
+		cmdCheck(t, global)
 	})
 }
 
@@ -425,7 +410,7 @@ func TestRestoreFilter(t *testing.T) {
 		}
 
 		cmdBackup(t, global, []string{env.testdata}, nil)
-		cmdFsck(t, global)
+		cmdCheck(t, global)
 
 		snapshotID := cmdList(t, global, "snapshots")[0]
 
@@ -471,7 +456,7 @@ func TestRestoreNoMetadataOnIgnoredIntermediateDirs(t *testing.T) {
 		OK(t, setZeroModTime(filepath.Join(env.testdata, "subdir1", "subdir2")))
 
 		cmdBackup(t, global, []string{env.testdata}, nil)
-		cmdFsck(t, global)
+		cmdCheck(t, global)
 
 		snapshotID := cmdList(t, global, "snapshots")[0]
 
diff --git a/repository/index.go b/repository/index.go
index 29a725872..d7bd4bf16 100644
--- a/repository/index.go
+++ b/repository/index.go
@@ -2,7 +2,6 @@ package repository
 
 import (
 	"encoding/json"
-	"errors"
 	"fmt"
 	"io"
 	"sync"
@@ -79,7 +78,7 @@ func (idx *Index) Lookup(id backend.ID) (packID backend.ID, tpe pack.BlobType, o
 	}
 
 	debug.Log("Index.Lookup", "id %v not found", id.Str())
-	return nil, pack.Data, 0, 0, errors.New("id not found")
+	return nil, pack.Data, 0, 0, fmt.Errorf("id %v not found in index", id)
 }
 
 // Has returns true iff the id is listed in the index.
@@ -108,13 +107,19 @@ func (idx *Index) Merge(other *Index) {
 	debug.Log("Index.Merge", "done merging index")
 }
 
+// PackedBlob is a blob already saved within a pack.
+type PackedBlob struct {
+	pack.Blob
+	PackID backend.ID
+}
+
 // Each returns a channel that yields all blobs known to the index. If done is
 // closed, the background goroutine terminates. This blocks any modification of
 // the index.
-func (idx *Index) Each(done chan struct{}) <-chan pack.Blob {
+func (idx *Index) Each(done chan struct{}) <-chan PackedBlob {
 	idx.m.Lock()
 
-	ch := make(chan pack.Blob)
+	ch := make(chan PackedBlob)
 
 	go func() {
 		defer idx.m.Unlock()
@@ -132,11 +137,14 @@ func (idx *Index) Each(done chan struct{}) <-chan pack.Blob {
 			select {
 			case <-done:
 				return
-			case ch <- pack.Blob{
-				ID:     id,
-				Offset: blob.offset,
-				Type:   blob.tpe,
-				Length: uint32(blob.length),
+			case ch <- PackedBlob{
+				Blob: pack.Blob{
+					ID:     id,
+					Offset: blob.offset,
+					Type:   blob.tpe,
+					Length: uint32(blob.length),
+				},
+				PackID: blob.packID,
 			}:
 			}
 		}
diff --git a/test/helpers.go b/test/helpers.go
index 544195d9b..a9306cf3a 100644
--- a/test/helpers.go
+++ b/test/helpers.go
@@ -35,6 +35,21 @@ func OK(tb testing.TB, err error) {
 	}
 }
 
+// OKs fails the test if any error from errs is not nil.
+func OKs(tb testing.TB, errs []error) {
+	errFound := false
+	for _, err := range errs {
+		if err != nil {
+			errFound = true
+			_, file, line, _ := runtime.Caller(1)
+			fmt.Printf("\033[31m%s:%d: unexpected error: %s\033[39m\n\n", filepath.Base(file), line, err.Error())
+		}
+	}
+	if errFound {
+		tb.FailNow()
+	}
+}
+
 // Equals fails the test if exp is not equal to act.
 func Equals(tb testing.TB, exp, act interface{}) {
 	if !reflect.DeepEqual(exp, act) {
diff --git a/tree.go b/tree.go
index 8afa6cc64..8b34f2bd6 100644
--- a/tree.go
+++ b/tree.go
@@ -89,3 +89,14 @@ func (t Tree) Find(name string) (*Node, error) {
 	_, node, err := t.binarySearch(name)
 	return node, err
 }
+
+// Subtrees returns a slice of all subtree IDs of the tree.
+func (t Tree) Subtrees() (trees backend.IDs) {
+	for _, node := range t.Nodes {
+		if node.Type == "dir" && node.Subtree != nil {
+			trees = append(trees, node.Subtree)
+		}
+	}
+
+	return trees
+}
diff --git a/walk.go b/walk.go
index 24b4a5294..304c133a5 100644
--- a/walk.go
+++ b/walk.go
@@ -21,7 +21,11 @@ func walkTree(repo *repository.Repository, path string, treeID backend.ID, done
 
 	t, err := LoadTree(repo, treeID)
 	if err != nil {
-		jobCh <- WalkTreeJob{Path: path, Error: err}
+		select {
+		case jobCh <- WalkTreeJob{Path: path, Error: err}:
+		case <-done:
+			return
+		}
 		return
 	}
 
@@ -30,11 +34,20 @@ func walkTree(repo *repository.Repository, path string, treeID backend.ID, done
 		if node.Type == "dir" {
 			walkTree(repo, p, node.Subtree, done, jobCh)
 		} else {
-			jobCh <- WalkTreeJob{Path: p, Node: node}
+			select {
+			case jobCh <- WalkTreeJob{Path: p, Node: node}:
+			case <-done:
+				return
+			}
 		}
 	}
 
-	jobCh <- WalkTreeJob{Path: path, Tree: t}
+	select {
+	case jobCh <- WalkTreeJob{Path: path, Tree: t}:
+	case <-done:
+		return
+	}
+
 	debug.Log("walkTree", "done for %q (%v)", path, treeID.Str())
 }