diff --git a/_examples/storage/README.md b/_examples/storage/README.md
index b7207ee11..fc72e6f6d 100644
--- a/_examples/storage/README.md
+++ b/_examples/storage/README.md
@@ -8,7 +8,7 @@
 ### and what this means ...
 *git* has as very well defined storage system, the `.git` directory, present on any repository. This is the place where `git` stores al the [`objects`](https://git-scm.com/book/en/v2/Git-Internals-Git-Objects), [`references`](https://git-scm.com/book/es/v2/Git-Internals-Git-References) and [`configuration`](https://git-scm.com/docs/git-config#_configuration_file). This information is stored in plain files.
 
-Our original **go-git** version was designed to work in memory, some time after we added support to read the `.git`, and now we have added support for fully customized  [storages](https://godoc.org/github.com/src-d/go-git#Storer).
+Our original **go-git** version was designed to work in memory, some time after we added support to read the `.git`, and now we have added support for fully customized [storages](https://godoc.org/gopkg.in/src-d/go-git.v4/storage#Storer).
 
 This means that the internal database of any repository can be saved and accessed on any support, databases, distributed filesystems, etc. This functionality is pretty similar to the [libgit2 backends](http://blog.deveo.com/your-git-repository-in-a-database-pluggable-backends-in-libgit2/)
 
diff --git a/config/config.go b/config/config.go
index 475045e64..477eb3570 100644
--- a/config/config.go
+++ b/config/config.go
@@ -6,6 +6,7 @@ import (
 	"errors"
 	"fmt"
 	"sort"
+	"strconv"
 
 	format "gopkg.in/src-d/go-git.v4/plumbing/format/config"
 )
@@ -40,6 +41,14 @@ type Config struct {
 		// Worktree is the path to the root of the working tree.
 		Worktree string
 	}
+
+	Pack struct {
+		// Window controls the size of the sliding window for delta
+		// compression.  The default is 10.  A value of 0 turns off
+		// delta compression entirely.
+		Window uint
+	}
+
 	// Remotes list of repository remotes, the key of the map is the name
 	// of the remote, should equal to RemoteConfig.Name.
 	Remotes map[string]*RemoteConfig
@@ -81,10 +90,14 @@ const (
 	remoteSection    = "remote"
 	submoduleSection = "submodule"
 	coreSection      = "core"
+	packSection      = "pack"
 	fetchKey         = "fetch"
 	urlKey           = "url"
 	bareKey          = "bare"
 	worktreeKey      = "worktree"
+	windowKey        = "window"
+
+	defaultPackWindow = uint(10)
 )
 
 // Unmarshal parses a git-config file and stores it.
@@ -98,6 +111,9 @@ func (c *Config) Unmarshal(b []byte) error {
 	}
 
 	c.unmarshalCore()
+	if err := c.unmarshalPack(); err != nil {
+		return err
+	}
 	c.unmarshalSubmodules()
 	return c.unmarshalRemotes()
 }
@@ -111,6 +127,21 @@ func (c *Config) unmarshalCore() {
 	c.Core.Worktree = s.Options.Get(worktreeKey)
 }
 
+func (c *Config) unmarshalPack() error {
+	s := c.Raw.Section(packSection)
+	window := s.Options.Get(windowKey)
+	if window == "" {
+		c.Pack.Window = defaultPackWindow
+	} else {
+		winUint, err := strconv.ParseUint(window, 10, 32)
+		if err != nil {
+			return err
+		}
+		c.Pack.Window = uint(winUint)
+	}
+	return nil
+}
+
 func (c *Config) unmarshalRemotes() error {
 	s := c.Raw.Section(remoteSection)
 	for _, sub := range s.Subsections {
@@ -138,6 +169,7 @@ func (c *Config) unmarshalSubmodules() {
 // Marshal returns Config encoded as a git-config file.
 func (c *Config) Marshal() ([]byte, error) {
 	c.marshalCore()
+	c.marshalPack()
 	c.marshalRemotes()
 	c.marshalSubmodules()
 
@@ -158,6 +190,13 @@ func (c *Config) marshalCore() {
 	}
 }
 
+func (c *Config) marshalPack() {
+	s := c.Raw.Section(packSection)
+	if c.Pack.Window != defaultPackWindow {
+		s.SetOption(windowKey, fmt.Sprintf("%d", c.Pack.Window))
+	}
+}
+
 func (c *Config) marshalRemotes() {
 	s := c.Raw.Section(remoteSection)
 	newSubsections := make(format.Subsections, 0, len(c.Remotes))
diff --git a/config/config_test.go b/config/config_test.go
index c27ee26be..019cee6f8 100644
--- a/config/config_test.go
+++ b/config/config_test.go
@@ -10,6 +10,8 @@ func (s *ConfigSuite) TestUnmarshall(c *C) {
 	input := []byte(`[core]
         bare = true
 		worktree = foo
+[pack]
+		window = 20
 [remote "origin"]
         url = git@github.com:mcuadros/go-git.git
         fetch = +refs/heads/*:refs/remotes/origin/*
@@ -33,6 +35,7 @@ func (s *ConfigSuite) TestUnmarshall(c *C) {
 
 	c.Assert(cfg.Core.IsBare, Equals, true)
 	c.Assert(cfg.Core.Worktree, Equals, "foo")
+	c.Assert(cfg.Pack.Window, Equals, uint(20))
 	c.Assert(cfg.Remotes, HasLen, 2)
 	c.Assert(cfg.Remotes["origin"].Name, Equals, "origin")
 	c.Assert(cfg.Remotes["origin"].URLs, DeepEquals, []string{"git@github.com:mcuadros/go-git.git"})
@@ -51,6 +54,8 @@ func (s *ConfigSuite) TestMarshall(c *C) {
 	output := []byte(`[core]
 	bare = true
 	worktree = bar
+[pack]
+	window = 20
 [remote "alt"]
 	url = git@github.com:mcuadros/go-git.git
 	url = git@github.com:src-d/go-git.git
@@ -65,6 +70,7 @@ func (s *ConfigSuite) TestMarshall(c *C) {
 	cfg := NewConfig()
 	cfg.Core.IsBare = true
 	cfg.Core.Worktree = "bar"
+	cfg.Pack.Window = 20
 	cfg.Remotes["origin"] = &RemoteConfig{
 		Name: "origin",
 		URLs: []string{"git@github.com:mcuadros/go-git.git"},
@@ -92,6 +98,8 @@ func (s *ConfigSuite) TestUnmarshallMarshall(c *C) {
 	bare = true
 	worktree = foo
 	custom = ignored
+[pack]
+	window = 20
 [remote "origin"]
 	url = git@github.com:mcuadros/go-git.git
 	fetch = +refs/heads/*:refs/remotes/origin/*
diff --git a/options.go b/options.go
index 9f10aae8c..4f840bfd2 100644
--- a/options.go
+++ b/options.go
@@ -95,6 +95,9 @@ type PullOptions struct {
 	// stored, if nil nothing is stored and the capability (if supported)
 	// no-progress, is sent to the server to avoid send this information.
 	Progress sideband.Progress
+	// Force allows the pull to update a local branch even when the remote
+	// branch does not descend from it.
+	Force bool
 }
 
 // Validate validates the fields and sets the default values.
@@ -142,6 +145,9 @@ type FetchOptions struct {
 	// Tags describe how the tags will be fetched from the remote repository,
 	// by default is TagFollowing.
 	Tags TagMode
+	// Force allows the fetch to update a local branch even when the remote
+	// branch does not descend from it.
+	Force bool
 }
 
 // Validate validates the fields and sets the default values.
diff --git a/plumbing/format/packfile/delta_selector.go b/plumbing/format/packfile/delta_selector.go
index 0b3539d78..77573ac7c 100644
--- a/plumbing/format/packfile/delta_selector.go
+++ b/plumbing/format/packfile/delta_selector.go
@@ -9,9 +9,6 @@ import (
 )
 
 const (
-	// How far back in the sorted list to search for deltas.  10 is
-	// the default in command line git.
-	deltaWindowSize = 10
 	// deltas based on deltas, how many steps we can do.
 	// 50 is the default value used in JGit
 	maxDepth = int64(50)
@@ -31,14 +28,24 @@ func newDeltaSelector(s storer.EncodedObjectStorer) *deltaSelector {
 	return &deltaSelector{s}
 }
 
-// ObjectsToPack creates a list of ObjectToPack from the hashes provided,
-// creating deltas if it's suitable, using an specific internal logic
-func (dw *deltaSelector) ObjectsToPack(hashes []plumbing.Hash) ([]*ObjectToPack, error) {
-	otp, err := dw.objectsToPack(hashes)
+// ObjectsToPack creates a list of ObjectToPack from the hashes
+// provided, creating deltas if it's suitable, using an specific
+// internal logic.  `packWindow` specifies the size of the sliding
+// window used to compare objects for delta compression; 0 turns off
+// delta compression entirely.
+func (dw *deltaSelector) ObjectsToPack(
+	hashes []plumbing.Hash,
+	packWindow uint,
+) ([]*ObjectToPack, error) {
+	otp, err := dw.objectsToPack(hashes, packWindow)
 	if err != nil {
 		return nil, err
 	}
 
+	if packWindow == 0 {
+		return otp, nil
+	}
+
 	dw.sort(otp)
 
 	var objectGroups [][]*ObjectToPack
@@ -60,7 +67,7 @@ func (dw *deltaSelector) ObjectsToPack(hashes []plumbing.Hash) ([]*ObjectToPack,
 		objs := objs
 		wg.Add(1)
 		go func() {
-			if walkErr := dw.walk(objs); walkErr != nil {
+			if walkErr := dw.walk(objs, packWindow); walkErr != nil {
 				once.Do(func() {
 					err = walkErr
 				})
@@ -77,10 +84,19 @@ func (dw *deltaSelector) ObjectsToPack(hashes []plumbing.Hash) ([]*ObjectToPack,
 	return otp, nil
 }
 
-func (dw *deltaSelector) objectsToPack(hashes []plumbing.Hash) ([]*ObjectToPack, error) {
+func (dw *deltaSelector) objectsToPack(
+	hashes []plumbing.Hash,
+	packWindow uint,
+) ([]*ObjectToPack, error) {
 	var objectsToPack []*ObjectToPack
 	for _, h := range hashes {
-		o, err := dw.encodedDeltaObject(h)
+		var o plumbing.EncodedObject
+		var err error
+		if packWindow == 0 {
+			o, err = dw.encodedObject(h)
+		} else {
+			o, err = dw.encodedDeltaObject(h)
+		}
 		if err != nil {
 			return nil, err
 		}
@@ -93,6 +109,10 @@ func (dw *deltaSelector) objectsToPack(hashes []plumbing.Hash) ([]*ObjectToPack,
 		objectsToPack = append(objectsToPack, otp)
 	}
 
+	if packWindow == 0 {
+		return objectsToPack, nil
+	}
+
 	if err := dw.fixAndBreakChains(objectsToPack); err != nil {
 		return nil, err
 	}
@@ -201,7 +221,10 @@ func (dw *deltaSelector) sort(objectsToPack []*ObjectToPack) {
 	sort.Sort(byTypeAndSize(objectsToPack))
 }
 
-func (dw *deltaSelector) walk(objectsToPack []*ObjectToPack) error {
+func (dw *deltaSelector) walk(
+	objectsToPack []*ObjectToPack,
+	packWindow uint,
+) error {
 	indexMap := make(map[plumbing.Hash]*deltaIndex)
 	for i := 0; i < len(objectsToPack); i++ {
 		target := objectsToPack[i]
@@ -218,7 +241,7 @@ func (dw *deltaSelector) walk(objectsToPack []*ObjectToPack) error {
 			continue
 		}
 
-		for j := i - 1; j >= 0 && i-j < deltaWindowSize; j-- {
+		for j := i - 1; j >= 0 && i-j < int(packWindow); j-- {
 			base := objectsToPack[j]
 			// Objects must use only the same type as their delta base.
 			// Since objectsToPack is sorted by type and size, once we find
diff --git a/plumbing/format/packfile/delta_selector_test.go b/plumbing/format/packfile/delta_selector_test.go
index ca4a96bc5..7d7fd0cf4 100644
--- a/plumbing/format/packfile/delta_selector_test.go
+++ b/plumbing/format/packfile/delta_selector_test.go
@@ -146,7 +146,8 @@ func (s *DeltaSelectorSuite) createTestObjects() {
 func (s *DeltaSelectorSuite) TestObjectsToPack(c *C) {
 	// Different type
 	hashes := []plumbing.Hash{s.hashes["base"], s.hashes["treeType"]}
-	otp, err := s.ds.ObjectsToPack(hashes)
+	deltaWindowSize := uint(10)
+	otp, err := s.ds.ObjectsToPack(hashes, deltaWindowSize)
 	c.Assert(err, IsNil)
 	c.Assert(len(otp), Equals, 2)
 	c.Assert(otp[0].Object, Equals, s.store.Objects[s.hashes["base"]])
@@ -154,7 +155,7 @@ func (s *DeltaSelectorSuite) TestObjectsToPack(c *C) {
 
 	// Size radically different
 	hashes = []plumbing.Hash{s.hashes["bigBase"], s.hashes["target"]}
-	otp, err = s.ds.ObjectsToPack(hashes)
+	otp, err = s.ds.ObjectsToPack(hashes, deltaWindowSize)
 	c.Assert(err, IsNil)
 	c.Assert(len(otp), Equals, 2)
 	c.Assert(otp[0].Object, Equals, s.store.Objects[s.hashes["bigBase"]])
@@ -162,7 +163,7 @@ func (s *DeltaSelectorSuite) TestObjectsToPack(c *C) {
 
 	// Delta Size Limit with no best delta yet
 	hashes = []plumbing.Hash{s.hashes["smallBase"], s.hashes["smallTarget"]}
-	otp, err = s.ds.ObjectsToPack(hashes)
+	otp, err = s.ds.ObjectsToPack(hashes, deltaWindowSize)
 	c.Assert(err, IsNil)
 	c.Assert(len(otp), Equals, 2)
 	c.Assert(otp[0].Object, Equals, s.store.Objects[s.hashes["smallBase"]])
@@ -170,7 +171,7 @@ func (s *DeltaSelectorSuite) TestObjectsToPack(c *C) {
 
 	// It will create the delta
 	hashes = []plumbing.Hash{s.hashes["base"], s.hashes["target"]}
-	otp, err = s.ds.ObjectsToPack(hashes)
+	otp, err = s.ds.ObjectsToPack(hashes, deltaWindowSize)
 	c.Assert(err, IsNil)
 	c.Assert(len(otp), Equals, 2)
 	c.Assert(otp[0].Object, Equals, s.store.Objects[s.hashes["target"]])
@@ -185,7 +186,7 @@ func (s *DeltaSelectorSuite) TestObjectsToPack(c *C) {
 		s.hashes["o2"],
 		s.hashes["o3"],
 	}
-	otp, err = s.ds.ObjectsToPack(hashes)
+	otp, err = s.ds.ObjectsToPack(hashes, deltaWindowSize)
 	c.Assert(err, IsNil)
 	c.Assert(len(otp), Equals, 3)
 	c.Assert(otp[0].Object, Equals, s.store.Objects[s.hashes["o1"]])
@@ -201,20 +202,32 @@ func (s *DeltaSelectorSuite) TestObjectsToPack(c *C) {
 	// a delta.
 	hashes = make([]plumbing.Hash, 0, deltaWindowSize+2)
 	hashes = append(hashes, s.hashes["base"])
-	for i := 0; i < deltaWindowSize; i++ {
+	for i := uint(0); i < deltaWindowSize; i++ {
 		hashes = append(hashes, s.hashes["smallTarget"])
 	}
 	hashes = append(hashes, s.hashes["target"])
 
 	// Don't sort so we can easily check the sliding window without
 	// creating a bunch of new objects.
-	otp, err = s.ds.objectsToPack(hashes)
+	otp, err = s.ds.objectsToPack(hashes, deltaWindowSize)
 	c.Assert(err, IsNil)
-	err = s.ds.walk(otp)
+	err = s.ds.walk(otp, deltaWindowSize)
 	c.Assert(err, IsNil)
-	c.Assert(len(otp), Equals, deltaWindowSize+2)
+	c.Assert(len(otp), Equals, int(deltaWindowSize)+2)
 	targetIdx := len(otp) - 1
 	c.Assert(otp[targetIdx].IsDelta(), Equals, false)
+
+	// Check that no deltas are created, and the objects are unsorted,
+	// if compression is off.
+	hashes = []plumbing.Hash{s.hashes["base"], s.hashes["target"]}
+	otp, err = s.ds.ObjectsToPack(hashes, 0)
+	c.Assert(err, IsNil)
+	c.Assert(len(otp), Equals, 2)
+	c.Assert(otp[0].Object, Equals, s.store.Objects[s.hashes["base"]])
+	c.Assert(otp[0].IsDelta(), Equals, false)
+	c.Assert(otp[1].Original, Equals, s.store.Objects[s.hashes["target"]])
+	c.Assert(otp[1].IsDelta(), Equals, false)
+	c.Assert(otp[1].Depth, Equals, 0)
 }
 
 func (s *DeltaSelectorSuite) TestMaxDepth(c *C) {
diff --git a/plumbing/format/packfile/encoder.go b/plumbing/format/packfile/encoder.go
index 142655904..7ee6546f4 100644
--- a/plumbing/format/packfile/encoder.go
+++ b/plumbing/format/packfile/encoder.go
@@ -14,10 +14,10 @@ import (
 // Encoder gets the data from the storage and write it into the writer in PACK
 // format
 type Encoder struct {
-	selector     *deltaSelector
-	w            *offsetWriter
-	zw           *zlib.Writer
-	hasher       plumbing.Hasher
+	selector *deltaSelector
+	w        *offsetWriter
+	zw       *zlib.Writer
+	hasher   plumbing.Hasher
 	// offsets is a map of object hashes to corresponding offsets in the packfile.
 	// It is used to determine offset of the base of a delta when a OFS_DELTA is
 	// used.
@@ -45,10 +45,15 @@ func NewEncoder(w io.Writer, s storer.EncodedObjectStorer, useRefDeltas bool) *E
 	}
 }
 
-// Encode creates a packfile containing all the objects referenced in hashes
-// and writes it to the writer in the Encoder.
-func (e *Encoder) Encode(hashes []plumbing.Hash) (plumbing.Hash, error) {
-	objects, err := e.selector.ObjectsToPack(hashes)
+// Encode creates a packfile containing all the objects referenced in
+// hashes and writes it to the writer in the Encoder.  `packWindow`
+// specifies the size of the sliding window used to compare objects
+// for delta compression; 0 turns off delta compression entirely.
+func (e *Encoder) Encode(
+	hashes []plumbing.Hash,
+	packWindow uint,
+) (plumbing.Hash, error) {
+	objects, err := e.selector.ObjectsToPack(hashes, packWindow)
 	if err != nil {
 		return plumbing.ZeroHash, err
 	}
@@ -137,7 +142,7 @@ func (e *Encoder) writeOfsDeltaHeader(deltaOffset int64, base plumbing.Hash) err
 
 	// for OFS_DELTA, offset of the base is interpreted as negative offset
 	// relative to the type-byte of the header of the ofs-delta entry.
-	relativeOffset := deltaOffset-baseOffset
+	relativeOffset := deltaOffset - baseOffset
 	if relativeOffset <= 0 {
 		return fmt.Errorf("bad offset for OFS_DELTA entry: %d", relativeOffset)
 	}
diff --git a/plumbing/format/packfile/encoder_advanced_test.go b/plumbing/format/packfile/encoder_advanced_test.go
index d92e2c410..39c07005d 100644
--- a/plumbing/format/packfile/encoder_advanced_test.go
+++ b/plumbing/format/packfile/encoder_advanced_test.go
@@ -27,12 +27,23 @@ func (s *EncoderAdvancedSuite) TestEncodeDecode(c *C) {
 	fixs.Test(c, func(f *fixtures.Fixture) {
 		storage, err := filesystem.NewStorage(f.DotGit())
 		c.Assert(err, IsNil)
-		s.testEncodeDecode(c, storage)
+		s.testEncodeDecode(c, storage, 10)
 	})
 
 }
 
-func (s *EncoderAdvancedSuite) testEncodeDecode(c *C, storage storer.Storer) {
+func (s *EncoderAdvancedSuite) TestEncodeDecodeNoDeltaCompression(c *C) {
+	fixs := fixtures.Basic().ByTag("packfile").ByTag(".git")
+	fixs = append(fixs, fixtures.ByURL("https://github.com/src-d/go-git.git").
+		ByTag("packfile").ByTag(".git").One())
+	fixs.Test(c, func(f *fixtures.Fixture) {
+		storage, err := filesystem.NewStorage(f.DotGit())
+		c.Assert(err, IsNil)
+		s.testEncodeDecode(c, storage, 0)
+	})
+}
+
+func (s *EncoderAdvancedSuite) testEncodeDecode(c *C, storage storer.Storer, packWindow uint) {
 
 	objIter, err := storage.IterEncodedObjects(plumbing.AnyObject)
 	c.Assert(err, IsNil)
@@ -57,7 +68,7 @@ func (s *EncoderAdvancedSuite) testEncodeDecode(c *C, storage storer.Storer) {
 
 	buf := bytes.NewBuffer(nil)
 	enc := NewEncoder(buf, storage, false)
-	_, err = enc.Encode(hashes)
+	_, err = enc.Encode(hashes, packWindow)
 	c.Assert(err, IsNil)
 
 	scanner := NewScanner(buf)
diff --git a/plumbing/format/packfile/encoder_test.go b/plumbing/format/packfile/encoder_test.go
index b5b0c4240..2cb9094ed 100644
--- a/plumbing/format/packfile/encoder_test.go
+++ b/plumbing/format/packfile/encoder_test.go
@@ -26,7 +26,7 @@ func (s *EncoderSuite) SetUpTest(c *C) {
 }
 
 func (s *EncoderSuite) TestCorrectPackHeader(c *C) {
-	hash, err := s.enc.Encode([]plumbing.Hash{})
+	hash, err := s.enc.Encode([]plumbing.Hash{}, 10)
 	c.Assert(err, IsNil)
 
 	hb := [20]byte(hash)
@@ -47,7 +47,7 @@ func (s *EncoderSuite) TestCorrectPackWithOneEmptyObject(c *C) {
 	_, err := s.store.SetEncodedObject(o)
 	c.Assert(err, IsNil)
 
-	hash, err := s.enc.Encode([]plumbing.Hash{o.Hash()})
+	hash, err := s.enc.Encode([]plumbing.Hash{o.Hash()}, 10)
 	c.Assert(err, IsNil)
 
 	// PACK + VERSION(2) + OBJECT NUMBER(1)
@@ -74,13 +74,13 @@ func (s *EncoderSuite) TestMaxObjectSize(c *C) {
 	o.SetType(plumbing.CommitObject)
 	_, err := s.store.SetEncodedObject(o)
 	c.Assert(err, IsNil)
-	hash, err := s.enc.Encode([]plumbing.Hash{o.Hash()})
+	hash, err := s.enc.Encode([]plumbing.Hash{o.Hash()}, 10)
 	c.Assert(err, IsNil)
 	c.Assert(hash.IsZero(), Not(Equals), true)
 }
 
 func (s *EncoderSuite) TestHashNotFound(c *C) {
-	h, err := s.enc.Encode([]plumbing.Hash{plumbing.NewHash("BAD")})
+	h, err := s.enc.Encode([]plumbing.Hash{plumbing.NewHash("BAD")}, 10)
 	c.Assert(h, Equals, plumbing.ZeroHash)
 	c.Assert(err, NotNil)
 	c.Assert(err, Equals, plumbing.ErrObjectNotFound)
diff --git a/plumbing/object/commit_walker.go b/plumbing/object/commit_walker.go
index 797c17a5f..40ad2582b 100644
--- a/plumbing/object/commit_walker.go
+++ b/plumbing/object/commit_walker.go
@@ -8,9 +8,10 @@ import (
 )
 
 type commitPreIterator struct {
-	seen  map[plumbing.Hash]bool
-	stack []CommitIter
-	start *Commit
+	seenExternal map[plumbing.Hash]bool
+	seen         map[plumbing.Hash]bool
+	stack        []CommitIter
+	start        *Commit
 }
 
 // NewCommitPreorderIter returns a CommitIter that walks the commit history,
@@ -20,16 +21,21 @@ type commitPreIterator struct {
 // and will return the error. Other errors might be returned if the history
 // cannot be traversed (e.g. missing objects). Ignore allows to skip some
 // commits from being iterated.
-func NewCommitPreorderIter(c *Commit, ignore []plumbing.Hash) CommitIter {
+func NewCommitPreorderIter(
+	c *Commit,
+	seenExternal map[plumbing.Hash]bool,
+	ignore []plumbing.Hash,
+) CommitIter {
 	seen := make(map[plumbing.Hash]bool)
 	for _, h := range ignore {
 		seen[h] = true
 	}
 
 	return &commitPreIterator{
-		seen:  seen,
-		stack: make([]CommitIter, 0),
-		start: c,
+		seenExternal: seenExternal,
+		seen:         seen,
+		stack:        make([]CommitIter, 0),
+		start:        c,
 	}
 }
 
@@ -57,7 +63,7 @@ func (w *commitPreIterator) Next() (*Commit, error) {
 			}
 		}
 
-		if w.seen[c.Hash] {
+		if w.seen[c.Hash] || w.seenExternal[c.Hash] {
 			continue
 		}
 
diff --git a/plumbing/object/commit_walker_test.go b/plumbing/object/commit_walker_test.go
index 48b504d41..a27104e1f 100644
--- a/plumbing/object/commit_walker_test.go
+++ b/plumbing/object/commit_walker_test.go
@@ -16,7 +16,7 @@ func (s *CommitWalkerSuite) TestCommitPreIterator(c *C) {
 	commit := s.commit(c, s.Fixture.Head)
 
 	var commits []*Commit
-	NewCommitPreorderIter(commit, nil).ForEach(func(c *Commit) error {
+	NewCommitPreorderIter(commit, nil, nil).ForEach(func(c *Commit) error {
 		commits = append(commits, c)
 		return nil
 	})
@@ -42,7 +42,7 @@ func (s *CommitWalkerSuite) TestCommitPreIteratorWithIgnore(c *C) {
 	commit := s.commit(c, s.Fixture.Head)
 
 	var commits []*Commit
-	NewCommitPreorderIter(commit, []plumbing.Hash{
+	NewCommitPreorderIter(commit, nil, []plumbing.Hash{
 		plumbing.NewHash("af2d6a6954d532f8ffb47615169c8fdf9d383a1a"),
 	}).ForEach(func(c *Commit) error {
 		commits = append(commits, c)
@@ -60,6 +60,30 @@ func (s *CommitWalkerSuite) TestCommitPreIteratorWithIgnore(c *C) {
 	}
 }
 
+func (s *CommitWalkerSuite) TestCommitPreIteratorWithSeenExternal(c *C) {
+	commit := s.commit(c, s.Fixture.Head)
+
+	var commits []*Commit
+	seenExternal := map[plumbing.Hash]bool{
+		plumbing.NewHash("af2d6a6954d532f8ffb47615169c8fdf9d383a1a"): true,
+	}
+	NewCommitPreorderIter(commit, seenExternal, nil).
+		ForEach(func(c *Commit) error {
+			commits = append(commits, c)
+			return nil
+		})
+
+	c.Assert(commits, HasLen, 2)
+
+	expected := []string{
+		"6ecf0ef2c2dffb796033e5a02219af86ec6584e5",
+		"918c48b83bd081e863dbe1b80f8998f058cd8294",
+	}
+	for i, commit := range commits {
+		c.Assert(commit.Hash.String(), Equals, expected[i])
+	}
+}
+
 func (s *CommitWalkerSuite) TestCommitPostIterator(c *C) {
 	commit := s.commit(c, s.Fixture.Head)
 
diff --git a/plumbing/revlist/revlist.go b/plumbing/revlist/revlist.go
index 10a58136c..0a9d1e812 100644
--- a/plumbing/revlist/revlist.go
+++ b/plumbing/revlist/revlist.go
@@ -37,6 +37,7 @@ func objects(
 ) ([]plumbing.Hash, error) {
 	seen := hashListToSet(ignore)
 	result := make(map[plumbing.Hash]bool)
+	visited := make(map[plumbing.Hash]bool)
 
 	walkerFunc := func(h plumbing.Hash) {
 		if !seen[h] {
@@ -46,7 +47,7 @@ func objects(
 	}
 
 	for _, h := range objects {
-		if err := processObject(s, h, seen, ignore, walkerFunc); err != nil {
+		if err := processObject(s, h, seen, visited, ignore, walkerFunc); err != nil {
 			if allowMissingObjects && err == plumbing.ErrObjectNotFound {
 				continue
 			}
@@ -63,6 +64,7 @@ func processObject(
 	s storer.EncodedObjectStorer,
 	h plumbing.Hash,
 	seen map[plumbing.Hash]bool,
+	visited map[plumbing.Hash]bool,
 	ignore []plumbing.Hash,
 	walkerFunc func(h plumbing.Hash),
 ) error {
@@ -82,12 +84,12 @@ func processObject(
 
 	switch do := do.(type) {
 	case *object.Commit:
-		return reachableObjects(do, seen, ignore, walkerFunc)
+		return reachableObjects(do, seen, visited, ignore, walkerFunc)
 	case *object.Tree:
 		return iterateCommitTrees(seen, do, walkerFunc)
 	case *object.Tag:
 		walkerFunc(do.Hash)
-		return processObject(s, do.Target, seen, ignore, walkerFunc)
+		return processObject(s, do.Target, seen, visited, ignore, walkerFunc)
 	case *object.Blob:
 		walkerFunc(do.Hash)
 	default:
@@ -105,10 +107,14 @@ func processObject(
 func reachableObjects(
 	commit *object.Commit,
 	seen map[plumbing.Hash]bool,
+	visited map[plumbing.Hash]bool,
 	ignore []plumbing.Hash,
 	cb func(h plumbing.Hash),
 ) error {
-	i := object.NewCommitPreorderIter(commit, ignore)
+	i := object.NewCommitPreorderIter(commit, seen, ignore)
+	pending := make(map[plumbing.Hash]bool)
+	addPendingParents(pending, visited, commit)
+
 	for {
 		commit, err := i.Next()
 		if err == io.EOF {
@@ -119,6 +125,16 @@ func reachableObjects(
 			return err
 		}
 
+		if pending[commit.Hash] {
+			delete(pending, commit.Hash)
+		}
+
+		addPendingParents(pending, visited, commit)
+
+		if visited[commit.Hash] && len(pending) == 0 {
+			break
+		}
+
 		if seen[commit.Hash] {
 			continue
 		}
@@ -138,6 +154,14 @@ func reachableObjects(
 	return nil
 }
 
+func addPendingParents(pending, visited map[plumbing.Hash]bool, commit *object.Commit) {
+	for _, p := range commit.ParentHashes {
+		if !visited[p] {
+			pending[p] = true
+		}
+	}
+}
+
 // iterateCommitTrees iterate all reachable trees from the given commit
 func iterateCommitTrees(
 	seen map[plumbing.Hash]bool,
diff --git a/plumbing/revlist/revlist_test.go b/plumbing/revlist/revlist_test.go
index dd1e8c1a1..643e3eb5f 100644
--- a/plumbing/revlist/revlist_test.go
+++ b/plumbing/revlist/revlist_test.go
@@ -217,3 +217,60 @@ func (s *RevListSuite) TestRevListObjectsNewBranch(c *C) {
 	}
 	c.Assert(len(remoteHist), Equals, len(revList))
 }
+
+// This tests will ensure that a5b8b09 and b8e471f will be visited even if
+// 35e8510 has already been visited and will not stop iterating until they
+// have been as well.
+//
+// * af2d6a6 some json
+// *   1669dce Merge branch 'master'
+// |\
+// | *   a5b8b09 Merge pull request #1
+// | |\
+// | | * b8e471f Creating changelog
+// | |/
+// * | 35e8510 binary file
+// |/
+// * b029517 Initial commit
+func (s *RevListSuite) TestReachableObjectsNoRevisit(c *C) {
+	obj, err := s.Storer.EncodedObject(plumbing.CommitObject, plumbing.NewHash("af2d6a6954d532f8ffb47615169c8fdf9d383a1a"))
+	c.Assert(err, IsNil)
+
+	do, err := object.DecodeObject(s.Storer, obj)
+	c.Assert(err, IsNil)
+
+	commit, ok := do.(*object.Commit)
+	c.Assert(ok, Equals, true)
+
+	var visited []plumbing.Hash
+	err = reachableObjects(
+		commit,
+		map[plumbing.Hash]bool{
+			plumbing.NewHash("35e85108805c84807bc66a02d91535e1e24b38b9"): true,
+		},
+		map[plumbing.Hash]bool{
+			plumbing.NewHash("35e85108805c84807bc66a02d91535e1e24b38b9"): true,
+		},
+		nil,
+		func(h plumbing.Hash) {
+			obj, err := s.Storer.EncodedObject(plumbing.AnyObject, h)
+			c.Assert(err, IsNil)
+
+			do, err := object.DecodeObject(s.Storer, obj)
+			c.Assert(err, IsNil)
+
+			if _, ok := do.(*object.Commit); ok {
+				visited = append(visited, h)
+			}
+		},
+	)
+	c.Assert(err, IsNil)
+
+	c.Assert(visited, DeepEquals, []plumbing.Hash{
+		plumbing.NewHash("af2d6a6954d532f8ffb47615169c8fdf9d383a1a"),
+		plumbing.NewHash("1669dce138d9b841a518c64b10914d88f5e488ea"),
+		plumbing.NewHash("a5b8b09e2f8fcb0bb99d3ccb0958157b40890d69"),
+		plumbing.NewHash("b029517f6300c2da0f4b651b8642506cd6aaf45d"),
+		plumbing.NewHash("b8e471f58bcbca63b07bda20e428190409c2db47"),
+	})
+}
diff --git a/plumbing/storer/reference.go b/plumbing/storer/reference.go
index 988c78466..34d28512d 100644
--- a/plumbing/storer/reference.go
+++ b/plumbing/storer/reference.go
@@ -16,6 +16,7 @@ var ErrMaxResolveRecursion = errors.New("max. recursion level reached")
 // ReferenceStorer is a generic storage of references.
 type ReferenceStorer interface {
 	SetReference(*plumbing.Reference) error
+	CheckAndSetReference(new, old *plumbing.Reference) error
 	Reference(plumbing.ReferenceName) (*plumbing.Reference, error)
 	IterReferences() (ReferenceIter, error)
 	RemoveReference(plumbing.ReferenceName) error
diff --git a/plumbing/transport/server/server.go b/plumbing/transport/server/server.go
index be36de5cf..f896f7a1d 100644
--- a/plumbing/transport/server/server.go
+++ b/plumbing/transport/server/server.go
@@ -165,7 +165,8 @@ func (s *upSession) UploadPack(ctx context.Context, req *packp.UploadPackRequest
 	pr, pw := io.Pipe()
 	e := packfile.NewEncoder(pw, s.storer, false)
 	go func() {
-		_, err := e.Encode(objs)
+		// TODO: plumb through a pack window.
+		_, err := e.Encode(objs, 10)
 		pw.CloseWithError(err)
 	}()
 
diff --git a/plumbing/transport/test/receive_pack.go b/plumbing/transport/test/receive_pack.go
index d29d9ca29..ed0f517ca 100644
--- a/plumbing/transport/test/receive_pack.go
+++ b/plumbing/transport/test/receive_pack.go
@@ -348,7 +348,7 @@ func (s *ReceivePackSuite) testSendPackDeleteReference(c *C) {
 func (s *ReceivePackSuite) emptyPackfile() io.ReadCloser {
 	var buf bytes.Buffer
 	e := packfile.NewEncoder(&buf, memory.NewStorage(), false)
-	_, err := e.Encode(nil)
+	_, err := e.Encode(nil, 10)
 	if err != nil {
 		panic(err)
 	}
diff --git a/remote.go b/remote.go
index 34ea7f57f..c57476613 100644
--- a/remote.go
+++ b/remote.go
@@ -25,6 +25,7 @@ import (
 var (
 	NoErrAlreadyUpToDate     = errors.New("already up-to-date")
 	ErrDeleteRefNotSupported = errors.New("server does not support delete-refs")
+	ErrForceNeeded           = errors.New("some refs were not updated")
 )
 
 // Remote represents a connection to a remote repository.
@@ -294,7 +295,7 @@ func (r *Remote) fetch(ctx context.Context, o *FetchOptions) (storer.ReferenceSt
 		}
 	}
 
-	updated, err := r.updateLocalReferenceStorage(o.RefSpecs, refs, remoteRefs, o.Tags)
+	updated, err := r.updateLocalReferenceStorage(o.RefSpecs, refs, remoteRefs, o.Tags, o.Force)
 	if err != nil {
 		return nil, err
 	}
@@ -615,7 +616,7 @@ func isFastForward(s storer.EncodedObjectStorer, old, new plumbing.Hash) (bool,
 	}
 
 	found := false
-	iter := object.NewCommitPreorderIter(c, nil)
+	iter := object.NewCommitPreorderIter(c, nil, nil)
 	return found, iter.ForEach(func(c *object.Commit) error {
 		if c.Hash != old {
 			return nil
@@ -682,8 +683,11 @@ func (r *Remote) updateLocalReferenceStorage(
 	specs []config.RefSpec,
 	fetchedRefs, remoteRefs memory.ReferenceStorage,
 	tagMode TagMode,
+	force bool,
 ) (updated bool, err error) {
 	isWildcard := true
+	forceNeeded := false
+
 	for _, spec := range specs {
 		if !spec.IsWildcard() {
 			isWildcard = false
@@ -698,9 +702,25 @@ func (r *Remote) updateLocalReferenceStorage(
 				continue
 			}
 
-			new := plumbing.NewHashReference(spec.Dst(ref.Name()), ref.Hash())
+			localName := spec.Dst(ref.Name())
+			old, _ := storer.ResolveReference(r.s, localName)
+			new := plumbing.NewHashReference(localName, ref.Hash())
+
+			// If the ref exists locally as a branch and force is not specified,
+			// only update if the new ref is an ancestor of the old
+			if old != nil && old.Name().IsBranch() && !force && !spec.IsForceUpdate() {
+				ff, err := isFastForward(r.s, old.Hash(), new.Hash())
+				if err != nil {
+					return updated, err
+				}
+
+				if !ff {
+					forceNeeded = true
+					continue
+				}
+			}
 
-			refUpdated, err := updateReferenceStorerIfNeeded(r.s, new)
+			refUpdated, err := checkAndUpdateReferenceStorerIfNeeded(r.s, new, old)
 			if err != nil {
 				return updated, err
 			}
@@ -728,6 +748,10 @@ func (r *Remote) updateLocalReferenceStorage(
 		updated = true
 	}
 
+	if err == nil && forceNeeded {
+		err = ErrForceNeeded
+	}
+
 	return
 }
 
@@ -797,17 +821,21 @@ func referencesToHashes(refs storer.ReferenceStorer) ([]plumbing.Hash, error) {
 func pushHashes(
 	ctx context.Context,
 	sess transport.ReceivePackSession,
-	sto storer.EncodedObjectStorer,
+	s storage.Storer,
 	req *packp.ReferenceUpdateRequest,
 	hs []plumbing.Hash,
 ) (*packp.ReportStatus, error) {
 
 	rd, wr := io.Pipe()
 	req.Packfile = rd
+	config, err := s.Config()
+	if err != nil {
+		return nil, err
+	}
 	done := make(chan error)
 	go func() {
-		e := packfile.NewEncoder(wr, sto, false)
-		if _, err := e.Encode(hs); err != nil {
+		e := packfile.NewEncoder(wr, s, false)
+		if _, err := e.Encode(hs, config.Pack.Window); err != nil {
 			done <- wr.CloseWithError(err)
 			return
 		}
diff --git a/repository.go b/repository.go
index 8114740ca..59af50914 100644
--- a/repository.go
+++ b/repository.go
@@ -625,9 +625,9 @@ func (r *Repository) calculateRemoteHeadReference(spec []config.RefSpec,
 	return refs
 }
 
-func updateReferenceStorerIfNeeded(
-	s storer.ReferenceStorer, r *plumbing.Reference) (updated bool, err error) {
-
+func checkAndUpdateReferenceStorerIfNeeded(
+	s storer.ReferenceStorer, r, old *plumbing.Reference) (
+	updated bool, err error) {
 	p, err := s.Reference(r.Name())
 	if err != nil && err != plumbing.ErrReferenceNotFound {
 		return false, err
@@ -635,7 +635,7 @@ func updateReferenceStorerIfNeeded(
 
 	// we use the string method to compare references, is the easiest way
 	if err == plumbing.ErrReferenceNotFound || r.String() != p.String() {
-		if err := s.SetReference(r); err != nil {
+		if err := s.CheckAndSetReference(r, old); err != nil {
 			return false, err
 		}
 
@@ -645,6 +645,11 @@ func updateReferenceStorerIfNeeded(
 	return false, nil
 }
 
+func updateReferenceStorerIfNeeded(
+	s storer.ReferenceStorer, r *plumbing.Reference) (updated bool, err error) {
+	return checkAndUpdateReferenceStorerIfNeeded(s, r, nil)
+}
+
 // Fetch fetches references along with the objects necessary to complete
 // their histories, from the remote named as FetchOptions.RemoteName.
 //
@@ -720,7 +725,7 @@ func (r *Repository) Log(o *LogOptions) (object.CommitIter, error) {
 		return nil, err
 	}
 
-	return object.NewCommitPreorderIter(commit, nil), nil
+	return object.NewCommitPreorderIter(commit, nil, nil), nil
 }
 
 // Tags returns all the References from Tags. This method returns all the tag
@@ -949,7 +954,7 @@ func (r *Repository) ResolveRevision(rev plumbing.Revision) (*plumbing.Hash, err
 				commit = c
 			}
 		case revision.CaretReg:
-			history := object.NewCommitPreorderIter(commit, nil)
+			history := object.NewCommitPreorderIter(commit, nil, nil)
 
 			re := item.(revision.CaretReg).Regexp
 			negate := item.(revision.CaretReg).Negate
@@ -979,7 +984,7 @@ func (r *Repository) ResolveRevision(rev plumbing.Revision) (*plumbing.Hash, err
 
 			commit = c
 		case revision.AtDate:
-			history := object.NewCommitPreorderIter(commit, nil)
+			history := object.NewCommitPreorderIter(commit, nil, nil)
 
 			date := item.(revision.AtDate).Date
 
diff --git a/storage/filesystem/internal/dotgit/dotgit.go b/storage/filesystem/internal/dotgit/dotgit.go
index 2840bc74d..c08e2b178 100644
--- a/storage/filesystem/internal/dotgit/dotgit.go
+++ b/storage/filesystem/internal/dotgit/dotgit.go
@@ -5,6 +5,7 @@ import (
 	"bufio"
 	"errors"
 	"fmt"
+	"io"
 	stdioutil "io/ioutil"
 	"os"
 	"strings"
@@ -242,7 +243,39 @@ func (d *DotGit) Object(h plumbing.Hash) (billy.File, error) {
 	return d.fs.Open(file)
 }
 
-func (d *DotGit) SetRef(r *plumbing.Reference) error {
+func (d *DotGit) readReferenceFrom(rd io.Reader, name string) (ref *plumbing.Reference, err error) {
+	b, err := stdioutil.ReadAll(rd)
+	if err != nil {
+		return nil, err
+	}
+
+	line := strings.TrimSpace(string(b))
+	return plumbing.NewReferenceFromStrings(name, line), nil
+}
+
+func (d *DotGit) checkReferenceAndTruncate(f billy.File, old *plumbing.Reference) error {
+	if old == nil {
+		return nil
+	}
+	ref, err := d.readReferenceFrom(f, old.Name().String())
+	if err != nil {
+		return err
+	}
+	if ref.Hash() != old.Hash() {
+		return fmt.Errorf("reference has changed concurrently")
+	}
+	_, err = f.Seek(0, io.SeekStart)
+	if err != nil {
+		return err
+	}
+	err = f.Truncate(0)
+	if err != nil {
+		return err
+	}
+	return nil
+}
+
+func (d *DotGit) SetRef(r, old *plumbing.Reference) error {
 	var content string
 	switch r.Type() {
 	case plumbing.SymbolicReference:
@@ -251,13 +284,34 @@ func (d *DotGit) SetRef(r *plumbing.Reference) error {
 		content = fmt.Sprintln(r.Hash().String())
 	}
 
-	f, err := d.fs.Create(r.Name().String())
+	// If we are not checking an old ref, just truncate the file.
+	mode := os.O_RDWR | os.O_CREATE
+	if old == nil {
+		mode |= os.O_TRUNC
+	}
+
+	f, err := d.fs.OpenFile(r.Name().String(), mode, 0666)
 	if err != nil {
 		return err
 	}
 
 	defer ioutil.CheckClose(f, &err)
 
+	// Lock is unlocked by the deferred Close above. This is because Unlock
+	// does not imply a fsync and thus there would be a race between
+	// Unlock+Close and other concurrent writers. Adding Sync to go-billy
+	// could work, but this is better (and avoids superfluous syncs).
+	err = f.Lock()
+	if err != nil {
+		return err
+	}
+
+	// this is a no-op to call even when old is nil.
+	err = d.checkReferenceAndTruncate(f, old)
+	if err != nil {
+		return err
+	}
+
 	_, err = f.Write([]byte(content))
 	return err
 }
@@ -512,13 +566,7 @@ func (d *DotGit) readReferenceFile(path, name string) (ref *plumbing.Reference,
 	}
 	defer ioutil.CheckClose(f, &err)
 
-	b, err := stdioutil.ReadAll(f)
-	if err != nil {
-		return nil, err
-	}
-
-	line := strings.TrimSpace(string(b))
-	return plumbing.NewReferenceFromStrings(name, line), nil
+	return d.readReferenceFrom(f, name)
 }
 
 // Module return a billy.Filesystem poiting to the module folder
diff --git a/storage/filesystem/reference.go b/storage/filesystem/reference.go
index 49627d354..54cdf560d 100644
--- a/storage/filesystem/reference.go
+++ b/storage/filesystem/reference.go
@@ -11,7 +11,11 @@ type ReferenceStorage struct {
 }
 
 func (r *ReferenceStorage) SetReference(ref *plumbing.Reference) error {
-	return r.dir.SetRef(ref)
+	return r.dir.SetRef(ref, nil)
+}
+
+func (r *ReferenceStorage) CheckAndSetReference(ref, old *plumbing.Reference) error {
+	return r.dir.SetRef(ref, old)
 }
 
 func (r *ReferenceStorage) Reference(n plumbing.ReferenceName) (*plumbing.Reference, error) {
diff --git a/storage/memory/storage.go b/storage/memory/storage.go
index 2380fedbe..69394af83 100644
--- a/storage/memory/storage.go
+++ b/storage/memory/storage.go
@@ -12,6 +12,7 @@ import (
 )
 
 var ErrUnsupportedObjectType = fmt.Errorf("unsupported object type")
+var ErrRefHasChanged = fmt.Errorf("reference has changed concurrently")
 
 // Storage is an implementation of git.Storer that stores data on memory, being
 // ephemeral. The use of this storage should be done in controlled envoriments,
@@ -202,6 +203,20 @@ func (r ReferenceStorage) SetReference(ref *plumbing.Reference) error {
 	return nil
 }
 
+func (r ReferenceStorage) CheckAndSetReference(ref, old *plumbing.Reference) error {
+	if ref != nil {
+		if old != nil {
+			tmp := r[ref.Name()]
+			if tmp != nil && tmp.Hash() != old.Hash() {
+				return ErrRefHasChanged
+			}
+		}
+		r[ref.Name()] = ref
+	}
+
+	return nil
+}
+
 func (r ReferenceStorage) Reference(n plumbing.ReferenceName) (*plumbing.Reference, error) {
 	ref, ok := r[n]
 	if !ok {
diff --git a/worktree.go b/worktree.go
index e2f8562a8..486d0d4b6 100644
--- a/worktree.go
+++ b/worktree.go
@@ -69,6 +69,7 @@ func (w *Worktree) PullContext(ctx context.Context, o *PullOptions) error {
 		Depth:      o.Depth,
 		Auth:       o.Auth,
 		Progress:   o.Progress,
+		Force:      o.Force,
 	})
 
 	updated := true