diff --git a/Documentation/config.txt b/Documentation/config.txt
index 6ecd70df0a..112041f407 100644
--- a/Documentation/config.txt
+++ b/Documentation/config.txt
@@ -2684,6 +2684,21 @@ Note that changing the compression level will not automatically recompress
 all existing objects. You can force recompression by passing the -F option
 to linkgit:git-repack[1].
 
+pack.island::
+	An extended regular expression configuring a set of delta
+	islands. See "DELTA ISLANDS" in linkgit:git-pack-objects[1]
+	for details.
+
+pack.islandCore::
+	Specify an island name which gets to have its objects be
+	packed first. This creates a kind of pseudo-pack at the front
+	of one pack, so that the objects from the specified island are
+	hopefully faster to copy into any pack that should be served
+	to a user requesting these objects. In practice this means
+	that the island specified should likely correspond to what is
+	the most commonly cloned in the repo. See also "DELTA ISLANDS"
+	in linkgit:git-pack-objects[1].
+
 pack.deltaCacheSize::
 	The maximum memory in bytes used for caching deltas in
 	linkgit:git-pack-objects[1] before writing them out to a pack.
@@ -3218,6 +3233,10 @@ repack.packKeptObjects::
 	index is being written (either via `--write-bitmap-index` or
 	`repack.writeBitmaps`).
 
+repack.useDeltaIslands::
+	If set to true, makes `git repack` act as if `--delta-islands`
+	was passed. Defaults to `false`.
+
 repack.writeBitmaps::
 	When true, git will write a bitmap index when packing all
 	objects to disk (e.g., when `git repack -a` is run).  This
diff --git a/Documentation/git-pack-objects.txt b/Documentation/git-pack-objects.txt
index d95b472d16..40c825c381 100644
--- a/Documentation/git-pack-objects.txt
+++ b/Documentation/git-pack-objects.txt
@@ -289,6 +289,103 @@ Unexpected missing object will raise an error.
 --unpack-unreachable::
 	Keep unreachable objects in loose form. This implies `--revs`.
 
+--delta-islands::
+	Restrict delta matches based on "islands". See DELTA ISLANDS
+	below.
+
+
+DELTA ISLANDS
+-------------
+
+When possible, `pack-objects` tries to reuse existing on-disk deltas to
+avoid having to search for new ones on the fly. This is an important
+optimization for serving fetches, because it means the server can avoid
+inflating most objects at all and just send the bytes directly from
+disk. This optimization can't work when an object is stored as a delta
+against a base which the receiver does not have (and which we are not
+already sending). In that case the server "breaks" the delta and has to
+find a new one, which has a high CPU cost. Therefore it's important for
+performance that the set of objects in on-disk delta relationships match
+what a client would fetch.
+
+In a normal repository, this tends to work automatically. The objects
+are mostly reachable from the branches and tags, and that's what clients
+fetch. Any deltas we find on the server are likely to be between objects
+the client has or will have.
+
+But in some repository setups, you may have several related but separate
+groups of ref tips, with clients tending to fetch those groups
+independently. For example, imagine that you are hosting several "forks"
+of a repository in a single shared object store, and letting clients
+view them as separate repositories through `GIT_NAMESPACE` or separate
+repos using the alternates mechanism. A naive repack may find that the
+optimal delta for an object is against a base that is only found in
+another fork. But when a client fetches, they will not have the base
+object, and we'll have to find a new delta on the fly.
+
+A similar situation may exist if you have many refs outside of
+`refs/heads/` and `refs/tags/` that point to related objects (e.g.,
+`refs/pull` or `refs/changes` used by some hosting providers). By
+default, clients fetch only heads and tags, and deltas against objects
+found only in those other groups cannot be sent as-is.
+
+Delta islands solve this problem by allowing you to group your refs into
+distinct "islands". Pack-objects computes which objects are reachable
+from which islands, and refuses to make a delta from an object `A`
+against a base which is not present in all of `A`'s islands. This
+results in slightly larger packs (because we miss some delta
+opportunities), but guarantees that a fetch of one island will not have
+to recompute deltas on the fly due to crossing island boundaries.
+
+When repacking with delta islands the delta window tends to get
+clogged with candidates that are forbidden by the config. Repacking
+with a big --window helps (and doesn't take as long as it otherwise
+might because we can reject some object pairs based on islands before
+doing any computation on the content).
+
+Islands are configured via the `pack.island` option, which can be
+specified multiple times. Each value is a left-anchored regular
+expressions matching refnames. For example:
+
+-------------------------------------------
+[pack]
+island = refs/heads/
+island = refs/tags/
+-------------------------------------------
+
+puts heads and tags into an island (whose name is the empty string; see
+below for more on naming). Any refs which do not match those regular
+expressions (e.g., `refs/pull/123`) is not in any island. Any object
+which is reachable only from `refs/pull/` (but not heads or tags) is
+therefore not a candidate to be used as a base for `refs/heads/`.
+
+Refs are grouped into islands based on their "names", and two regexes
+that produce the same name are considered to be in the same
+island. The names are computed from the regexes by concatenating any
+capture groups from the regex, with a '-' dash in between. (And if
+there are no capture groups, then the name is the empty string, as in
+the above example.) This allows you to create arbitrary numbers of
+islands. Only up to 14 such capture groups are supported though.
+
+For example, imagine you store the refs for each fork in
+`refs/virtual/ID`, where `ID` is a numeric identifier. You might then
+configure:
+
+-------------------------------------------
+[pack]
+island = refs/virtual/([0-9]+)/heads/
+island = refs/virtual/([0-9]+)/tags/
+island = refs/virtual/([0-9]+)/(pull)/
+-------------------------------------------
+
+That puts the heads and tags for each fork in their own island (named
+"1234" or similar), and the pull refs for each go into their own
+"1234-pull".
+
+Note that we pick a single island for each regex to go into, using "last
+one wins" ordering (which allows repo-specific config to take precedence
+over user-wide config, and so forth).
+
 SEE ALSO
 --------
 linkgit:git-rev-list[1]
diff --git a/Documentation/git-repack.txt b/Documentation/git-repack.txt
index d056250968..aa0cc8bd44 100644
--- a/Documentation/git-repack.txt
+++ b/Documentation/git-repack.txt
@@ -160,6 +160,11 @@ depth is 4095.
 	being removed. In addition, any unreachable loose objects will
 	be packed (and their loose counterparts removed).
 
+-i::
+--delta-islands::
+	Pass the `--delta-islands` option to `git-pack-objects`, see
+	linkgit:git-pack-objects[1].
+
 Configuration
 -------------
 
diff --git a/Makefile b/Makefile
index d6cd9a5355..7cf6820013 100644
--- a/Makefile
+++ b/Makefile
@@ -850,6 +850,7 @@ LIB_OBJS += csum-file.o
 LIB_OBJS += ctype.o
 LIB_OBJS += date.o
 LIB_OBJS += decorate.o
+LIB_OBJS += delta-islands.o
 LIB_OBJS += diffcore-break.o
 LIB_OBJS += diffcore-delta.o
 LIB_OBJS += diffcore-order.o
diff --git a/builtin/pack-objects.c b/builtin/pack-objects.c
index f434e1fb0c..425bdc8ac5 100644
--- a/builtin/pack-objects.c
+++ b/builtin/pack-objects.c
@@ -24,6 +24,7 @@
 #include "streaming.h"
 #include "thread-utils.h"
 #include "pack-bitmap.h"
+#include "delta-islands.h"
 #include "reachable.h"
 #include "sha1-array.h"
 #include "argv-array.h"
@@ -62,6 +63,7 @@ static struct packing_data to_pack;
 static struct pack_idx_entry **written_list;
 static uint32_t nr_result, nr_written, nr_seen;
 static struct bitmap_index *bitmap_git;
+static uint32_t write_layer;
 
 static int non_empty;
 static int reuse_delta = 1, reuse_object = 1;
@@ -97,6 +99,8 @@ static uint16_t write_bitmap_options;
 
 static int exclude_promisor_objects;
 
+static int use_delta_islands;
+
 static unsigned long delta_cache_size = 0;
 static unsigned long max_delta_cache_size = DEFAULT_DELTA_CACHE_SIZE;
 static unsigned long cache_max_small_delta_size = 1000;
@@ -616,7 +620,7 @@ static inline void add_to_write_order(struct object_entry **wo,
 			       unsigned int *endp,
 			       struct object_entry *e)
 {
-	if (e->filled)
+	if (e->filled || oe_layer(&to_pack, e) != write_layer)
 		return;
 	wo[(*endp)++] = e;
 	e->filled = 1;
@@ -676,9 +680,58 @@ static void add_family_to_write_order(struct object_entry **wo,
 	add_descendants_to_write_order(wo, endp, root);
 }
 
+static void compute_layer_order(struct object_entry **wo, unsigned int *wo_end)
+{
+	unsigned int i, last_untagged;
+	struct object_entry *objects = to_pack.objects;
+
+	for (i = 0; i < to_pack.nr_objects; i++) {
+		if (objects[i].tagged)
+			break;
+		add_to_write_order(wo, wo_end, &objects[i]);
+	}
+	last_untagged = i;
+
+	/*
+	 * Then fill all the tagged tips.
+	 */
+	for (; i < to_pack.nr_objects; i++) {
+		if (objects[i].tagged)
+			add_to_write_order(wo, wo_end, &objects[i]);
+	}
+
+	/*
+	 * And then all remaining commits and tags.
+	 */
+	for (i = last_untagged; i < to_pack.nr_objects; i++) {
+		if (oe_type(&objects[i]) != OBJ_COMMIT &&
+		    oe_type(&objects[i]) != OBJ_TAG)
+			continue;
+		add_to_write_order(wo, wo_end, &objects[i]);
+	}
+
+	/*
+	 * And then all the trees.
+	 */
+	for (i = last_untagged; i < to_pack.nr_objects; i++) {
+		if (oe_type(&objects[i]) != OBJ_TREE)
+			continue;
+		add_to_write_order(wo, wo_end, &objects[i]);
+	}
+
+	/*
+	 * Finally all the rest in really tight order
+	 */
+	for (i = last_untagged; i < to_pack.nr_objects; i++) {
+		if (!objects[i].filled && oe_layer(&to_pack, &objects[i]) == write_layer)
+			add_family_to_write_order(wo, wo_end, &objects[i]);
+	}
+}
+
 static struct object_entry **compute_write_order(void)
 {
-	unsigned int i, wo_end, last_untagged;
+	uint32_t max_layers = 1;
+	unsigned int i, wo_end;
 
 	struct object_entry **wo;
 	struct object_entry *objects = to_pack.objects;
@@ -709,52 +762,14 @@ static struct object_entry **compute_write_order(void)
 	 */
 	for_each_tag_ref(mark_tagged, NULL);
 
-	/*
-	 * Give the objects in the original recency order until
-	 * we see a tagged tip.
-	 */
+	if (use_delta_islands)
+		max_layers = compute_pack_layers(&to_pack);
+
 	ALLOC_ARRAY(wo, to_pack.nr_objects);
-	for (i = wo_end = 0; i < to_pack.nr_objects; i++) {
-		if (objects[i].tagged)
-			break;
-		add_to_write_order(wo, &wo_end, &objects[i]);
-	}
-	last_untagged = i;
+	wo_end = 0;
 
-	/*
-	 * Then fill all the tagged tips.
-	 */
-	for (; i < to_pack.nr_objects; i++) {
-		if (objects[i].tagged)
-			add_to_write_order(wo, &wo_end, &objects[i]);
-	}
-
-	/*
-	 * And then all remaining commits and tags.
-	 */
-	for (i = last_untagged; i < to_pack.nr_objects; i++) {
-		if (oe_type(&objects[i]) != OBJ_COMMIT &&
-		    oe_type(&objects[i]) != OBJ_TAG)
-			continue;
-		add_to_write_order(wo, &wo_end, &objects[i]);
-	}
-
-	/*
-	 * And then all the trees.
-	 */
-	for (i = last_untagged; i < to_pack.nr_objects; i++) {
-		if (oe_type(&objects[i]) != OBJ_TREE)
-			continue;
-		add_to_write_order(wo, &wo_end, &objects[i]);
-	}
-
-	/*
-	 * Finally all the rest in really tight order
-	 */
-	for (i = last_untagged; i < to_pack.nr_objects; i++) {
-		if (!objects[i].filled)
-			add_family_to_write_order(wo, &wo_end, &objects[i]);
-	}
+	for (; write_layer < max_layers; ++write_layer)
+		compute_layer_order(wo, &wo_end);
 
 	if (wo_end != to_pack.nr_objects)
 		die(_("ordered %u objects, expected %"PRIu32),
@@ -1544,7 +1559,8 @@ static void check_object(struct object_entry *entry)
 		if (base_ref && (
 		    (base_entry = packlist_find(&to_pack, base_ref, NULL)) ||
 		    (thin &&
-		     bitmap_has_sha1_in_uninteresting(bitmap_git, base_ref)))) {
+		     bitmap_has_sha1_in_uninteresting(bitmap_git, base_ref))) &&
+		    in_same_island(&entry->idx.oid, &base_entry->idx.oid)) {
 			/*
 			 * If base_ref was set above that means we wish to
 			 * reuse delta data, and either we found that object in
@@ -1867,6 +1883,11 @@ static int type_size_sort(const void *_a, const void *_b)
 		return -1;
 	if (a->preferred_base < b->preferred_base)
 		return 1;
+	if (use_delta_islands) {
+		int island_cmp = island_delta_cmp(&a->idx.oid, &b->idx.oid);
+		if (island_cmp)
+			return island_cmp;
+	}
 	if (a_size > b_size)
 		return -1;
 	if (a_size < b_size)
@@ -2027,6 +2048,9 @@ static int try_delta(struct unpacked *trg, struct unpacked *src,
 	if (trg_size < src_size / 32)
 		return 0;
 
+	if (!in_same_island(&trg->entry->idx.oid, &src->entry->idx.oid))
+		return 0;
+
 	/* Load data if not already done */
 	if (!trg->data) {
 		read_lock();
@@ -2569,6 +2593,9 @@ static void prepare_pack(int window, int depth)
 	uint32_t i, nr_deltas;
 	unsigned n;
 
+	if (use_delta_islands)
+		resolve_tree_islands(progress, &to_pack);
+
 	get_object_details();
 
 	/*
@@ -2732,6 +2759,9 @@ static void show_commit(struct commit *commit, void *data)
 
 	if (write_bitmap_index)
 		index_commit_for_bitmap(commit);
+
+	if (use_delta_islands)
+		propagate_island_marks(commit);
 }
 
 static void show_object(struct object *obj, const char *name, void *data)
@@ -2739,6 +2769,19 @@ static void show_object(struct object *obj, const char *name, void *data)
 	add_preferred_base_object(name);
 	add_object_entry(&obj->oid, obj->type, name, 0);
 	obj->flags |= OBJECT_ADDED;
+
+	if (use_delta_islands) {
+		const char *p;
+		unsigned depth = 0;
+		struct object_entry *ent;
+
+		for (p = strchr(name, '/'); p; p = strchr(p + 1, '/'))
+			depth++;
+
+		ent = packlist_find(&to_pack, obj->oid.hash, NULL);
+		if (ent && depth > oe_tree_depth(&to_pack, ent))
+			oe_set_tree_depth(&to_pack, ent, depth);
+	}
 }
 
 static void show_object__ma_allow_any(struct object *obj, const char *name, void *data)
@@ -3064,6 +3107,9 @@ static void get_object_list(int ac, const char **av)
 	if (use_bitmap_index && !get_object_list_from_bitmap(&revs))
 		return;
 
+	if (use_delta_islands)
+		load_delta_islands();
+
 	if (prepare_revision_walk(&revs))
 		die(_("revision walk setup failed"));
 	mark_edges_uninteresting(&revs, show_edge);
@@ -3242,6 +3288,8 @@ int cmd_pack_objects(int argc, const char **argv, const char *prefix)
 		  option_parse_missing_action },
 		OPT_BOOL(0, "exclude-promisor-objects", &exclude_promisor_objects,
 			 N_("do not pack objects in promisor packfiles")),
+		OPT_BOOL(0, "delta-islands", &use_delta_islands,
+			 N_("respect islands during delta compression")),
 		OPT_END(),
 	};
 
@@ -3368,6 +3416,9 @@ int cmd_pack_objects(int argc, const char **argv, const char *prefix)
 	if (pack_to_stdout || !rev_list_all)
 		write_bitmap_index = 0;
 
+	if (use_delta_islands)
+		argv_array_push(&rp, "--topo-order");
+
 	if (progress && all_progress_implied)
 		progress = 2;
 
diff --git a/builtin/repack.c b/builtin/repack.c
index 42be88e86c..c6a7943d5c 100644
--- a/builtin/repack.c
+++ b/builtin/repack.c
@@ -15,6 +15,7 @@
 static int delta_base_offset = 1;
 static int pack_kept_objects = -1;
 static int write_bitmaps;
+static int use_delta_islands;
 static char *packdir, *packtmp;
 
 static const char *const git_repack_usage[] = {
@@ -43,6 +44,10 @@ static int repack_config(const char *var, const char *value, void *cb)
 		write_bitmaps = git_config_bool(var, value);
 		return 0;
 	}
+	if (!strcmp(var, "repack.usedeltaislands")) {
+		use_delta_islands = git_config_bool(var, value);
+		return 0;
+	}
 	return git_default_config(var, value, cb);
 }
 
@@ -303,6 +308,8 @@ int cmd_repack(int argc, const char **argv, const char *prefix)
 				N_("pass --local to git-pack-objects")),
 		OPT_BOOL('b', "write-bitmap-index", &write_bitmaps,
 				N_("write bitmap index")),
+		OPT_BOOL('i', "delta-islands", &use_delta_islands,
+				N_("pass --delta-islands to git-pack-objects")),
 		OPT_STRING(0, "unpack-unreachable", &unpack_unreachable, N_("approxidate"),
 				N_("with -A, do not loosen objects older than this")),
 		OPT_BOOL('k', "keep-unreachable", &keep_unreachable,
@@ -363,6 +370,8 @@ int cmd_repack(int argc, const char **argv, const char *prefix)
 		argv_array_push(&cmd.args, "--exclude-promisor-objects");
 	if (write_bitmaps)
 		argv_array_push(&cmd.args, "--write-bitmap-index");
+	if (use_delta_islands)
+		argv_array_push(&cmd.args, "--delta-islands");
 
 	if (pack_everything & ALL_INTO_ONE) {
 		get_non_kept_pack_filenames(&existing_packs, &keep_pack_list);
diff --git a/delta-islands.c b/delta-islands.c
new file mode 100644
index 0000000000..8e5018e406
--- /dev/null
+++ b/delta-islands.c
@@ -0,0 +1,502 @@
+#include "cache.h"
+#include "attr.h"
+#include "object.h"
+#include "blob.h"
+#include "commit.h"
+#include "tag.h"
+#include "tree.h"
+#include "delta.h"
+#include "pack.h"
+#include "tree-walk.h"
+#include "diff.h"
+#include "revision.h"
+#include "list-objects.h"
+#include "progress.h"
+#include "refs.h"
+#include "khash.h"
+#include "pack-bitmap.h"
+#include "pack-objects.h"
+#include "delta-islands.h"
+#include "sha1-array.h"
+#include "config.h"
+
+KHASH_INIT(str, const char *, void *, 1, kh_str_hash_func, kh_str_hash_equal)
+
+static khash_sha1 *island_marks;
+static unsigned island_counter;
+static unsigned island_counter_core;
+
+static kh_str_t *remote_islands;
+
+struct remote_island {
+	uint64_t hash;
+	struct oid_array oids;
+};
+
+struct island_bitmap {
+	uint32_t refcount;
+	uint32_t bits[FLEX_ARRAY];
+};
+
+static uint32_t island_bitmap_size;
+
+/*
+ * Allocate a new bitmap; if "old" is not NULL, the new bitmap will be a copy
+ * of "old". Otherwise, the new bitmap is empty.
+ */
+static struct island_bitmap *island_bitmap_new(const struct island_bitmap *old)
+{
+	size_t size = sizeof(struct island_bitmap) + (island_bitmap_size * 4);
+	struct island_bitmap *b = xcalloc(1, size);
+
+	if (old)
+		memcpy(b, old, size);
+
+	b->refcount = 1;
+	return b;
+}
+
+static void island_bitmap_or(struct island_bitmap *a, const struct island_bitmap *b)
+{
+	uint32_t i;
+
+	for (i = 0; i < island_bitmap_size; ++i)
+		a->bits[i] |= b->bits[i];
+}
+
+static int island_bitmap_is_subset(struct island_bitmap *self,
+		struct island_bitmap *super)
+{
+	uint32_t i;
+
+	if (self == super)
+		return 1;
+
+	for (i = 0; i < island_bitmap_size; ++i) {
+		if ((self->bits[i] & super->bits[i]) != self->bits[i])
+			return 0;
+	}
+
+	return 1;
+}
+
+#define ISLAND_BITMAP_BLOCK(x) (x / 32)
+#define ISLAND_BITMAP_MASK(x) (1 << (x % 32))
+
+static void island_bitmap_set(struct island_bitmap *self, uint32_t i)
+{
+	self->bits[ISLAND_BITMAP_BLOCK(i)] |= ISLAND_BITMAP_MASK(i);
+}
+
+static int island_bitmap_get(struct island_bitmap *self, uint32_t i)
+{
+	return (self->bits[ISLAND_BITMAP_BLOCK(i)] & ISLAND_BITMAP_MASK(i)) != 0;
+}
+
+int in_same_island(const struct object_id *trg_oid, const struct object_id *src_oid)
+{
+	khiter_t trg_pos, src_pos;
+
+	/* If we aren't using islands, assume everything goes together. */
+	if (!island_marks)
+		return 1;
+
+	/*
+	 * If we don't have a bitmap for the target, we can delta it
+	 * against anything -- it's not an important object
+	 */
+	trg_pos = kh_get_sha1(island_marks, trg_oid->hash);
+	if (trg_pos >= kh_end(island_marks))
+		return 1;
+
+	/*
+	 * if the source (our delta base) doesn't have a bitmap,
+	 * we don't want to base any deltas on it!
+	 */
+	src_pos = kh_get_sha1(island_marks, src_oid->hash);
+	if (src_pos >= kh_end(island_marks))
+		return 0;
+
+	return island_bitmap_is_subset(kh_value(island_marks, trg_pos),
+				kh_value(island_marks, src_pos));
+}
+
+int island_delta_cmp(const struct object_id *a, const struct object_id *b)
+{
+	khiter_t a_pos, b_pos;
+	struct island_bitmap *a_bitmap = NULL, *b_bitmap = NULL;
+
+	if (!island_marks)
+		return 0;
+
+	a_pos = kh_get_sha1(island_marks, a->hash);
+	if (a_pos < kh_end(island_marks))
+		a_bitmap = kh_value(island_marks, a_pos);
+
+	b_pos = kh_get_sha1(island_marks, b->hash);
+	if (b_pos < kh_end(island_marks))
+		b_bitmap = kh_value(island_marks, b_pos);
+
+	if (a_bitmap) {
+		if (!b_bitmap || !island_bitmap_is_subset(a_bitmap, b_bitmap))
+			return -1;
+	}
+	if (b_bitmap) {
+		if (!a_bitmap || !island_bitmap_is_subset(b_bitmap, a_bitmap))
+			return 1;
+	}
+
+	return 0;
+}
+
+static struct island_bitmap *create_or_get_island_marks(struct object *obj)
+{
+	khiter_t pos;
+	int hash_ret;
+
+	pos = kh_put_sha1(island_marks, obj->oid.hash, &hash_ret);
+	if (hash_ret)
+		kh_value(island_marks, pos) = island_bitmap_new(NULL);
+
+	return kh_value(island_marks, pos);
+}
+
+static void set_island_marks(struct object *obj, struct island_bitmap *marks)
+{
+	struct island_bitmap *b;
+	khiter_t pos;
+	int hash_ret;
+
+	pos = kh_put_sha1(island_marks, obj->oid.hash, &hash_ret);
+	if (hash_ret) {
+		/*
+		 * We don't have one yet; make a copy-on-write of the
+		 * parent.
+		 */
+		marks->refcount++;
+		kh_value(island_marks, pos) = marks;
+		return;
+	}
+
+	/*
+	 * We do have it. Make sure we split any copy-on-write before
+	 * updating.
+	 */
+	b = kh_value(island_marks, pos);
+	if (b->refcount > 1) {
+		b->refcount--;
+		b = kh_value(island_marks, pos) = island_bitmap_new(b);
+	}
+	island_bitmap_or(b, marks);
+}
+
+static void mark_remote_island_1(struct remote_island *rl, int is_core_island)
+{
+	uint32_t i;
+
+	for (i = 0; i < rl->oids.nr; ++i) {
+		struct island_bitmap *marks;
+		struct object *obj = parse_object(the_repository, &rl->oids.oid[i]);
+
+		if (!obj)
+			continue;
+
+		marks = create_or_get_island_marks(obj);
+		island_bitmap_set(marks, island_counter);
+
+		if (is_core_island && obj->type == OBJ_COMMIT)
+			obj->flags |= NEEDS_BITMAP;
+
+		/* If it was a tag, also make sure we hit the underlying object. */
+		while (obj && obj->type == OBJ_TAG) {
+			obj = ((struct tag *)obj)->tagged;
+			if (obj) {
+				parse_object(the_repository, &obj->oid);
+				marks = create_or_get_island_marks(obj);
+				island_bitmap_set(marks, island_counter);
+			}
+		}
+	}
+
+	if (is_core_island)
+		island_counter_core = island_counter;
+
+	island_counter++;
+}
+
+struct tree_islands_todo {
+	struct object_entry *entry;
+	unsigned int depth;
+};
+
+static int tree_depth_compare(const void *a, const void *b)
+{
+	const struct tree_islands_todo *todo_a = a;
+	const struct tree_islands_todo *todo_b = b;
+
+	return todo_a->depth - todo_b->depth;
+}
+
+void resolve_tree_islands(int progress, struct packing_data *to_pack)
+{
+	struct progress *progress_state = NULL;
+	struct tree_islands_todo *todo;
+	int nr = 0;
+	int i;
+
+	if (!island_marks)
+		return;
+
+	/*
+	 * We process only trees, as commits and tags have already been handled
+	 * (and passed their marks on to root trees, as well. We must make sure
+	 * to process them in descending tree-depth order so that marks
+	 * propagate down the tree properly, even if a sub-tree is found in
+	 * multiple parent trees.
+	 */
+	ALLOC_ARRAY(todo, to_pack->nr_objects);
+	for (i = 0; i < to_pack->nr_objects; i++) {
+		if (oe_type(&to_pack->objects[i]) == OBJ_TREE) {
+			todo[nr].entry = &to_pack->objects[i];
+			todo[nr].depth = oe_tree_depth(to_pack, &to_pack->objects[i]);
+			nr++;
+		}
+	}
+	QSORT(todo, nr, tree_depth_compare);
+
+	if (progress)
+		progress_state = start_progress(_("Propagating island marks"), nr);
+
+	for (i = 0; i < nr; i++) {
+		struct object_entry *ent = todo[i].entry;
+		struct island_bitmap *root_marks;
+		struct tree *tree;
+		struct tree_desc desc;
+		struct name_entry entry;
+		khiter_t pos;
+
+		pos = kh_get_sha1(island_marks, ent->idx.oid.hash);
+		if (pos >= kh_end(island_marks))
+			continue;
+
+		root_marks = kh_value(island_marks, pos);
+
+		tree = lookup_tree(the_repository, &ent->idx.oid);
+		if (!tree || parse_tree(tree) < 0)
+			die(_("bad tree object %s"), oid_to_hex(&ent->idx.oid));
+
+		init_tree_desc(&desc, tree->buffer, tree->size);
+		while (tree_entry(&desc, &entry)) {
+			struct object *obj;
+
+			if (S_ISGITLINK(entry.mode))
+				continue;
+
+			obj = lookup_object(the_repository, entry.oid->hash);
+			if (!obj)
+				continue;
+
+			set_island_marks(obj, root_marks);
+		}
+
+		free_tree_buffer(tree);
+
+		display_progress(progress_state, i+1);
+	}
+
+	stop_progress(&progress_state);
+	free(todo);
+}
+
+static regex_t *island_regexes;
+static unsigned int island_regexes_alloc, island_regexes_nr;
+static const char *core_island_name;
+
+static int island_config_callback(const char *k, const char *v, void *cb)
+{
+	if (!strcmp(k, "pack.island")) {
+		struct strbuf re = STRBUF_INIT;
+
+		if (!v)
+			return config_error_nonbool(k);
+
+		ALLOC_GROW(island_regexes, island_regexes_nr + 1, island_regexes_alloc);
+
+		if (*v != '^')
+			strbuf_addch(&re, '^');
+		strbuf_addstr(&re, v);
+
+		if (regcomp(&island_regexes[island_regexes_nr], re.buf, REG_EXTENDED))
+			die(_("failed to load island regex for '%s': %s"), k, re.buf);
+
+		strbuf_release(&re);
+		island_regexes_nr++;
+		return 0;
+	}
+
+	if (!strcmp(k, "pack.islandcore"))
+		return git_config_string(&core_island_name, k, v);
+
+	return 0;
+}
+
+static void add_ref_to_island(const char *island_name, const struct object_id *oid)
+{
+	uint64_t sha_core;
+	struct remote_island *rl = NULL;
+
+	int hash_ret;
+	khiter_t pos = kh_put_str(remote_islands, island_name, &hash_ret);
+
+	if (hash_ret) {
+		kh_key(remote_islands, pos) = xstrdup(island_name);
+		kh_value(remote_islands, pos) = xcalloc(1, sizeof(struct remote_island));
+	}
+
+	rl = kh_value(remote_islands, pos);
+	oid_array_append(&rl->oids, oid);
+
+	memcpy(&sha_core, oid->hash, sizeof(uint64_t));
+	rl->hash += sha_core;
+}
+
+static int find_island_for_ref(const char *refname, const struct object_id *oid,
+			       int flags, void *data)
+{
+	/*
+	 * We should advertise 'ARRAY_SIZE(matches) - 2' as the max,
+	 * so we can diagnose below a config with more capture groups
+	 * than we support.
+	 */
+	regmatch_t matches[16];
+	int i, m;
+	struct strbuf island_name = STRBUF_INIT;
+
+	/* walk backwards to get last-one-wins ordering */
+	for (i = island_regexes_nr - 1; i >= 0; i--) {
+		if (!regexec(&island_regexes[i], refname,
+			     ARRAY_SIZE(matches), matches, 0))
+			break;
+	}
+
+	if (i < 0)
+		return 0;
+
+	if (matches[ARRAY_SIZE(matches) - 1].rm_so != -1)
+		warning(_("island regex from config has "
+			  "too many capture groups (max=%d)"),
+			(int)ARRAY_SIZE(matches) - 2);
+
+	for (m = 1; m < ARRAY_SIZE(matches); m++) {
+		regmatch_t *match = &matches[m];
+
+		if (match->rm_so == -1)
+			continue;
+
+		if (island_name.len)
+			strbuf_addch(&island_name, '-');
+
+		strbuf_add(&island_name, refname + match->rm_so, match->rm_eo - match->rm_so);
+	}
+
+	add_ref_to_island(island_name.buf, oid);
+	strbuf_release(&island_name);
+	return 0;
+}
+
+static struct remote_island *get_core_island(void)
+{
+	if (core_island_name) {
+		khiter_t pos = kh_get_str(remote_islands, core_island_name);
+		if (pos < kh_end(remote_islands))
+			return kh_value(remote_islands, pos);
+	}
+
+	return NULL;
+}
+
+static void deduplicate_islands(void)
+{
+	struct remote_island *island, *core = NULL, **list;
+	unsigned int island_count, dst, src, ref, i = 0;
+
+	island_count = kh_size(remote_islands);
+	ALLOC_ARRAY(list, island_count);
+
+	kh_foreach_value(remote_islands, island, {
+		list[i++] = island;
+	});
+
+	for (ref = 0; ref + 1 < island_count; ref++) {
+		for (src = ref + 1, dst = src; src < island_count; src++) {
+			if (list[ref]->hash == list[src]->hash)
+				continue;
+
+			if (src != dst)
+				list[dst] = list[src];
+
+			dst++;
+		}
+		island_count = dst;
+	}
+
+	island_bitmap_size = (island_count / 32) + 1;
+	core = get_core_island();
+
+	for (i = 0; i < island_count; ++i) {
+		mark_remote_island_1(list[i], core && list[i]->hash == core->hash);
+	}
+
+	free(list);
+}
+
+void load_delta_islands(void)
+{
+	island_marks = kh_init_sha1();
+	remote_islands = kh_init_str();
+
+	git_config(island_config_callback, NULL);
+	for_each_ref(find_island_for_ref, NULL);
+	deduplicate_islands();
+
+	fprintf(stderr, _("Marked %d islands, done.\n"), island_counter);
+}
+
+void propagate_island_marks(struct commit *commit)
+{
+	khiter_t pos = kh_get_sha1(island_marks, commit->object.oid.hash);
+
+	if (pos < kh_end(island_marks)) {
+		struct commit_list *p;
+		struct island_bitmap *root_marks = kh_value(island_marks, pos);
+
+		parse_commit(commit);
+		set_island_marks(&get_commit_tree(commit)->object, root_marks);
+		for (p = commit->parents; p; p = p->next)
+			set_island_marks(&p->item->object, root_marks);
+	}
+}
+
+int compute_pack_layers(struct packing_data *to_pack)
+{
+	uint32_t i;
+
+	if (!core_island_name || !island_marks)
+		return 1;
+
+	for (i = 0; i < to_pack->nr_objects; ++i) {
+		struct object_entry *entry = &to_pack->objects[i];
+		khiter_t pos = kh_get_sha1(island_marks, entry->idx.oid.hash);
+
+		oe_set_layer(to_pack, entry, 1);
+
+		if (pos < kh_end(island_marks)) {
+			struct island_bitmap *bitmap = kh_value(island_marks, pos);
+
+			if (island_bitmap_get(bitmap, island_counter_core))
+				oe_set_layer(to_pack, entry, 0);
+		}
+	}
+
+	return 2;
+}
diff --git a/delta-islands.h b/delta-islands.h
new file mode 100644
index 0000000000..f9725730f4
--- /dev/null
+++ b/delta-islands.h
@@ -0,0 +1,11 @@
+#ifndef DELTA_ISLANDS_H
+#define DELTA_ISLANDS_H
+
+int island_delta_cmp(const struct object_id *a, const struct object_id *b);
+int in_same_island(const struct object_id *, const struct object_id *);
+void resolve_tree_islands(int progress, struct packing_data *to_pack);
+void load_delta_islands(void);
+void propagate_island_marks(struct commit *commit);
+int compute_pack_layers(struct packing_data *to_pack);
+
+#endif /* DELTA_ISLANDS_H */
diff --git a/pack-objects.c b/pack-objects.c
index 3462858772..7f7b7dddf6 100644
--- a/pack-objects.c
+++ b/pack-objects.c
@@ -164,6 +164,12 @@ struct object_entry *packlist_alloc(struct packing_data *pdata,
 			REALLOC_ARRAY(pdata->in_pack, pdata->nr_alloc);
 		if (pdata->delta_size)
 			REALLOC_ARRAY(pdata->delta_size, pdata->nr_alloc);
+
+		if (pdata->tree_depth)
+			REALLOC_ARRAY(pdata->tree_depth, pdata->nr_alloc);
+
+		if (pdata->layer)
+			REALLOC_ARRAY(pdata->layer, pdata->nr_alloc);
 	}
 
 	new_entry = pdata->objects + pdata->nr_objects++;
@@ -179,6 +185,12 @@ struct object_entry *packlist_alloc(struct packing_data *pdata,
 	if (pdata->in_pack)
 		pdata->in_pack[pdata->nr_objects - 1] = NULL;
 
+	if (pdata->tree_depth)
+		pdata->tree_depth[pdata->nr_objects - 1] = 0;
+
+	if (pdata->layer)
+		pdata->layer[pdata->nr_objects - 1] = 0;
+
 	return new_entry;
 }
 
diff --git a/pack-objects.h b/pack-objects.h
index b0c1f137c6..2ca39cfcfe 100644
--- a/pack-objects.h
+++ b/pack-objects.h
@@ -103,6 +103,7 @@ struct object_entry {
 	unsigned no_try_delta:1;
 	unsigned type_:TYPE_BITS;
 	unsigned in_pack_type:TYPE_BITS; /* could be delta */
+
 	unsigned preferred_base:1; /*
 				    * we do not pack this, but is available
 				    * to be used as the base object to delta
@@ -158,6 +159,10 @@ struct packing_data {
 
 	uintmax_t oe_size_limit;
 	uintmax_t oe_delta_size_limit;
+
+	/* delta islands */
+	unsigned int *tree_depth;
+	unsigned char *layer;
 };
 
 void prepare_packing_data(struct packing_data *pdata);
@@ -400,4 +405,38 @@ static inline void oe_set_delta_size(struct packing_data *pack,
 	}
 }
 
+static inline unsigned int oe_tree_depth(struct packing_data *pack,
+					 struct object_entry *e)
+{
+	if (!pack->tree_depth)
+		return 0;
+	return pack->tree_depth[e - pack->objects];
+}
+
+static inline void oe_set_tree_depth(struct packing_data *pack,
+				     struct object_entry *e,
+				     unsigned int tree_depth)
+{
+	if (!pack->tree_depth)
+		ALLOC_ARRAY(pack->tree_depth, pack->nr_objects);
+	pack->tree_depth[e - pack->objects] = tree_depth;
+}
+
+static inline unsigned char oe_layer(struct packing_data *pack,
+				     struct object_entry *e)
+{
+	if (!pack->layer)
+		return 0;
+	return pack->layer[e - pack->objects];
+}
+
+static inline void oe_set_layer(struct packing_data *pack,
+				struct object_entry *e,
+				unsigned char layer)
+{
+	if (!pack->layer)
+		ALLOC_ARRAY(pack->layer, pack->nr_objects);
+	pack->layer[e - pack->objects] = layer;
+}
+
 #endif
diff --git a/t/t5320-delta-islands.sh b/t/t5320-delta-islands.sh
new file mode 100755
index 0000000000..fea92a5777
--- /dev/null
+++ b/t/t5320-delta-islands.sh
@@ -0,0 +1,143 @@
+#!/bin/sh
+
+test_description='exercise delta islands'
+. ./test-lib.sh
+
+# returns true iff $1 is a delta based on $2
+is_delta_base () {
+	delta_base=$(echo "$1" | git cat-file --batch-check='%(deltabase)') &&
+	echo >&2 "$1 has base $delta_base" &&
+	test "$delta_base" = "$2"
+}
+
+# generate a commit on branch $1 with a single file, "file", whose
+# content is mostly based on the seed $2, but with a unique bit
+# of content $3 appended. This should allow us to see whether
+# blobs of different refs delta against each other.
+commit() {
+	blob=$({ test-tool genrandom "$2" 10240 && echo "$3"; } |
+	       git hash-object -w --stdin) &&
+	tree=$(printf '100644 blob %s\tfile\n' "$blob" | git mktree) &&
+	commit=$(echo "$2-$3" | git commit-tree "$tree" ${4:+-p "$4"}) &&
+	git update-ref "refs/heads/$1" "$commit" &&
+	eval "$1"'=$(git rev-parse $1:file)' &&
+	eval "echo >&2 $1=\$$1"
+}
+
+test_expect_success 'setup commits' '
+	commit one seed 1 &&
+	commit two seed 12
+'
+
+# Note: This is heavily dependent on the "prefer larger objects as base"
+# heuristic.
+test_expect_success 'vanilla repack deltas one against two' '
+	git repack -adf &&
+	is_delta_base $one $two
+'
+
+test_expect_success 'island repack with no island definition is vanilla' '
+	git repack -adfi &&
+	is_delta_base $one $two
+'
+
+test_expect_success 'island repack with no matches is vanilla' '
+	git -c "pack.island=refs/foo" repack -adfi &&
+	is_delta_base $one $two
+'
+
+test_expect_success 'separate islands disallows delta' '
+	git -c "pack.island=refs/heads/(.*)" repack -adfi &&
+	! is_delta_base $one $two &&
+	! is_delta_base $two $one
+'
+
+test_expect_success 'same island allows delta' '
+	git -c "pack.island=refs/heads" repack -adfi &&
+	is_delta_base $one $two
+'
+
+test_expect_success 'coalesce same-named islands' '
+	git \
+		-c "pack.island=refs/(.*)/one" \
+		-c "pack.island=refs/(.*)/two" \
+		repack -adfi &&
+	is_delta_base $one $two
+'
+
+test_expect_success 'island restrictions drop reused deltas' '
+	git repack -adfi &&
+	is_delta_base $one $two &&
+	git -c "pack.island=refs/heads/(.*)" repack -adi &&
+	! is_delta_base $one $two &&
+	! is_delta_base $two $one
+'
+
+test_expect_success 'island regexes are left-anchored' '
+	git -c "pack.island=heads/(.*)" repack -adfi &&
+	is_delta_base $one $two
+'
+
+test_expect_success 'island regexes follow last-one-wins scheme' '
+	git \
+		-c "pack.island=refs/heads/(.*)" \
+		-c "pack.island=refs/heads/" \
+		repack -adfi &&
+	is_delta_base $one $two
+'
+
+test_expect_success 'setup shared history' '
+	commit root shared root &&
+	commit one shared 1 root &&
+	commit two shared 12-long root
+'
+
+# We know that $two will be preferred as a base from $one,
+# because we can transform it with a pure deletion.
+#
+# We also expect $root as a delta against $two by the "longest is base" rule.
+test_expect_success 'vanilla delta goes between branches' '
+	git repack -adf &&
+	is_delta_base $one $two &&
+	is_delta_base $root $two
+'
+
+# Here we should allow $one to base itself on $root; even though
+# they are in different islands, the objects in $root are in a superset
+# of islands compared to those in $one.
+#
+# Similarly, $two can delta against $root by our rules. And unlike $one,
+# in which we are just allowing it, the island rules actually put $root
+# as a possible base for $two, which it would not otherwise be (due to the size
+# sorting).
+test_expect_success 'deltas allowed against superset islands' '
+	git -c "pack.island=refs/heads/(.*)" repack -adfi &&
+	is_delta_base $one $root &&
+	is_delta_base $two $root
+'
+
+# We are going to test the packfile order here, so we again have to make some
+# assumptions. We assume that "$root", as part of our core "one", must come
+# before "$two". This should be guaranteed by the island code. However, for
+# this test to fail without islands, we are also assuming that it would not
+# otherwise do so. This is true by the current write order, which will put
+# commits (and their contents) before their parents.
+test_expect_success 'island core places core objects first' '
+	cat >expect <<-EOF &&
+	$root
+	$two
+	EOF
+	git -c "pack.island=refs/heads/(.*)" \
+	    -c "pack.islandcore=one" \
+	    repack -adfi &&
+	git verify-pack -v .git/objects/pack/*.pack |
+	cut -d" " -f1 |
+	egrep "$root|$two" >actual &&
+	test_cmp expect actual
+'
+
+test_expect_success 'unmatched island core is not fatal' '
+	git -c "pack.islandcore=one" repack -adfi
+'
+
+test_done