diff --git a/Documentation/technical/pack-format.txt b/Documentation/technical/pack-format.txt
index 96d2fc589f..8833b71c8b 100644
--- a/Documentation/technical/pack-format.txt
+++ b/Documentation/technical/pack-format.txt
@@ -274,6 +274,26 @@ Pack file entry: <+
 
     Index checksum of all of the above.
 
+== pack-*.rev files have the format:
+
+  - A 4-byte magic number '0x52494458' ('RIDX').
+
+  - A 4-byte version identifier (= 1).
+
+  - A 4-byte hash function identifier (= 1 for SHA-1, 2 for SHA-256).
+
+  - A table of index positions (one per packed object, num_objects in
+    total, each a 4-byte unsigned integer in network order), sorted by
+    their corresponding offsets in the packfile.
+
+  - A trailer, containing a:
+
+    checksum of the corresponding packfile, and
+
+    a checksum of all of the above.
+
+All 4-byte numbers are in network order.
+
 == multi-pack-index (MIDX) files have the following format:
 
 The multi-pack-index files refer to multiple pack-files and loose objects.
diff --git a/builtin/repack.c b/builtin/repack.c
index 2158b48f4c..01440de2d5 100644
--- a/builtin/repack.c
+++ b/builtin/repack.c
@@ -209,6 +209,7 @@ static struct {
 } exts[] = {
 	{".pack"},
 	{".idx"},
+	{".rev", 1},
 	{".bitmap", 1},
 	{".promisor", 1},
 };
diff --git a/object-store.h b/object-store.h
index c4fc9dd74e..541dab0858 100644
--- a/object-store.h
+++ b/object-store.h
@@ -85,6 +85,9 @@ struct packed_git {
 		 multi_pack_index:1;
 	unsigned char hash[GIT_MAX_RAWSZ];
 	struct revindex_entry *revindex;
+	const uint32_t *revindex_data;
+	const uint32_t *revindex_map;
+	size_t revindex_size;
 	/* something like ".git/objects/pack/xxxxx.pack" */
 	char pack_name[FLEX_ARRAY]; /* more */
 };
diff --git a/pack-revindex.c b/pack-revindex.c
index 5e69bc7372..a174fa5388 100644
--- a/pack-revindex.c
+++ b/pack-revindex.c
@@ -164,16 +164,130 @@ static void create_pack_revindex(struct packed_git *p)
 	sort_revindex(p->revindex, num_ent, p->pack_size);
 }
 
-int load_pack_revindex(struct packed_git *p)
+static int create_pack_revindex_in_memory(struct packed_git *p)
 {
-	if (!p->revindex) {
-		if (open_pack_index(p))
-			return -1;
-		create_pack_revindex(p);
-	}
+	if (open_pack_index(p))
+		return -1;
+	create_pack_revindex(p);
 	return 0;
 }
 
+static char *pack_revindex_filename(struct packed_git *p)
+{
+	size_t len;
+	if (!strip_suffix(p->pack_name, ".pack", &len))
+		BUG("pack_name does not end in .pack");
+	return xstrfmt("%.*s.rev", (int)len, p->pack_name);
+}
+
+#define RIDX_HEADER_SIZE (12)
+#define RIDX_MIN_SIZE (RIDX_HEADER_SIZE + (2 * the_hash_algo->rawsz))
+
+struct revindex_header {
+	uint32_t signature;
+	uint32_t version;
+	uint32_t hash_id;
+};
+
+static int load_revindex_from_disk(char *revindex_name,
+				   uint32_t num_objects,
+				   const uint32_t **data_p, size_t *len_p)
+{
+	int fd, ret = 0;
+	struct stat st;
+	void *data = NULL;
+	size_t revindex_size;
+	struct revindex_header *hdr;
+
+	fd = git_open(revindex_name);
+
+	if (fd < 0) {
+		ret = -1;
+		goto cleanup;
+	}
+	if (fstat(fd, &st)) {
+		ret = error_errno(_("failed to read %s"), revindex_name);
+		goto cleanup;
+	}
+
+	revindex_size = xsize_t(st.st_size);
+
+	if (revindex_size < RIDX_MIN_SIZE) {
+		ret = error(_("reverse-index file %s is too small"), revindex_name);
+		goto cleanup;
+	}
+
+	if (revindex_size - RIDX_MIN_SIZE != st_mult(sizeof(uint32_t), num_objects)) {
+		ret = error(_("reverse-index file %s is corrupt"), revindex_name);
+		goto cleanup;
+	}
+
+	data = xmmap(NULL, revindex_size, PROT_READ, MAP_PRIVATE, fd, 0);
+	hdr = data;
+
+	if (ntohl(hdr->signature) != RIDX_SIGNATURE) {
+		ret = error(_("reverse-index file %s has unknown signature"), revindex_name);
+		goto cleanup;
+	}
+	if (ntohl(hdr->version) != 1) {
+		ret = error(_("reverse-index file %s has unsupported version %"PRIu32),
+			    revindex_name, ntohl(hdr->version));
+		goto cleanup;
+	}
+	if (!(ntohl(hdr->hash_id) == 1 || ntohl(hdr->hash_id) == 2)) {
+		ret = error(_("reverse-index file %s has unsupported hash id %"PRIu32),
+			    revindex_name, ntohl(hdr->hash_id));
+		goto cleanup;
+	}
+
+cleanup:
+	if (ret) {
+		if (data)
+			munmap(data, revindex_size);
+	} else {
+		*len_p = revindex_size;
+		*data_p = (const uint32_t *)data;
+	}
+
+	close(fd);
+	return ret;
+}
+
+static int load_pack_revindex_from_disk(struct packed_git *p)
+{
+	char *revindex_name;
+	int ret;
+	if (open_pack_index(p))
+		return -1;
+
+	revindex_name = pack_revindex_filename(p);
+
+	ret = load_revindex_from_disk(revindex_name,
+				      p->num_objects,
+				      &p->revindex_map,
+				      &p->revindex_size);
+	if (ret)
+		goto cleanup;
+
+	p->revindex_data = (const uint32_t *)((const char *)p->revindex_map + RIDX_HEADER_SIZE);
+
+cleanup:
+	free(revindex_name);
+	return ret;
+}
+
+int load_pack_revindex(struct packed_git *p)
+{
+	if (p->revindex || p->revindex_data)
+		return 0;
+
+	if (!load_pack_revindex_from_disk(p))
+		return 0;
+	else if (!create_pack_revindex_in_memory(p))
+		return 0;
+	return -1;
+}
+
 int offset_to_pack_pos(struct packed_git *p, off_t ofs, uint32_t *pos)
 {
 	unsigned lo, hi;
@@ -203,18 +317,28 @@ int offset_to_pack_pos(struct packed_git *p, off_t ofs, uint32_t *pos)
 
 uint32_t pack_pos_to_index(struct packed_git *p, uint32_t pos)
 {
-	if (!p->revindex)
+	if (!(p->revindex || p->revindex_data))
 		BUG("pack_pos_to_index: reverse index not yet loaded");
 	if (p->num_objects <= pos)
 		BUG("pack_pos_to_index: out-of-bounds object at %"PRIu32, pos);
-	return p->revindex[pos].nr;
+
+	if (p->revindex)
+		return p->revindex[pos].nr;
+	else
+		return get_be32(p->revindex_data + pos);
 }
 
 off_t pack_pos_to_offset(struct packed_git *p, uint32_t pos)
 {
-	if (!p->revindex)
+	if (!(p->revindex || p->revindex_data))
 		BUG("pack_pos_to_index: reverse index not yet loaded");
 	if (p->num_objects < pos)
 		BUG("pack_pos_to_offset: out-of-bounds object at %"PRIu32, pos);
-	return p->revindex[pos].offset;
+
+	if (p->revindex)
+		return p->revindex[pos].offset;
+	else if (pos == p->num_objects)
+		return p->pack_size - the_hash_algo->rawsz;
+	else
+		return nth_packed_object_offset(p, pack_pos_to_index(p, pos));
 }
diff --git a/pack-revindex.h b/pack-revindex.h
index 6e0320b08b..61b2f3ab75 100644
--- a/pack-revindex.h
+++ b/pack-revindex.h
@@ -16,11 +16,17 @@
  *   can be found
  */
 
+#define RIDX_SIGNATURE 0x52494458 /* "RIDX" */
+#define RIDX_VERSION 1
+
 struct packed_git;
 
 /*
  * load_pack_revindex populates the revindex's internal data-structures for the
  * given pack, returning zero on success and a negative value otherwise.
+ *
+ * If a '.rev' file is present it is mmap'd, and pointers are assigned into it
+ * (instead of using the in-memory variant).
  */
 int load_pack_revindex(struct packed_git *p);
 
@@ -55,7 +61,9 @@ uint32_t pack_pos_to_index(struct packed_git *p, uint32_t pos);
  * If the reverse index has not yet been loaded, or the position is out of
  * bounds, this function aborts.
  *
- * This function runs in constant time.
+ * This function runs in constant time under both in-memory and on-disk reverse
+ * indexes, but an additional step is taken to consult the corresponding .idx
+ * file when using the on-disk format.
  */
 off_t pack_pos_to_offset(struct packed_git *p, uint32_t pos);
 
diff --git a/packfile.c b/packfile.c
index 4b938b4372..1fec12ac5f 100644
--- a/packfile.c
+++ b/packfile.c
@@ -324,11 +324,21 @@ void close_pack_index(struct packed_git *p)
 	}
 }
 
+void close_pack_revindex(struct packed_git *p) {
+	if (!p->revindex_map)
+		return;
+
+	munmap((void *)p->revindex_map, p->revindex_size);
+	p->revindex_map = NULL;
+	p->revindex_data = NULL;
+}
+
 void close_pack(struct packed_git *p)
 {
 	close_pack_windows(p);
 	close_pack_fd(p);
 	close_pack_index(p);
+	close_pack_revindex(p);
 }
 
 void close_object_store(struct raw_object_store *o)
@@ -351,7 +361,7 @@ void close_object_store(struct raw_object_store *o)
 
 void unlink_pack_path(const char *pack_name, int force_delete)
 {
-	static const char *exts[] = {".pack", ".idx", ".keep", ".bitmap", ".promisor"};
+	static const char *exts[] = {".pack", ".idx", ".rev", ".keep", ".bitmap", ".promisor"};
 	int i;
 	struct strbuf buf = STRBUF_INIT;
 	size_t plen;
@@ -853,6 +863,7 @@ static void prepare_pack(const char *full_name, size_t full_name_len,
 	if (!strcmp(file_name, "multi-pack-index"))
 		return;
 	if (ends_with(file_name, ".idx") ||
+	    ends_with(file_name, ".rev") ||
 	    ends_with(file_name, ".pack") ||
 	    ends_with(file_name, ".bitmap") ||
 	    ends_with(file_name, ".keep") ||
diff --git a/packfile.h b/packfile.h
index a58fc738e0..4cfec9e8d3 100644
--- a/packfile.h
+++ b/packfile.h
@@ -90,6 +90,7 @@ uint32_t get_pack_fanout(struct packed_git *p, uint32_t value);
 
 unsigned char *use_pack(struct packed_git *, struct pack_window **, off_t, unsigned long *);
 void close_pack_windows(struct packed_git *);
+void close_pack_revindex(struct packed_git *);
 void close_pack(struct packed_git *);
 void close_object_store(struct raw_object_store *o);
 void unuse_pack(struct pack_window **);
diff --git a/tmp-objdir.c b/tmp-objdir.c
index 42ed4db5d3..b8d880e362 100644
--- a/tmp-objdir.c
+++ b/tmp-objdir.c
@@ -185,9 +185,11 @@ static int pack_copy_priority(const char *name)
 		return 1;
 	if (ends_with(name, ".pack"))
 		return 2;
-	if (ends_with(name, ".idx"))
+	if (ends_with(name, ".rev"))
 		return 3;
-	return 4;
+	if (ends_with(name, ".idx"))
+		return 4;
+	return 5;
 }
 
 static int pack_copy_cmp(const char *a, const char *b)
