diff --git a/cache.h b/cache.h
index e1f0e182ad..1ad914084b 100644
--- a/cache.h
+++ b/cache.h
@@ -11,6 +11,7 @@
 #include "string-list.h"
 #include "pack-revindex.h"
 #include "hash.h"
+#include "sha1-array.h"
 
 #ifndef platform_SHA_CTX
 /*
@@ -1579,6 +1580,16 @@ extern struct alternate_object_database {
 	struct strbuf scratch;
 	size_t base_len;
 
+	/*
+	 * Used to store the results of readdir(3) calls when searching
+	 * for unique abbreviated hashes.  This cache is never
+	 * invalidated, thus it's racy and not necessarily accurate.
+	 * That's fine for its purpose; don't use it for tasks requiring
+	 * greater accuracy!
+	 */
+	char loose_objects_subdir_seen[256];
+	struct oid_array loose_objects_cache;
+
 	char path[FLEX_ARRAY];
 } *alt_odb_list;
 extern void prepare_alt_odb(void);
@@ -1797,6 +1808,12 @@ typedef int each_loose_cruft_fn(const char *basename,
 typedef int each_loose_subdir_fn(int nr,
 				 const char *path,
 				 void *data);
+int for_each_file_in_obj_subdir(int subdir_nr,
+				struct strbuf *path,
+				each_loose_object_fn obj_cb,
+				each_loose_cruft_fn cruft_cb,
+				each_loose_subdir_fn subdir_cb,
+				void *data);
 int for_each_loose_file_in_objdir(const char *path,
 				  each_loose_object_fn obj_cb,
 				  each_loose_cruft_fn cruft_cb,
diff --git a/sha1_file.c b/sha1_file.c
index 59a4ed2ed3..5e0ee2b68b 100644
--- a/sha1_file.c
+++ b/sha1_file.c
@@ -3735,12 +3735,12 @@ void assert_sha1_type(const unsigned char *sha1, enum object_type expect)
 		    typename(expect));
 }
 
-static int for_each_file_in_obj_subdir(int subdir_nr,
-				       struct strbuf *path,
-				       each_loose_object_fn obj_cb,
-				       each_loose_cruft_fn cruft_cb,
-				       each_loose_subdir_fn subdir_cb,
-				       void *data)
+int for_each_file_in_obj_subdir(int subdir_nr,
+				struct strbuf *path,
+				each_loose_object_fn obj_cb,
+				each_loose_cruft_fn cruft_cb,
+				each_loose_subdir_fn subdir_cb,
+				void *data)
 {
 	size_t baselen = path->len;
 	DIR *dir = opendir(path->buf);
diff --git a/sha1_name.c b/sha1_name.c
index 8eec9f7c1b..76cb76a844 100644
--- a/sha1_name.c
+++ b/sha1_name.c
@@ -77,10 +77,19 @@ static void update_candidates(struct disambiguate_state *ds, const struct object
 	/* otherwise, current can be discarded and candidate is still good */
 }
 
+static int append_loose_object(const struct object_id *oid, const char *path,
+			       void *data)
+{
+	oid_array_append(data, oid);
+	return 0;
+}
+
+static int match_sha(unsigned, const unsigned char *, const unsigned char *);
+
 static void find_short_object_filename(struct disambiguate_state *ds)
 {
+	int subdir_nr = ds->bin_pfx.hash[0];
 	struct alternate_object_database *alt;
-	char hex[GIT_MAX_HEXSZ];
 	static struct alternate_object_database *fakeent;
 
 	if (!fakeent) {
@@ -95,29 +104,30 @@ static void find_short_object_filename(struct disambiguate_state *ds)
 	}
 	fakeent->next = alt_odb_list;
 
-	xsnprintf(hex, sizeof(hex), "%.2s", ds->hex_pfx);
 	for (alt = fakeent; alt && !ds->ambiguous; alt = alt->next) {
-		struct strbuf *buf = alt_scratch_buf(alt);
-		struct dirent *de;
-		DIR *dir;
+		int pos;
 
-		strbuf_addf(buf, "%.2s/", ds->hex_pfx);
-		dir = opendir(buf->buf);
-		if (!dir)
-			continue;
-
-		while (!ds->ambiguous && (de = readdir(dir)) != NULL) {
-			struct object_id oid;
-
-			if (strlen(de->d_name) != GIT_SHA1_HEXSZ - 2)
-				continue;
-			if (memcmp(de->d_name, ds->hex_pfx + 2, ds->len - 2))
-				continue;
-			memcpy(hex + 2, de->d_name, GIT_SHA1_HEXSZ - 2);
-			if (!get_oid_hex(hex, &oid))
-				update_candidates(ds, &oid);
+		if (!alt->loose_objects_subdir_seen[subdir_nr]) {
+			struct strbuf *buf = alt_scratch_buf(alt);
+			strbuf_addf(buf, "%02x/", subdir_nr);
+			for_each_file_in_obj_subdir(subdir_nr, buf,
+						    append_loose_object,
+						    NULL, NULL,
+						    &alt->loose_objects_cache);
+			alt->loose_objects_subdir_seen[subdir_nr] = 1;
+		}
+
+		pos = oid_array_lookup(&alt->loose_objects_cache, &ds->bin_pfx);
+		if (pos < 0)
+			pos = -1 - pos;
+		while (!ds->ambiguous && pos < alt->loose_objects_cache.nr) {
+			const struct object_id *oid;
+			oid = alt->loose_objects_cache.oid + pos;
+			if (!match_sha(ds->len, ds->bin_pfx.hash, oid->hash))
+				break;
+			update_candidates(ds, oid);
+			pos++;
 		}
-		closedir(dir);
 	}
 }