diff --git a/read-cache.c b/read-cache.c
index b0a06db5c5..a78b88a41b 100644
--- a/read-cache.c
+++ b/read-cache.c
@@ -2400,9 +2400,21 @@ int read_index_from(struct index_state *istate, const char *path,
 	base_path = xstrfmt("%s/sharedindex.%s", gitdir, base_oid_hex);
 	trace2_region_enter_printf("index", "shared/do_read_index",
 				   the_repository, "%s", base_path);
-	ret = do_read_index(split_index->base, base_path, 1);
+	ret = do_read_index(split_index->base, base_path, 0);
 	trace2_region_leave_printf("index", "shared/do_read_index",
 				   the_repository, "%s", base_path);
+	if (!ret) {
+		char *path_copy = xstrdup(path);
+		const char *base_path2 = xstrfmt("%s/sharedindex.%s",
+						 dirname(path_copy),
+						 base_oid_hex);
+		free(path_copy);
+		trace2_region_enter_printf("index", "shared/do_read_index",
+					   the_repository, "%s", base_path2);
+		ret = do_read_index(split_index->base, base_path2, 1);
+		trace2_region_leave_printf("index", "shared/do_read_index",
+					   the_repository, "%s", base_path2);
+	}
 	if (!oideq(&split_index->base_oid, &split_index->base->oid))
 		die(_("broken index, expect %s in %s, got %s"),
 		    base_oid_hex, base_path,
@@ -2821,11 +2833,8 @@ static int do_write_index(struct index_state *istate, struct tempfile *tempfile,
 		}
 	}
 
-	if (!istate->version) {
+	if (!istate->version)
 		istate->version = get_index_format_default(the_repository);
-		if (git_env_bool("GIT_TEST_SPLIT_INDEX", 0))
-			init_split_index(istate);
-	}
 
 	/* demote version 3 to version 2 when the latter suffices */
 	if (istate->version == 3 || istate->version == 2)
@@ -3252,7 +3261,7 @@ static int too_many_not_shared_entries(struct index_state *istate)
 int write_locked_index(struct index_state *istate, struct lock_file *lock,
 		       unsigned flags)
 {
-	int new_shared_index, ret;
+	int new_shared_index, ret, test_split_index_env;
 	struct split_index *si = istate->split_index;
 
 	if (git_env_bool("GIT_TEST_CHECK_CACHE_TREE", 0))
@@ -3267,7 +3276,10 @@ int write_locked_index(struct index_state *istate, struct lock_file *lock,
 	if (istate->fsmonitor_last_update)
 		fill_fsmonitor_bitmap(istate);
 
-	if (!si || alternate_index_output ||
+	test_split_index_env = git_env_bool("GIT_TEST_SPLIT_INDEX", 0);
+
+	if ((!si && !test_split_index_env) ||
+	    alternate_index_output ||
 	    (istate->cache_changed & ~EXTMASK)) {
 		if (si)
 			oidclr(&si->base_oid);
@@ -3275,10 +3287,15 @@ int write_locked_index(struct index_state *istate, struct lock_file *lock,
 		goto out;
 	}
 
-	if (git_env_bool("GIT_TEST_SPLIT_INDEX", 0)) {
-		int v = si->base_oid.hash[0];
-		if ((v & 15) < 6)
+	if (test_split_index_env) {
+		if (!si) {
+			si = init_split_index(istate);
 			istate->cache_changed |= SPLIT_INDEX_ORDERED;
+		} else {
+			int v = si->base_oid.hash[0];
+			if ((v & 15) < 6)
+				istate->cache_changed |= SPLIT_INDEX_ORDERED;
+		}
 	}
 	if (too_many_not_shared_entries(istate))
 		istate->cache_changed |= SPLIT_INDEX_ORDERED;
diff --git a/t/t1091-sparse-checkout-builtin.sh b/t/t1091-sparse-checkout-builtin.sh
index 71236981e6..8675ad5154 100755
--- a/t/t1091-sparse-checkout-builtin.sh
+++ b/t/t1091-sparse-checkout-builtin.sh
@@ -206,16 +206,21 @@ test_expect_success 'sparse-checkout disable' '
 '
 
 test_expect_success 'sparse-index enabled and disabled' '
-	git -C repo sparse-checkout init --cone --sparse-index &&
-	test_cmp_config -C repo true index.sparse &&
-	test-tool -C repo read-cache --table >cache &&
-	grep " tree " cache &&
+	(
+		sane_unset GIT_TEST_SPLIT_INDEX &&
+		git -C repo update-index --no-split-index &&
 
-	git -C repo sparse-checkout disable &&
-	test-tool -C repo read-cache --table >cache &&
-	! grep " tree " cache &&
-	git -C repo config --list >config &&
-	! grep index.sparse config
+		git -C repo sparse-checkout init --cone --sparse-index &&
+		test_cmp_config -C repo true index.sparse &&
+		test-tool -C repo read-cache --table >cache &&
+		grep " tree " cache &&
+
+		git -C repo sparse-checkout disable &&
+		test-tool -C repo read-cache --table >cache &&
+		! grep " tree " cache &&
+		git -C repo config --list >config &&
+		! grep index.sparse config
+	)
 '
 
 test_expect_success 'cone mode: init and set' '
diff --git a/t/t1600-index.sh b/t/t1600-index.sh
index c9b9e718b8..46329c488b 100755
--- a/t/t1600-index.sh
+++ b/t/t1600-index.sh
@@ -4,6 +4,8 @@ test_description='index file specific tests'
 
 . ./test-lib.sh
 
+sane_unset GIT_TEST_SPLIT_INDEX
+
 test_expect_success 'setup' '
 	echo 1 >a
 '
@@ -13,7 +15,8 @@ test_expect_success 'bogus GIT_INDEX_VERSION issues warning' '
 		rm -f .git/index &&
 		GIT_INDEX_VERSION=2bogus &&
 		export GIT_INDEX_VERSION &&
-		git add a 2>&1 | sed "s/[0-9]//" >actual.err &&
+		git add a 2>err &&
+		sed "s/[0-9]//" err >actual.err &&
 		sed -e "s/ Z$/ /" <<-\EOF >expect.err &&
 			warning: GIT_INDEX_VERSION set, but the value is invalid.
 			Using version Z
@@ -27,7 +30,8 @@ test_expect_success 'out of bounds GIT_INDEX_VERSION issues warning' '
 		rm -f .git/index &&
 		GIT_INDEX_VERSION=1 &&
 		export GIT_INDEX_VERSION &&
-		git add a 2>&1 | sed "s/[0-9]//" >actual.err &&
+		git add a 2>err &&
+		sed "s/[0-9]//" err >actual.err &&
 		sed -e "s/ Z$/ /" <<-\EOF >expect.err &&
 			warning: GIT_INDEX_VERSION set, but the value is invalid.
 			Using version Z
@@ -50,7 +54,8 @@ test_expect_success 'out of bounds index.version issues warning' '
 		sane_unset GIT_INDEX_VERSION &&
 		rm -f .git/index &&
 		git config --add index.version 1 &&
-		git add a 2>&1 | sed "s/[0-9]//" >actual.err &&
+		git add a 2>err &&
+		sed "s/[0-9]//" err >actual.err &&
 		sed -e "s/ Z$/ /" <<-\EOF >expect.err &&
 			warning: index.version set, but the value is invalid.
 			Using version Z
@@ -79,7 +84,7 @@ test_index_version () {
 		else
 			unset GIT_INDEX_VERSION
 		fi &&
-		git add a 2>&1 &&
+		git add a &&
 		echo $EXPECTED_OUTPUT_VERSION >expect &&
 		test-tool index-version <.git/index >actual &&
 		test_cmp expect actual
diff --git a/t/t1700-split-index.sh b/t/t1700-split-index.sh
index 986baa612e..decd2527ed 100755
--- a/t/t1700-split-index.sh
+++ b/t/t1700-split-index.sh
@@ -510,4 +510,38 @@ test_expect_success 'do not refresh null base index' '
 	)
 '
 
+test_expect_success 'reading split index at alternate location' '
+	git init reading-alternate-location &&
+	(
+		cd reading-alternate-location &&
+		>file-in-alternate &&
+		git update-index --split-index --add file-in-alternate
+	) &&
+	echo file-in-alternate >expect &&
+
+	# Should be able to find the shared index both right next to
+	# the specified split index file ...
+	GIT_INDEX_FILE=./reading-alternate-location/.git/index \
+	git ls-files --cached >actual &&
+	test_cmp expect actual &&
+
+	# ... and, for backwards compatibility, in the current GIT_DIR
+	# as well.
+	mv -v ./reading-alternate-location/.git/sharedindex.* .git &&
+	GIT_INDEX_FILE=./reading-alternate-location/.git/index \
+	git ls-files --cached >actual &&
+	test_cmp expect actual
+'
+
+test_expect_success 'GIT_TEST_SPLIT_INDEX works' '
+	git init git-test-split-index &&
+	(
+		cd git-test-split-index &&
+		>file &&
+		GIT_TEST_SPLIT_INDEX=1 git update-index --add file &&
+		ls -l .git/sharedindex.* >actual &&
+		test_line_count = 1 actual
+	)
+'
+
 test_done
diff --git a/t/t7519-status-fsmonitor.sh b/t/t7519-status-fsmonitor.sh
index f1463197b9..f488d930df 100755
--- a/t/t7519-status-fsmonitor.sh
+++ b/t/t7519-status-fsmonitor.sh
@@ -399,41 +399,45 @@ check_sparse_index_behavior () {
 }
 
 test_expect_success 'status succeeds with sparse index' '
-	git clone . full &&
-	git clone --sparse . sparse &&
-	git -C sparse sparse-checkout init --cone --sparse-index &&
-	git -C sparse sparse-checkout set dir1 dir2 &&
+	(
+		sane_unset GIT_TEST_SPLIT_INDEX &&
 
-	write_script .git/hooks/fsmonitor-test <<-\EOF &&
-		printf "last_update_token\0"
-	EOF
-	git -C full config core.fsmonitor ../.git/hooks/fsmonitor-test &&
-	git -C sparse config core.fsmonitor ../.git/hooks/fsmonitor-test &&
-	check_sparse_index_behavior ! &&
+		git clone . full &&
+		git clone --sparse . sparse &&
+		git -C sparse sparse-checkout init --cone --sparse-index &&
+		git -C sparse sparse-checkout set dir1 dir2 &&
 
-	write_script .git/hooks/fsmonitor-test <<-\EOF &&
-		printf "last_update_token\0"
-		printf "dir1/modified\0"
-	EOF
-	check_sparse_index_behavior ! &&
+		write_script .git/hooks/fsmonitor-test <<-\EOF &&
+			printf "last_update_token\0"
+		EOF
+		git -C full config core.fsmonitor ../.git/hooks/fsmonitor-test &&
+		git -C sparse config core.fsmonitor ../.git/hooks/fsmonitor-test &&
+		check_sparse_index_behavior ! &&
 
-	git -C sparse sparse-checkout add dir1a &&
+		write_script .git/hooks/fsmonitor-test <<-\EOF &&
+			printf "last_update_token\0"
+			printf "dir1/modified\0"
+		EOF
+		check_sparse_index_behavior ! &&
 
-	for repo in full sparse
-	do
-		cp -r $repo/dir1 $repo/dir1a &&
-		git -C $repo add dir1a &&
-		git -C $repo commit -m "add dir1a" || return 1
-	done &&
-	git -C sparse sparse-checkout set dir1 dir2 &&
+		git -C sparse sparse-checkout add dir1a &&
 
-	# This one modifies outside the sparse-checkout definition
-	# and hence we expect to expand the sparse-index.
-	write_script .git/hooks/fsmonitor-test <<-\EOF &&
-		printf "last_update_token\0"
-		printf "dir1a/modified\0"
-	EOF
-	check_sparse_index_behavior
+		for repo in full sparse
+		do
+			cp -r $repo/dir1 $repo/dir1a &&
+			git -C $repo add dir1a &&
+			git -C $repo commit -m "add dir1a" || return 1
+		done &&
+		git -C sparse sparse-checkout set dir1 dir2 &&
+
+		# This one modifies outside the sparse-checkout definition
+		# and hence we expect to expand the sparse-index.
+		write_script .git/hooks/fsmonitor-test <<-\EOF &&
+			printf "last_update_token\0"
+			printf "dir1a/modified\0"
+		EOF
+		check_sparse_index_behavior
+	)
 '
 
 test_done