diff --git a/block-cache/io_engine.cc b/block-cache/io_engine.cc
index 7837c33..a4211cb 100644
--- a/block-cache/io_engine.cc
+++ b/block-cache/io_engine.cc
@@ -75,9 +75,11 @@ aio_engine::~aio_engine()
 }
 
 aio_engine::handle
-aio_engine::open_file(std::string const &path, mode m)
+aio_engine::open_file(std::string const &path, mode m, sharing s)
 {
 	int flags = (m == READ_ONLY) ? O_RDONLY : O_RDWR;
+	if (s == EXCLUSIVE)
+		flags |= O_EXCL;
 	int fd = ::open(path.c_str(), O_DIRECT | flags);
 	if (fd < 0) {
 		ostringstream out;
diff --git a/block-cache/io_engine.h b/block-cache/io_engine.h
index 55b556d..f9e4c9c 100644
--- a/block-cache/io_engine.h
+++ b/block-cache/io_engine.h
@@ -27,12 +27,17 @@ namespace bcache {
 			WRITE
 		};
 
+		enum sharing {
+			EXCLUSIVE,
+			SHARED
+		};
+
 		io_engine() {}
 		virtual ~io_engine() {}
 
 		using handle = unsigned;
 
-		virtual handle open_file(std::string const &path, mode m) = 0;
+		virtual handle open_file(std::string const &path, mode m, sharing s = EXCLUSIVE) = 0;
 		virtual void close_file(handle h) = 0;
 
 		// returns false if there are insufficient resources to
@@ -79,8 +84,7 @@ namespace bcache {
 
 		using handle = unsigned;
 
-		// FIXME: open exclusive?
-		virtual handle open_file(std::string const &path, mode m);
+		virtual handle open_file(std::string const &path, mode m, sharing s = EXCLUSIVE);
 		virtual void close_file(handle h);
 
 		// Returns false if queueing the io failed
diff --git a/unit-tests/copier_t.cc b/unit-tests/copier_t.cc
index e95819f..0185d9e 100644
--- a/unit-tests/copier_t.cc
+++ b/unit-tests/copier_t.cc
@@ -33,7 +33,7 @@ using namespace testing;
 namespace {
 	class io_engine_mock : public io_engine {
 	public:
-		MOCK_METHOD2(open_file, handle(string const &, mode));
+		MOCK_METHOD3(open_file, handle(string const &, mode, sharing));
 		MOCK_METHOD1(close_file, void(handle));
 		MOCK_METHOD6(issue_io, bool(handle, dir, sector_t, sector_t, void *, unsigned));
 
@@ -51,9 +51,9 @@ namespace {
 		}
 
 		unique_ptr<copier> make_copier() {
-			EXPECT_CALL(engine_, open_file(src_file_, io_engine::READ_ONLY)).
+			EXPECT_CALL(engine_, open_file(src_file_, io_engine::READ_ONLY, io_engine::EXCLUSIVE)).
 				WillOnce(Return(SRC_HANDLE));
-			EXPECT_CALL(engine_, open_file(dest_file_, io_engine::READ_WRITE)).
+			EXPECT_CALL(engine_, open_file(dest_file_, io_engine::READ_WRITE, io_engine::EXCLUSIVE)).
 				WillOnce(Return(DEST_HANDLE));
 
 			EXPECT_CALL(engine_, close_file(SRC_HANDLE)).Times(1);