diff --git a/persistent-data/space-maps/disk.cc b/persistent-data/space-maps/disk.cc
index e915eb4..9b1f9cc 100644
--- a/persistent-data/space-maps/disk.cc
+++ b/persistent-data/space-maps/disk.cc
@@ -773,4 +773,15 @@ persistent_data::open_metadata_sm(transaction_manager &tm, void *root)
 			checked_space_map::ptr(new sm_disk(store, tm, v))));
 }
 
+block_address
+persistent_data::get_nr_blocks_in_data_sm(transaction_manager &tm, void *root)
+{
+	sm_root_disk d;
+	sm_root v;
+
+	::memcpy(&d, root, sizeof(d));
+	sm_root_traits::unpack(d, v);
+	return v.nr_blocks_;
+}
+
 //----------------------------------------------------------------
diff --git a/persistent-data/space-maps/disk.h b/persistent-data/space-maps/disk.h
index 0a69f04..cb747e8 100644
--- a/persistent-data/space-maps/disk.h
+++ b/persistent-data/space-maps/disk.h
@@ -36,6 +36,11 @@ namespace persistent_data {
 
 	checked_space_map::ptr
 	open_metadata_sm(transaction_manager &tm, void *root);
+
+	// Get the number of data blocks with minimal IO.  Used when
+	// repairing to avoid the bulk of the space maps.
+	block_address
+	get_nr_blocks_in_data_sm(transaction_manager &tm, void *root);
 }
 
 //----------------------------------------------------------------