diff --git a/persistent-data/space-maps/disk.cc b/persistent-data/space-maps/disk.cc
index e641c89..e915eb4 100644
--- a/persistent-data/space-maps/disk.cc
+++ b/persistent-data/space-maps/disk.cc
@@ -747,6 +747,11 @@ persistent_data::create_metadata_sm(transaction_manager &tm, block_address nr_bl
 {
 	index_store::ptr store(new metadata_index_store(tm));
 	checked_space_map::ptr sm(new sm_disk(store, tm));
+
+	if (nr_blocks > MAX_METADATA_BLOCKS) {
+		cerr << "truncating metadata device to " << MAX_METADATA_BLOCKS << " 4k blocks\n";
+		nr_blocks = MAX_METADATA_BLOCKS;
+	}
 	sm->extend(nr_blocks);
 	sm->commit();
 	return create_careful_alloc_sm(
diff --git a/persistent-data/space-maps/disk_structures.h b/persistent-data/space-maps/disk_structures.h
index 1429d36..0a57e61 100644
--- a/persistent-data/space-maps/disk_structures.h
+++ b/persistent-data/space-maps/disk_structures.h
@@ -61,6 +61,7 @@ namespace persistent_data {
 		};
 
 		unsigned const MAX_METADATA_BITMAPS = 255;
+		unsigned const MAX_METADATA_BLOCKS = (255 * ((1 << 14) - 64));
 		unsigned const ENTRIES_PER_BYTE = 4;
 
 		struct metadata_index {