diff --git a/Makefile b/Makefile
index 13a69dd..fb3afe2 100644
--- a/Makefile
+++ b/Makefile
@@ -16,6 +16,7 @@ SOURCE=\
 	space_map.cc \
 	space_map_disk.cc \
 	space_map_recursive.cc \
+	space_map_transactional.cc \
 	thin_pool.cc \
 	transaction_manager.cc \
 	xml_format.cc
diff --git a/space_map_transactional.cc b/space_map_transactional.cc
new file mode 100644
index 0000000..dfbec48
--- /dev/null
+++ b/space_map_transactional.cc
@@ -0,0 +1,96 @@
+#include "space_map_transactional.h"
+
+//----------------------------------------------------------------
+
+namespace {
+	class sm_transactional : public checked_space_map {
+	public:
+		typedef shared_ptr<sm_transactional> ptr;
+
+		sm_transactional(checked_space_map::ptr sm)
+			: sm_(sm),
+			  committed_(sm_->clone()),
+			  allocated_(0),
+			  search_start_(0) {
+		}
+
+		virtual block_address get_nr_blocks() const {
+			return committed_->get_nr_blocks();
+		}
+
+		virtual block_address get_nr_free() const {
+			return committed_->get_nr_free() - allocated_;
+		}
+
+		virtual ref_t get_count(block_address b) const {
+			return sm_->get_count(b);
+		}
+
+		virtual void set_count(block_address b, ref_t c) {
+			sm_->set_count(b, c);
+		}
+
+		virtual void commit() {
+			sm_->commit();
+			committed_ = sm_->clone();
+			allocated_ = 0;
+			search_start_ = 0;
+		}
+
+		virtual void inc(block_address b) {
+			// FIXME: this may do an implicit allocation, so
+			// search_start_ and allocated_ will be wrong.
+			sm_->inc(b);
+		}
+
+		virtual void dec(block_address b) {
+			sm_->dec(b);
+		}
+
+		virtual maybe_block new_block() {
+			return new_block(0, sm_->get_nr_blocks());
+		}
+
+		virtual maybe_block new_block(block_address begin, block_address end) {
+			if (end <= search_start_)
+				return maybe_block();
+
+			maybe_block mb = sm_->new_block(max(search_start_, begin), end);
+			if (mb) {
+				allocated_++;
+				search_start_ = *mb + 1;
+			} else
+				search_start_ = end;
+
+			return mb;
+		}
+
+		virtual bool count_possibly_greater_than_one(block_address b) const {
+			return sm_->count_possibly_greater_than_one(b);
+		}
+
+		virtual void extend(block_address extra_blocks) {
+			return sm_->extend(extra_blocks);
+		}
+
+		virtual size_t root_size() const {
+			return sm_->root_size();
+		}
+
+		virtual void copy_root(void *dest, size_t len) const {
+			return sm_->copy_root(dest, len);
+		}
+
+		virtual checked_space_map::ptr clone() const {
+			return checked_space_map::ptr(new sm_transactional(sm_));
+		}
+
+	private:
+		checked_space_map::ptr sm_;
+		checked_space_map::ptr committed_;
+		block_address allocated_;
+		block_address search_start_;
+	};
+}
+
+//----------------------------------------------------------------
diff --git a/space_map_transactional.h b/space_map_transactional.h
new file mode 100644
index 0000000..4cd502b
--- /dev/null
+++ b/space_map_transactional.h
@@ -0,0 +1,19 @@
+#ifndef SPACE_MAP_TRANSACTIONAL_H
+#define SPACE_MAP_TRANSACTIONAL_H
+
+#include "space_map.h"
+
+//----------------------------------------------------------------
+
+namespace persistent_data {
+	
+	// FIXME: change name 'transactional' is so vague.
+
+	// This space map ensures no blocks are allocated which have been
+	// freed within the current transaction.
+	checked_space_map::ptr create_transactional_sm(checked_space_map::ptr sm);
+}
+
+//----------------------------------------------------------------
+
+#endif