diff --git a/Makefile b/Makefile
index 250f8e1..ad03c7a 100644
--- a/Makefile
+++ b/Makefile
@@ -17,7 +17,7 @@ SOURCE=\
 
 OBJECTS=$(subst .cc,.o,$(SOURCE))
 TOP_DIR:=$(PWD)
-CPPFLAGS=-Wall -g -I$(TOP_DIR)
+CPPFLAGS=-Wall -g -I$(TOP_DIR) -O8
 #CPPFLAGS=-Wall -std=c++0x -g -I$(TOP_DIR)
 LIBS=-lstdc++ -lboost_program_options
 
diff --git a/metadata.cc b/metadata.cc
index 851d81d..cc3d189 100644
--- a/metadata.cc
+++ b/metadata.cc
@@ -3,6 +3,7 @@
 #include "btree_checker.h"
 #include "core_map.h"
 #include "math_utils.h"
+#include "space_map_disk.h"
 
 #include <stdexcept>
 #include <sstream>
@@ -396,26 +397,37 @@ metadata::device_exists(thin_dev_t dev) const
 }
 
 namespace {
-	optional<error_set::ptr>
-	check_ref_counts(string const &desc, block_counter const &counts,
-			 space_map::ptr sm) {
-		error_set::ptr errors(new error_set(desc));
+	struct check_count : public space_map::iterator {
+		check_count(string const &desc, block_counter const &expected)
+			: bad_(false),
+			  expected_(expected),
+			  errors_(new error_set(desc)) {
+		}
 
-		bool bad = false;
-		for (block_address b = 0; b < sm->get_nr_blocks(); b++) {
-			uint32_t actual = sm->get_count(b);
-			uint32_t expected = counts.get_count(b);
+		virtual void operator() (block_address b, ref_t actual) {
+			ref_t expected = expected_.get_count(b);
 
 			if (actual != expected) {
 				ostringstream out;
 				out << b << ": was " << actual
 				    << ", expected " << expected;
-				errors->add_child(out.str());
-				bad = true;
+				errors_->add_child(out.str());
+				bad_ = true;
 			}
 		}
 
-		return bad ? optional<error_set::ptr>(errors) : optional<error_set::ptr>();
+		bool bad_;
+		block_counter const &expected_;
+		error_set::ptr errors_;
+	};
+
+	optional<error_set::ptr>
+	check_ref_counts(string const &desc, block_counter const &counts,
+			 space_map::ptr sm) {
+
+		check_count checker(desc, counts);
+		sm->iterate(checker);
+		return checker.bad_ ? optional<error_set::ptr>(checker.errors_) : optional<error_set::ptr>();
 	}
 }
 
diff --git a/space_map.h b/space_map.h
index a6735d9..13a4f29 100644
--- a/space_map.h
+++ b/space_map.h
@@ -29,6 +29,16 @@ namespace persistent_data {
 		virtual bool count_possibly_greater_than_one(block_address b) const = 0;
 
 		virtual void extend(block_address extra_blocks) = 0;
+
+		struct iterator {
+			virtual ~iterator() {}
+
+			virtual void operator() (block_address b, ref_t c) = 0;
+		};
+
+		virtual void iterate(iterator &it) const {
+			throw std::runtime_error("not implemented");
+		}
 	};
 
 	class persistent_space_map : public space_map {
diff --git a/space_map_disk.cc b/space_map_disk.cc
index 74cb2b9..26a4e9e 100644
--- a/space_map_disk.cc
+++ b/space_map_disk.cc
@@ -80,6 +80,19 @@ namespace {
 			return ie_;
 		}
 
+		void iterate(block_address offset, block_address hi, space_map::iterator &it) const {
+			read_ref rr = tm_->read_lock(ie_.blocknr_);
+			void const *bits = bitmap_data(rr);
+
+			for (unsigned b = 0; b < hi; b++) {
+				ref_t b1 = test_bit_le(bits, b * 2);
+				ref_t b2 = test_bit_le(bits, b * 2 + 1);
+				ref_t result = b2 ? 1 : 0;
+				result |= b1 ? 0b10 : 0;
+				it(offset + b, result);
+			}
+		}
+
 	private:
 		void check_crc(transaction_manager::read_ref &rr) {
 			crc32c sum(240779);
@@ -248,6 +261,34 @@ namespace {
 			ref_counts_.visit(v);
 		}
 
+		struct look_aside_iterator : public iterator {
+			look_aside_iterator(sm_disk_base const &smd, iterator &it)
+				: smd_(smd),
+				  it_(it) {
+			}
+
+			virtual void operator () (block_address b, ref_t c) {
+				it_(b, c == 3 ? smd_.lookup_ref_count(b) : c);
+			}
+
+			sm_disk_base const &smd_;
+			iterator &it_;
+		};
+
+		friend struct look_aside_iterator;
+
+		virtual void iterate(iterator &it) const {
+			look_aside_iterator wrapper(*this, it);
+			unsigned nr_indexes = div_up<block_address>(nr_blocks_, entries_per_block_);
+
+			for (unsigned i = 0; i < nr_indexes; i++) {
+				unsigned hi = (i == nr_indexes - 1) ? (nr_blocks_ % entries_per_block_) : entries_per_block_;
+				index_entry ie = find_ie(i);
+				bitmap bm(tm_, ie);
+				bm.iterate(i * entries_per_block_, hi, wrapper);
+			}
+		}
+
 	protected:
 		transaction_manager::ptr get_tm() const {
 			return tm_;