diff --git a/Makefile.in b/Makefile.in
index d56c1ca..1d37887 100644
--- a/Makefile.in
+++ b/Makefile.in
@@ -51,6 +51,7 @@ SOURCE=\
 	thin-provisioning/device_tree.cc \
 	thin-provisioning/file_utils.cc \
 	thin-provisioning/human_readable_format.cc \
+	thin-provisioning/mapping_tree.cc \
 	thin-provisioning/metadata.cc \
 	thin-provisioning/metadata_checker.cc \
 	thin-provisioning/metadata_disk_structures.cc \
diff --git a/thin-provisioning/device_tree.cc b/thin-provisioning/device_tree.cc
index 5684ab0..fa32969 100644
--- a/thin-provisioning/device_tree.cc
+++ b/thin-provisioning/device_tree.cc
@@ -1,10 +1,6 @@
 #include "thin-provisioning/device_tree.h"
 
 #include "persistent-data/data-structures/btree_damage_visitor.h"
-#include "persistent-data/transaction_manager.h"
-#include "persistent-data/space-maps/core.h"
-#include "thin-provisioning/metadata.h"
-#include "thin-provisioning/metadata_disk_structures.h"
 
 using namespace persistent_data;
 using namespace thin_provisioning;
@@ -22,12 +18,13 @@ namespace {
 
 	class ll_damage_visitor {
 	public:
+		// FIXME: is the namespace needed on here?
 		ll_damage_visitor(device_tree_detail::damage_visitor &v)
 		: v_(v) {
 		}
 
 		virtual void visit(btree_path const &path, btree_detail::damage const &d) {
-			v_.visit(path, missing_devices(d.desc_, d.lost_keys_));
+			v_.visit(missing_devices(d.desc_, d.lost_keys_));
 		}
 
 	private:
diff --git a/thin-provisioning/device_tree.h b/thin-provisioning/device_tree.h
index c088c3b..1aad86a 100644
--- a/thin-provisioning/device_tree.h
+++ b/thin-provisioning/device_tree.h
@@ -54,7 +54,7 @@ namespace thin_provisioning {
 				d.visit(*this);
 			}
 
-			virtual void visit(btree_path const &path, missing_devices const &d) = 0;
+			virtual void visit(missing_devices const &d) = 0;
 		};
 
 		// FIXME: need to add some more damage types for bad leaf data
diff --git a/thin-provisioning/mapping_tree.cc b/thin-provisioning/mapping_tree.cc
new file mode 100644
index 0000000..d3b20c4
--- /dev/null
+++ b/thin-provisioning/mapping_tree.cc
@@ -0,0 +1,177 @@
+#include "thin-provisioning/mapping_tree.h"
+
+#include "persistent-data/data-structures/btree_damage_visitor.h"
+#include "persistent-data/space_map.h"
+
+using namespace persistent_data;
+
+//----------------------------------------------------------------
+
+namespace thin_provisioning {
+	namespace mapping_tree_detail {
+		space_map_ref_counter::space_map_ref_counter(space_map::ptr sm)
+			: sm_(sm)
+		{
+		}
+
+		void
+		space_map_ref_counter::inc(block_address b)
+		{
+			sm_->inc(b);
+		}
+
+		void
+		space_map_ref_counter::dec(block_address b)
+		{
+			sm_->dec(b);
+		}
+
+		//--------------------------------
+
+		block_time_ref_counter::block_time_ref_counter(space_map::ptr sm)
+			: sm_(sm)
+		{
+		}
+
+		void
+		block_time_ref_counter::inc(block_time bt)
+		{
+			sm_->inc(bt.block_);
+		}
+
+		void
+		block_time_ref_counter::dec(block_time bt)
+		{
+			sm_->dec(bt.block_);
+		}
+
+		//--------------------------------
+
+		void
+		block_traits::unpack(disk_type const &disk, value_type &value)
+		{
+			uint64_t v = to_cpu<uint64_t>(disk);
+			value.block_ = v >> 24;
+			value.time_ = v & ((1 << 24) - 1);
+		}
+
+		void
+		block_traits::pack(value_type const &value, disk_type &disk)
+		{
+			uint64_t v = (value.block_ << 24) | value.time_;
+			disk = base::to_disk<base::le64>(v);
+		}
+
+		//--------------------------------
+
+		mtree_ref_counter::mtree_ref_counter(transaction_manager::ptr tm)
+			: tm_(tm)
+		{
+		}
+
+		void
+		mtree_ref_counter::inc(block_address b)
+		{
+		}
+
+		void
+		mtree_ref_counter::dec(block_address b)
+		{
+		}
+
+		//--------------------------------
+
+		void
+		mtree_traits::unpack(disk_type const &disk, value_type &value)
+		{
+			value = base::to_cpu<uint64_t>(disk);
+		}
+
+		void
+		mtree_traits::pack(value_type const &value, disk_type &disk)
+		{
+			disk = base::to_disk<base::le64>(value);
+		}
+
+		//--------------------------------
+
+		missing_devices::missing_devices(std::string const &desc, range<uint64_t> const &keys)
+		: desc_(desc),
+		  keys_(keys)
+		{
+		}
+
+		void
+		missing_devices::visit(damage_visitor &v) const
+		{
+			v.visit(*this);
+		}
+
+		//--------------------------------
+
+		missing_mappings::missing_mappings(std::string const &desc, uint64_t thin_dev,
+						   range<uint64_t> const &keys)
+			: desc_(desc),
+			thin_dev_(thin_dev),
+			keys_(keys)
+		{
+		}
+
+		void
+		missing_mappings::visit(damage_visitor &v) const
+		{
+			v.visit(*this);
+		}
+	}
+}
+
+//----------------------------------------------------------------
+
+namespace {
+	using namespace thin_provisioning;
+	using namespace mapping_tree_detail;
+
+	struct leaf_visitor {
+		virtual void visit(btree_path const &, block_time const &) {
+		}
+	};
+
+	class ll_damage_visitor {
+	public:
+		ll_damage_visitor(damage_visitor &v)
+		: v_(v) {
+		}
+
+		virtual void visit(btree_path const &path, btree_detail::damage const &d) {
+			switch (path.size()) {
+			case 0:
+				v_.visit(missing_devices(d.desc_, d.lost_keys_));
+				break;
+
+			case 1:
+				v_.visit(missing_mappings(d.desc_, path[0], d.lost_keys_));
+				break;
+
+			default:
+				// shouldn't get here.
+				throw std::runtime_error("ll_damage_visitor: path too long");
+			}
+		}
+
+	private:
+		damage_visitor &v_;
+	};
+}
+
+void
+thin_provisioning::check_mapping_tree(mapping_tree const &tree,
+				      mapping_tree_detail::damage_visitor &visitor)
+{
+	block_counter counter; //FIXME: get rid of this counter arg
+	leaf_visitor vv;
+	ll_damage_visitor dv(visitor);
+
+	btree_visit_values(tree, counter, vv, dv);
+}
+
+//----------------------------------------------------------------
diff --git a/thin-provisioning/mapping_tree.h b/thin-provisioning/mapping_tree.h
index 0464a53..0b23389 100644
--- a/thin-provisioning/mapping_tree.h
+++ b/thin-provisioning/mapping_tree.h
@@ -10,17 +10,10 @@ namespace thin_provisioning {
 	namespace mapping_tree_detail {
 		class space_map_ref_counter {
 		public:
-			space_map_ref_counter(space_map::ptr sm)
-				: sm_(sm) {
-			}
+			space_map_ref_counter(space_map::ptr sm);
 
-			void inc(block_address b) {
-				sm_->inc(b);
-			}
-
-			void dec(block_address b) {
-				sm_->dec(b);
-			}
+			void inc(block_address b);
+			void dec(block_address b);
 
 		private:
 			space_map::ptr sm_;
@@ -33,17 +26,9 @@ namespace thin_provisioning {
 
 		class block_time_ref_counter {
 		public:
-			block_time_ref_counter(space_map::ptr sm)
-				: sm_(sm) {
-			}
-
-			void inc(block_time bt) {
-				sm_->inc(bt.block_);
-			}
-
-			void dec(block_time bt) {
-				sm_->dec(bt.block_);
-			}
+			block_time_ref_counter(space_map::ptr sm);
+			void inc(block_time bt);
+			void dec(block_time bt);
 
 		private:
 			space_map::ptr sm_;
@@ -54,29 +39,16 @@ namespace thin_provisioning {
 			typedef block_time value_type;
 			typedef block_time_ref_counter ref_counter;
 
-			static void unpack(disk_type const &disk, value_type &value) {
-				uint64_t v = to_cpu<uint64_t>(disk);
-				value.block_ = v >> 24;
-				value.time_ = v & ((1 << 24) - 1);
-			}
-
-			static void pack(value_type const &value, disk_type &disk) {
-				uint64_t v = (value.block_ << 24) | value.time_;
-				disk = base::to_disk<base::le64>(v);
-			}
+			static void unpack(disk_type const &disk, value_type &value);
+			static void pack(value_type const &value, disk_type &disk);
 		};
 
 		class mtree_ref_counter {
 		public:
-			mtree_ref_counter(transaction_manager::ptr tm)
-				: tm_(tm) {
-			}
+			mtree_ref_counter(transaction_manager::ptr tm);
 
-			void inc(block_address b) {
-			}
-
-			void dec(block_address b) {
-			}
+			void inc(block_address b);
+			void dec(block_address b);
 
 		private:
 			transaction_manager::ptr tm_;
@@ -87,20 +59,58 @@ namespace thin_provisioning {
 			typedef uint64_t value_type;
 			typedef mtree_ref_counter ref_counter;
 
-			static void unpack(disk_type const &disk, value_type &value) {
-				value = base::to_cpu<uint64_t>(disk);
+			static void unpack(disk_type const &disk, value_type &value);
+			static void pack(value_type const &value, disk_type &disk);
+		};
+
+		//--------------------------------
+
+		class damage_visitor;
+
+		struct damage {
+			virtual ~damage() {}
+			virtual void visit(damage_visitor &v) const = 0;
+		};
+
+		struct missing_devices : public damage {
+			missing_devices(std::string const &desc, range<uint64_t> const &keys);
+			virtual void visit(damage_visitor &v) const;
+
+			std::string desc_;
+			range<uint64_t> keys_;
+		};
+
+		struct missing_mappings : public damage {
+			missing_mappings(std::string const &desc, uint64_t thin_dev,
+					 range<uint64_t> const &keys);
+			virtual void visit(damage_visitor &v) const;
+
+			std::string desc_;
+			uint64_t thin_dev_;
+			range<uint64_t> keys_;
+		};
+
+		class damage_visitor {
+		public:
+			virtual ~damage_visitor() {}
+
+			void visit(damage const &d) {
+				d.visit(*this);
 			}
 
-			static void pack(value_type const &value, disk_type &disk) {
-				disk = base::to_disk<base::le64>(value);
-			}
+			virtual void visit(missing_devices const &d) = 0;
+			virtual void visit(missing_mappings const &d) = 0;
 		};
+
 	}
 
 	typedef persistent_data::btree<2, mapping_tree_detail::block_traits> mapping_tree;
 	typedef persistent_data::btree<1, mapping_tree_detail::mtree_traits> dev_tree;
 	typedef persistent_data::btree<1, mapping_tree_detail::block_traits> single_mapping_tree;
-};
+
+	void check_mapping_tree(mapping_tree const &tree,
+				mapping_tree_detail::damage_visitor &visitor);
+}
 
 //----------------------------------------------------------------