diff --git a/caching/cache_check.cc b/caching/cache_check.cc
index 419169f..8f6ab7d 100644
--- a/caching/cache_check.cc
+++ b/caching/cache_check.cc
@@ -17,6 +17,7 @@
 #include "caching/metadata.h"
 #include "persistent-data/block.h"
 #include "persistent-data/file_utils.h"
+#include "persistent-data/space_map.h"
 #include "persistent-data/space-maps/core.h"
 #include "version.h"
 
@@ -158,6 +159,15 @@ namespace {
 		using reporter_base::get_error;
 	};
 
+	class space_map_reporter : public space_map_detail::visitor, reporter_base {
+	public:
+		space_map_reporter(nested_output &o)
+		: reporter_base(o) {
+		}
+
+		using reporter_base::get_error;
+	};
+
 	//--------------------------------
 
 	transaction_manager::ptr open_tm(block_manager<>::ptr bm) {
diff --git a/persistent-data/data-structures/array.h b/persistent-data/data-structures/array.h
index 654902d..6c76bf2 100644
--- a/persistent-data/data-structures/array.h
+++ b/persistent-data/data-structures/array.h
@@ -21,6 +21,7 @@
 
 #include "persistent-data/math_utils.h"
 #include "persistent-data/data-structures/btree.h"
+#include "persistent-data/data-structures/btree_counter.h"
 #include "persistent-data/data-structures/btree_damage_visitor.h"
 #include "persistent-data/data-structures/array_block.h"
 
@@ -275,10 +276,9 @@ namespace persistent_data {
 		template <typename ValueVisitor, typename DamageVisitor>
 		void visit_values(ValueVisitor &value_visitor,
 				  DamageVisitor &damage_visitor) const {
-			block_counter counter;
 			block_value_visitor<ValueVisitor> bvisitor(*this, value_visitor);
 			block_damage_visitor<DamageVisitor> dvisitor(damage_visitor, entries_per_block_);
-			btree_visit_values(block_tree_, counter, bvisitor, dvisitor);
+			btree_visit_values(block_tree_, bvisitor, dvisitor);
 
 			// check that all blocks were seen
 			unsigned h = bvisitor.get_highest_seen();
@@ -288,6 +288,26 @@ namespace persistent_data {
 			}
 		}
 
+		//--------------------------------
+
+		struct ablock_counter {
+			ablock_counter(block_counter &bc)
+			: bc_(bc) {
+			}
+
+			void visit(btree_detail::node_location const &loc, block_address b) {
+				bc_.inc(b);
+			}
+
+		private:
+			block_counter &bc_;
+		};
+
+		void count_metadata_blocks(block_counter &bc) const {
+			ablock_counter vc(bc);
+			count_btree_blocks(block_tree_, bc, vc);
+		}
+
 	private:
 
 		struct resizer {
diff --git a/persistent-data/data-structures/btree_damage_visitor.h b/persistent-data/data-structures/btree_damage_visitor.h
index 67addaa..c8eee3b 100644
--- a/persistent-data/data-structures/btree_damage_visitor.h
+++ b/persistent-data/data-structures/btree_damage_visitor.h
@@ -136,11 +136,9 @@ namespace persistent_data {
 			typedef run<block_address> run64;
 			typedef boost::optional<run64> maybe_run64;
 
-			btree_damage_visitor(block_counter &counter,
-					     ValueVisitor &value_visitor,
+			btree_damage_visitor(ValueVisitor &value_visitor,
 					     DamageVisitor &damage_visitor)
-				: counter_(counter),
-				  avoid_repeated_visits_(true),
+				: avoid_repeated_visits_(true),
 				  value_visitor_(value_visitor),
 				  damage_visitor_(damage_visitor) {
 			}
@@ -243,8 +241,6 @@ namespace persistent_data {
 			bool already_visited(node const &n) {
 				block_address b = n.get_location();
 
-				counter_.inc(b);
-
 				if (avoid_repeated_visits_) {
 					if (seen_.count(b) > 0)
 						return true;
@@ -441,7 +437,6 @@ namespace persistent_data {
 
 			//--------------------------------
 
-			block_counter &counter_;
 			bool avoid_repeated_visits_;
 
 			ValueVisitor &value_visitor_;
@@ -458,11 +453,10 @@ namespace persistent_data {
 
 	template <unsigned Levels, typename ValueTraits, typename ValueVisitor, typename DamageVisitor>
 	void btree_visit_values(btree<Levels, ValueTraits> const &tree,
-				block_counter &counter,
 				ValueVisitor &value_visitor,
 				DamageVisitor &damage_visitor) {
 		btree_detail::btree_damage_visitor<ValueVisitor, DamageVisitor, Levels, ValueTraits>
-			v(counter, value_visitor, damage_visitor);
+			v(value_visitor, damage_visitor);
 		tree.visit_depth_first(v);
 	}
 }
diff --git a/persistent-data/space-maps/careful_alloc.cc b/persistent-data/space-maps/careful_alloc.cc
index a0fb180..a25653c 100644
--- a/persistent-data/space-maps/careful_alloc.cc
+++ b/persistent-data/space-maps/careful_alloc.cc
@@ -97,8 +97,8 @@ namespace {
 			return sm_->copy_root(dest, len);
 		}
 
-		virtual void check(block_counter &counter) const {
-			return sm_->check(counter);
+		virtual void visit(space_map_detail::visitor &v) const {
+			return sm_->visit(v);
 		}
 
 		virtual checked_space_map::ptr clone() const {
diff --git a/persistent-data/space-maps/disk.cc b/persistent-data/space-maps/disk.cc
index 36937ec..df3ed43 100644
--- a/persistent-data/space-maps/disk.cc
+++ b/persistent-data/space-maps/disk.cc
@@ -21,7 +21,7 @@
 #include "persistent-data/space-maps/recursive.h"
 #include "persistent-data/space-maps/careful_alloc.h"
 
-#include "persistent-data/data-structures/btree_checker.h"
+#include "persistent-data/data-structures/btree_damage_visitor.h"
 #include "persistent-data/checksum.h"
 #include "persistent-data/endian_utils.h"
 #include "persistent-data/math_utils.h"
@@ -200,6 +200,7 @@ namespace {
 		}
 	};
 
+#if 0
 	class ref_count_checker : public btree_checker<1, ref_count_traits> {
 	public:
 		typedef boost::shared_ptr<ref_count_checker> ptr;
@@ -208,6 +209,14 @@ namespace {
 			: btree_checker<1, ref_count_traits>(counter) {
 		}
 	};
+#endif
+
+	class index_entry_visitor {
+	public:
+		virtual ~index_entry_visitor() {}
+		virtual void visit(index_entry const &ie) = 0;
+		virtual void visit(run<block_address> const &missing) = 0;
+	};
 
 	class index_store {
 	public:
@@ -219,7 +228,7 @@ namespace {
 		virtual void commit_ies() = 0;
 		virtual ptr clone() const = 0;
 		virtual block_address get_root() const = 0;
-		virtual void check(block_counter &counter, block_address nr_index_entries) const = 0;
+		virtual void visit(index_entry_visitor &v, block_address nr_index_entries) const = 0;
 	};
 
 	unsigned const ENTRIES_PER_BLOCK = (MD_BLOCK_SIZE - sizeof(bitmap_header)) * 4;
@@ -358,12 +367,14 @@ namespace {
 			nr_blocks_ = nr_blocks;
 		}
 
-		virtual void check(block_counter &counter) const {
-			ref_count_checker v(counter);
-			ref_counts_.visit_depth_first(v);
+		virtual void visit(space_map_detail::visitor &v) const {
+#if 0
+			ref_count_checker rcv(v);
+			ref_counts_.visit_depth_first(rcv);
 
 			block_address nr_entries = div_up<block_address>(get_nr_blocks(), ENTRIES_PER_BLOCK);
-			indexes_->check(counter, nr_entries);
+			indexes_->visit(v, nr_entries);
+#endif
 		}
 
 		struct look_aside_iterator : public iterator {
@@ -498,56 +509,34 @@ namespace {
 		btree<1, ref_count_traits> ref_counts_;
 	};
 
-	class bitmap_tree_validator : public btree_checker<1, index_entry_traits> {
+	//--------------------------------
+
+	class ie_value_visitor {
 	public:
-		typedef btree_detail::node_location node_location;
-		typedef boost::shared_ptr<bitmap_tree_validator> ptr;
-
-		bitmap_tree_validator(block_counter &counter)
-			: btree_checker<1, index_entry_traits>(counter) {
+		ie_value_visitor(index_entry_visitor &v)
+			: v_(v) {
 		}
 
-		bool visit_leaf(node_location const &loc,
-				btree_detail::node_ref<index_entry_traits> const &n) {
-			bool r = btree_checker<1, index_entry_traits>::visit_leaf(loc, n);
-			if (!r)
-				return r;
-
-			for (unsigned i = 0; i < n.get_nr_entries(); i++) {
-				if (seen_indexes_.count(n.key_at(i)) > 0) {
-					ostringstream out;
-					out << "index entry " << i << " is present twice";
-					throw runtime_error(out.str());
-				}
-
-				seen_indexes_.insert(n.key_at(i));
-				btree_checker<1, index_entry_traits>::get_counter().inc(n.value_at(i).blocknr_);
-			}
-
-			return true;
-		}
-
-		void check_all_index_entries_present(block_address nr_entries) {
-			for (block_address i = 0; i < nr_entries; i++) {
-				if (seen_indexes_.count(i) == 0) {
-					ostringstream out;
-					out << "missing index entry " << i;
-					throw runtime_error(out.str());
-				}
-			}
-
-			set<block_address>::const_iterator it;
-			for (it = seen_indexes_.begin(); it != seen_indexes_.end(); ++it) {
-				if (*it >= nr_entries) {
-					ostringstream out;
-					out << "unexpected index entry " << *it;
-					throw runtime_error(out.str());
-				}
-			}
+		virtual void visit(btree_path const &path, sm_disk_detail::index_entry const &ie) {
+			// FIXME: finish
 		}
 
 	private:
-		set<block_address> seen_indexes_;
+		index_entry_visitor &v_;
+	};
+
+	class ie_damage_visitor {
+	public:
+		ie_damage_visitor(index_entry_visitor &v)
+			: v_(v) {
+		}
+
+		virtual void visit(btree_path const &path, btree_detail::damage const &d) {
+			// FIXME: finish
+		}
+
+	private:
+		index_entry_visitor &v_;
 	};
 
 	class btree_index_store : public index_store {
@@ -595,10 +584,10 @@ namespace {
 			return bitmaps_.get_root();
 		}
 
-		virtual void check(block_counter &counter, block_address nr_index_entries) const {
-			bitmap_tree_validator v(counter);
-			bitmaps_.visit_depth_first(v);
-			v.check_all_index_entries_present(nr_index_entries);
+		virtual void visit(index_entry_visitor &v, block_address nr_index_entries) const {
+			ie_value_visitor vv(v);
+			ie_damage_visitor dv(v);
+			btree_visit_values(bitmaps_, vv, dv);
 		}
 
 	private:
@@ -654,12 +643,18 @@ namespace {
 			return bitmap_root_;
 		}
 
-		virtual void check(block_counter &counter, block_address nr_index_entries) const {
+		virtual void visit(index_entry_visitor &vv, block_address nr_index_entries) const {
+			for (unsigned i = 0; i < entries_.size(); i++)
+				if (entries_[i].blocknr_ != 0)
+					vv.visit(entries_[i]);
+
+#if 0
 			counter.inc(bitmap_root_);
 			for (unsigned i = 0; i < entries_.size(); i++)
 				// FIXME: this looks like a hack
 				if (entries_[i].blocknr_ != 0) // superblock
 					counter.inc(entries_[i].blocknr_);
+#endif
 		}
 
 	private:
diff --git a/persistent-data/space-maps/recursive.cc b/persistent-data/space-maps/recursive.cc
index f7939e9..6533c20 100644
--- a/persistent-data/space-maps/recursive.cc
+++ b/persistent-data/space-maps/recursive.cc
@@ -188,10 +188,10 @@ namespace {
 			return sm_->copy_root(dest, len);
 		}
 
-		virtual void check(persistent_data::block_counter &counter) const {
+		virtual void visit(space_map_detail::visitor &v) const {
 			cant_recurse("check");
 			recursing_const_lock lock(*this);
-			return sm_->check(counter);
+			return sm_->visit(v);
 		}
 
 		virtual checked_space_map::ptr clone() const {
diff --git a/persistent-data/space_map.h b/persistent-data/space_map.h
index 8f37b0b..5395c90 100644
--- a/persistent-data/space_map.h
+++ b/persistent-data/space_map.h
@@ -19,8 +19,9 @@
 #ifndef SPACE_MAP_H
 #define SPACE_MAP_H
 
-#include "block.h"
-#include "block_counter.h"
+#include "persistent-data/block.h"
+#include "persistent-data/block_counter.h"
+#include "persistent-data/run.h"
 
 #include <boost/shared_ptr.hpp>
 #include <boost/optional.hpp>
@@ -114,6 +115,28 @@ namespace persistent_data {
 		}
 	};
 
+	//--------------------------------
+
+	namespace space_map_detail {
+		class damage {
+			virtual ~damage() {}
+		};
+
+		class missing_counts : public damage {
+		public:
+			missing_counts(base::run<block_address> &lost);
+		};
+
+		class visitor {
+		public:
+			virtual ~visitor() {}
+			virtual void visit(missing_counts const &mc) = 0;
+			virtual void visit(block_address b, uint32_t count) = 0;
+		};
+	}
+
+	//--------------------------------
+
 	class persistent_space_map : public space_map {
 	public:
 		typedef boost::shared_ptr<persistent_space_map> ptr;
@@ -126,8 +149,8 @@ namespace persistent_data {
 	public:
 		typedef boost::shared_ptr<checked_space_map> ptr;
 
-		virtual void check(block_counter &counter) const {
-			throw std::runtime_error("'check' not implemented");
+		virtual void visit(space_map_detail::visitor &v) const {
+			throw std::runtime_error("space_map.visit not implemented");
 		}
 
 		// FIXME: should this be in the base space_map class?
diff --git a/thin-provisioning/device_tree.cc b/thin-provisioning/device_tree.cc
index ef5d097..223d816 100644
--- a/thin-provisioning/device_tree.cc
+++ b/thin-provisioning/device_tree.cc
@@ -84,10 +84,9 @@ thin_provisioning::walk_device_tree(device_tree const &tree,
 				    device_tree_detail::device_visitor &vv,
 				    device_tree_detail::damage_visitor &dv)
 {
-	block_counter counter;
 	visitor_adapter av(vv);
 	ll_damage_visitor ll_dv(dv);
-	btree_visit_values(tree, counter, av, ll_dv);
+	btree_visit_values(tree, av, ll_dv);
 }
 
 void
diff --git a/thin-provisioning/mapping_tree.cc b/thin-provisioning/mapping_tree.cc
index 007d1dd..ea3292f 100644
--- a/thin-provisioning/mapping_tree.cc
+++ b/thin-provisioning/mapping_tree.cc
@@ -173,9 +173,8 @@ thin_provisioning::walk_mapping_tree(dev_tree const &tree,
 				     mapping_tree_detail::device_visitor &dev_v,
 				     mapping_tree_detail::damage_visitor &dv)
 {
-	block_counter counter;
 	ll_damage_visitor ll_dv(dv);
-	btree_visit_values(tree, counter, dev_v, ll_dv);
+	btree_visit_values(tree, dev_v, ll_dv);
 }
 
 void
@@ -191,9 +190,8 @@ thin_provisioning::walk_mapping_tree(mapping_tree const &tree,
 				     mapping_tree_detail::mapping_visitor &mv,
 				     mapping_tree_detail::damage_visitor &dv)
 {
-	block_counter counter;
 	ll_damage_visitor ll_dv(dv);
-	btree_visit_values(tree, counter, mv, ll_dv);
+	btree_visit_values(tree, mv, ll_dv);
 }
 
 void
@@ -209,9 +207,8 @@ thin_provisioning::walk_mapping_tree(single_mapping_tree const &tree,
 				     mapping_tree_detail::mapping_visitor &mv,
 				     mapping_tree_detail::damage_visitor &dv)
 {
-	block_counter counter;
 	ll_damage_visitor ll_dv(dv);
-	btree_visit_values(tree, counter, mv, ll_dv);
+	btree_visit_values(tree, mv, ll_dv);
 }
 
 void
diff --git a/thin-provisioning/thin_pool.cc b/thin-provisioning/thin_pool.cc
index cb2655e..23725d4 100644
--- a/thin-provisioning/thin_pool.cc
+++ b/thin-provisioning/thin_pool.cc
@@ -18,8 +18,6 @@
 
 #include "thin-provisioning/thin_pool.h"
 
-#include "persistent-data/data-structures/btree_checker.h"
-
 #include <stdexcept>
 #include <sstream>
 #include <iostream>
diff --git a/thin-provisioning/thin_rmap.cc b/thin-provisioning/thin_rmap.cc
index bfbd67f..9211459 100644
--- a/thin-provisioning/thin_rmap.cc
+++ b/thin-provisioning/thin_rmap.cc
@@ -63,7 +63,6 @@ namespace {
 	}
 
 	int rmap(string const &path, vector<region> const &regions) {
-		block_counter counter; // FIXME: get rid of this counter arg
 		damage_visitor dv;
 		rmap_visitor rv;
 
@@ -79,7 +78,7 @@ namespace {
 			mapping_tree mtree(tm, sb.data_mapping_root_,
 					   mapping_tree_detail::block_traits::ref_counter(tm->get_sm()));
 
-			btree_visit_values(mtree, counter, rv, dv);
+			btree_visit_values(mtree, rv, dv);
 			rv.complete();
 			display_rmap(cout, rv.get_rmap());
 
diff --git a/unit-tests/btree_damage_visitor_t.cc b/unit-tests/btree_damage_visitor_t.cc
index 175f0a7..d6eb9c4 100644
--- a/unit-tests/btree_damage_visitor_t.cc
+++ b/unit-tests/btree_damage_visitor_t.cc
@@ -388,8 +388,7 @@ namespace {
 		}
 
 		virtual void run_() {
-			block_counter counter;
-			btree_visit_values(*tree_, counter, value_visitor_, damage_visitor_);
+			btree_visit_values(*tree_, value_visitor_, damage_visitor_);
 		}
 	};
 
@@ -470,8 +469,7 @@ namespace {
 		}
 
 		virtual void run_() {
-			block_counter counter;
-			btree_visit_values(*tree_, counter, value_visitor_, damage_visitor_);
+			btree_visit_values(*tree_, value_visitor_, damage_visitor_);
 		}
 	};
 }