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_checker.h b/persistent-data/data-structures/btree_checker.h
deleted file mode 100644
index d78505f..0000000
--- a/persistent-data/data-structures/btree_checker.h
+++ /dev/null
@@ -1,277 +0,0 @@
-// Copyright (C) 2011 Red Hat, Inc. All rights reserved.
-//
-// This file is part of the thin-provisioning-tools source.
-//
-// thin-provisioning-tools is free software: you can redistribute it
-// and/or modify it under the terms of the GNU General Public License
-// as published by the Free Software Foundation, either version 3 of
-// the License, or (at your option) any later version.
-//
-// thin-provisioning-tools is distributed in the hope that it will be
-// useful, but WITHOUT ANY WARRANTY; without even the implied warranty
-// of MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
-// GNU General Public License for more details.
-//
-// You should have received a copy of the GNU General Public License along
-// with thin-provisioning-tools.  If not, see
-// <http://www.gnu.org/licenses/>.
-
-#ifndef BTREE_CHECKER_H
-#define BTREE_CHECKER_H
-
-#include "btree.h"
-
-#include "persistent-data/block_counter.h"
-#include "persistent-data/checksum.h"
-#include "persistent-data/error_set.h"
-
-#include <sstream>
-#include <map>
-#include <set>
-
-using namespace persistent_data;
-using namespace std;
-
-//----------------------------------------------------------------
-
-namespace persistent_data {
-	//----------------------------------------------------------------
-	// This class implements consistency checking for the btrees in
-	// general.  Derive from this if you want some additional checks.
-	// It's worth summarising what is checked:
-	//
-	// Implemented
-	// -----------
-	//
-	// - block_nr
-	// - nr_entries < max_entries
-	// - max_entries fits in block
-	// - max_entries is divisible by 3
-	// - nr_entries > minimum (except for root nodes)
-	//
-	// Not implemented
-	// ---------------
-	//
-	// - leaf | internal flags (this can be inferred from siblings)
-	//----------------------------------------------------------------
-	template <uint32_t Levels, typename ValueTraits>
-	class btree_checker : public btree<Levels, ValueTraits>::visitor {
-	public:
-		typedef btree_detail::node_location node_location;
-
-		btree_checker(block_counter &counter, bool avoid_repeated_visits = true)
-			: counter_(counter),
-			  errs_(new error_set("btree errors")),
-			  avoid_repeated_visits_(avoid_repeated_visits) {
-		}
-
-		bool visit_internal(node_location const &loc,
-				    btree_detail::node_ref<block_traits> const &n) {
-			return check_internal(loc, n);
-		}
-
-		bool visit_internal_leaf(node_location const &loc,
-					 btree_detail::node_ref<block_traits> const &n) {
-			return check_leaf(loc, n);
-		}
-
-		bool visit_leaf(node_location const &loc,
-				btree_detail::node_ref<ValueTraits> const &n) {
-			return check_leaf(loc, n);
-		}
-
-		error_set::ptr get_errors() const {
-			return errs_;
-		}
-
-	protected:
-		block_counter &get_counter() {
-			return counter_;
-		}
-
-	private:
-		bool check_internal(node_location const &loc,
-				    btree_detail::node_ref<block_traits> const &n) {
-			if (!already_visited(n) &&
-			    check_block_nr(n) &&
-			    check_max_entries(n) &&
-			    check_nr_entries(n, loc.is_sub_root()) &&
-			    check_ordered_keys(n) &&
-			    check_parent_key(loc.is_sub_root() ? boost::optional<uint64_t>() : loc.key, n)) {
-				if (loc.is_sub_root())
-					new_root(loc.level());
-
-				return true;
-			}
-
-			return false;
-		}
-
-		template <typename ValueTraits2>
-		bool check_leaf(node_location const &loc,
-				btree_detail::node_ref<ValueTraits2> const &n) {
-			if (!already_visited(n) &&
-			    check_block_nr(n) &&
-			    check_max_entries(n) &&
-			    check_nr_entries(n, loc.is_sub_root()) &&
-			    check_ordered_keys(n) &&
-			    check_parent_key(loc.is_sub_root() ? boost::optional<uint64_t>() : loc.key, n)) {
-				if (loc.is_sub_root())
-					new_root(loc.level());
-
-				return check_leaf_key(loc.level(), n);
-			}
-
-			return false;
-		}
-
-
-		template <typename node>
-		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;
-
-				seen_.insert(b);
-			}
-
-			return false;
-		}
-
-		template <typename node>
-		bool check_block_nr(node const &n) const {
-			if (n.get_location() != n.get_block_nr()) {
-				std::ostringstream out;
-				out << "block number mismatch: actually "
-				    << n.get_location()
-				    << ", claims " << n.get_block_nr();
-				errs_->add_child(out.str());
-				return false;
-			}
-
-			return true;
-		}
-
-		template <typename node>
-		bool check_max_entries(node const &n) const {
-			size_t elt_size = sizeof(uint64_t) + n.get_value_size();
-			if (elt_size * n.get_max_entries() + sizeof(node_header) > MD_BLOCK_SIZE) {
-				std::ostringstream out;
-				out << "max entries too large: " << n.get_max_entries();
-				errs_->add_child(out.str());
-				return false;
-			}
-
-			if (n.get_max_entries() % 3) {
-				std::ostringstream out;
-				out << "max entries is not divisible by 3: " << n.get_max_entries();
-				errs_->add_child(out.str());
-				return false;
-			}
-
-			return true;
-		}
-
-		template <typename node>
-		bool check_nr_entries(node const &n, bool is_root) const {
-			if (n.get_nr_entries() > n.get_max_entries()) {
-				std::ostringstream out;
-				out << "bad nr_entries: "
-				    << n.get_nr_entries() << " < "
-				    << n.get_max_entries();
-				errs_->add_child(out.str());
-				return false;
-			}
-
-			block_address min = n.get_max_entries() / 3;
-			if (!is_root && (n.get_nr_entries() < min)) {
-				ostringstream out;
-				out << "too few entries in btree: "
-				    << n.get_nr_entries()
-				    << ", expected at least "
-				    << min
-				    << "(max_entries = " << n.get_max_entries() << ")";
-				errs_->add_child(out.str());
-				return false;
-			}
-
-			return true;
-		}
-
-		template <typename node>
-		bool check_ordered_keys(node const &n) const {
-			unsigned nr_entries = n.get_nr_entries();
-
-			if (nr_entries == 0)
-				return true; // can only happen if a root node
-
-			uint64_t last_key = n.key_at(0);
-
-			for (unsigned i = 1; i < nr_entries; i++) {
-				uint64_t k = n.key_at(i);
-				if (k <= last_key) {
-					ostringstream out;
-					out << "keys are out of order, " << k << " <= " << last_key;
-					errs_->add_child(out.str());
-					return false;
-				}
-				last_key = k;
-			}
-
-			return true;
-		}
-
-		template <typename node>
-		bool check_parent_key(boost::optional<uint64_t> key, node const &n) const {
-			if (!key)
-				return true;
-
-			if (*key > n.key_at(0)) {
-				ostringstream out;
-				out << "parent key mismatch: parent was " << *key
-				    << ", but lowest in node was " << n.key_at(0);
-				errs_->add_child(out.str());
-				return false;
-			}
-
-			return true;
-		}
-
-		template <typename node>
-		bool check_leaf_key(unsigned level, node const &n) {
-			if (n.get_nr_entries() == 0)
-				return true; // can only happen if a root node
-
-			if (last_leaf_key_[level] && *last_leaf_key_[level] >= n.key_at(0)) {
-				ostringstream out;
-				out << "the last key of the previous leaf was " << *last_leaf_key_[level]
-				    << " and the first key of this leaf is " << n.key_at(0);
-				errs_->add_child(out.str());
-				return false;
-			}
-
-			last_leaf_key_[level] = n.key_at(n.get_nr_entries() - 1);
-			return true;
-		}
-
-     	        void new_root(unsigned level) {
-			// we're starting a new subtree, so should
-			// reset the last_leaf value.
-			last_leaf_key_[level] = boost::optional<uint64_t>();
-	        }
-
-		block_counter &counter_;
-		std::set<block_address> seen_;
-		error_set::ptr errs_;
-		boost::optional<uint64_t> last_leaf_key_[Levels];
-		bool avoid_repeated_visits_;
-	};
-}
-
-//----------------------------------------------------------------
-
-#endif
diff --git a/persistent-data/data-structures/btree_counter.h b/persistent-data/data-structures/btree_counter.h
new file mode 100644
index 0000000..2f00173
--- /dev/null
+++ b/persistent-data/data-structures/btree_counter.h
@@ -0,0 +1,81 @@
+#ifndef PERSISTENT_DATA_DATA_STRUCTURES_BTREE_COUNTER_H
+#define PERSISTENT_DATA_DATA_STRUCTURES_BTREE_COUNTER_H
+
+#include "persistent-data/block_counter.h"
+
+//----------------------------------------------------------------
+
+namespace persistent_data {
+	namespace btree_count_detail {
+		template <unsigned Levels, typename ValueTraits, typename ValueCounter>
+		class counting_visitor : public btree<Levels, ValueTraits>::visitor {
+		public:
+			typedef btree<Levels, ValueTraits> tree;
+
+			counting_visitor(block_counter &bc, ValueCounter &vc)
+				: bc_(bc),
+				  vc_(vc) {
+			}
+
+			virtual bool visit_internal(node_location const &l,
+						    typename tree::internal_node const &n) {
+				return visit_node(n);
+			}
+
+			virtual bool visit_internal_leaf(node_location const &l,
+							 typename tree::internal_node const &n) {
+				return visit_node(n);
+			}
+
+			virtual bool visit_leaf(node_location const &l,
+						typename tree::leaf_node const &n) {
+				if (visit_node(n)) {
+					unsigned nr = n.get_nr_entries();
+
+					for (unsigned i = 0; i < nr; i++) {
+						// FIXME: confirm l2 is correct
+						node_location l2(l);
+						l2.push_key(i);
+						vc_.visit(l2, n.value_at(i));
+					}
+
+					return true;
+				}
+
+				return false;
+			}
+
+		private:
+			template <typename Node>
+			bool visit_node(Node const &n) {
+				block_address b = n.get_location();
+				bool seen = bc_.get_count(b);
+				bc_.inc(b);
+				return !seen;
+			}
+
+			block_counter &bc_;
+			ValueCounter &vc_;
+		};
+	}
+
+	template <typename T>
+	struct noop_value_counter {
+		void visit(btree_detail::node_location const &loc, T const &v) {
+		}
+	};
+
+	// Counts how many times each metadata block is referenced in the
+	// tree.  Blocks already referenced in the block counter are not
+	// walked.  This walk should only be done once you're sure the tree
+	// is not corrupt.
+	template <unsigned Levels, typename ValueTraits, typename ValueCounter>
+	void count_btree_blocks(btree<Levels, ValueTraits> const &tree, block_counter &bc, ValueCounter &vc) {
+		btree_count_detail::counting_visitor<Levels, ValueTraits, ValueCounter> v(bc, vc);
+		tree.visit_depth_first(v);
+	}
+}
+
+//----------------------------------------------------------------
+
+#endif
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/Makefile.in b/unit-tests/Makefile.in
index db790e6..d2506ec 100644
--- a/unit-tests/Makefile.in
+++ b/unit-tests/Makefile.in
@@ -51,6 +51,7 @@ TEST_SOURCE=\
 	unit-tests/bitset_t.cc \
 	unit-tests/block_t.cc \
 	unit-tests/btree_t.cc \
+	unit-tests/btree_counter_t.cc \
 	unit-tests/btree_damage_visitor_t.cc \
 	unit-tests/buffer_t.cc \
 	unit-tests/cache_t.cc \
diff --git a/unit-tests/btree_counter_t.cc b/unit-tests/btree_counter_t.cc
new file mode 100644
index 0000000..5f4a7d0
--- /dev/null
+++ b/unit-tests/btree_counter_t.cc
@@ -0,0 +1,84 @@
+#include "gmock/gmock.h"
+
+#include "test_utils.h"
+
+#include "persistent-data/data-structures/btree.h"
+#include "persistent-data/data-structures/btree_counter.h"
+#include "persistent-data/space-maps/core.h"
+
+using namespace base;
+using namespace std;
+using namespace persistent_data;
+using namespace test;
+using namespace testing;
+
+//----------------------------------------------------------------
+
+namespace {
+	block_address const BLOCK_SIZE = 4096;
+	block_address const NR_BLOCKS = 102400;
+	block_address const SUPERBLOCK = 0;
+
+	class BTreeCounterTests : public Test {
+	public:
+		BTreeCounterTests()
+			: bm_(create_bm<BLOCK_SIZE>(NR_BLOCKS)),
+			  sm_(setup_core_map()),
+			  tm_(new transaction_manager(bm_, sm_)) {
+		}
+
+		void check_nr_metadata_blocks_is_ge(unsigned n) {
+			block_counter bc;
+			noop_value_counter<uint64_t> vc;
+			count_btree_blocks(*tree_, bc, vc);
+			ASSERT_THAT(bc.get_counts().size(), Ge(n));
+		}
+
+		with_temp_directory dir_;
+		block_manager<>::ptr bm_;
+		space_map::ptr sm_;
+		transaction_manager::ptr tm_;
+		uint64_traits::ref_counter rc_;
+
+		btree<1, uint64_traits>::ptr tree_;
+
+	private:
+		space_map::ptr setup_core_map() {
+			space_map::ptr sm(new core_map(NR_BLOCKS));
+			sm->inc(SUPERBLOCK);
+			return sm;
+		}
+
+		void commit() {
+			block_manager<>::write_ref superblock(bm_->superblock(SUPERBLOCK));
+		}
+	};
+
+	void dump_counts(block_counter const &bc) {
+		block_counter::count_map::const_iterator it, end = bc.get_counts().end();
+		for (it = bc.get_counts().begin(); it != end; ++it)
+			cout << it->first << " -> " << it->second << endl;
+	}
+}
+
+//----------------------------------------------------------------
+
+TEST_F(BTreeCounterTests, count_empty_tree)
+{
+	tree_.reset(new btree<1, uint64_traits>(tm_, rc_));
+	check_nr_metadata_blocks_is_ge(1);
+}
+
+TEST_F(BTreeCounterTests, count_populated_tree)
+{
+	tree_.reset(new btree<1, uint64_traits>(tm_, rc_));
+
+	for (unsigned i = 0; i < 10000; i++) {
+		uint64_t key[1] = {i};
+		tree_->insert(key, 0ull);
+	}
+
+	check_nr_metadata_blocks_is_ge(40);
+}
+
+//----------------------------------------------------------------
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_);
 		}
 	};
 }