diff --git a/Makefile b/Makefile
index 8daa325..13a69dd 100644
--- a/Makefile
+++ b/Makefile
@@ -38,7 +38,7 @@ test-programs: $(TEST_PROGRAMS)
 .SUFFIXES: .cc .o .d
 
 %.d: %.cc
-	g++ -MM -MT $(subst .cc,.o,$<) $(CPPFLAGS) $< > $@.$$$$;                  \
+	g++ -MM -MT $(subst .cc,.o,$<) $(CPPFLAGS) $< > $@.$$$$; \
 	sed 's,\([^ :]*\)\.o[ :]*,\1.o $@ : ,g' < $@.$$$$ > $@; \
 	rm -f $@.$$$$
 
diff --git a/space_map_disk.cc b/space_map_disk.cc
index 6cb92d7..3a947b6 100644
--- a/space_map_disk.cc
+++ b/space_map_disk.cc
@@ -197,24 +197,41 @@ namespace {
 		}
 	};
 
+	class index_store {
+	public:
+		typedef boost::shared_ptr<index_store> ptr;
+
+		virtual void resize(block_address nr_indexes) = 0;
+		virtual index_entry find_ie(block_address b) const = 0;
+		virtual void save_ie(block_address b, struct index_entry ie) = 0;
+		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;
+	};
+
+	unsigned const ENTRIES_PER_BLOCK = (MD_BLOCK_SIZE - sizeof(bitmap_header)) * 4;
+
 	class sm_disk_base : public checked_space_map {
 	public:
 		typedef boost::shared_ptr<sm_disk_base> ptr;
 		typedef transaction_manager::read_ref read_ref;
 		typedef transaction_manager::write_ref write_ref;
 
-		sm_disk_base(transaction_manager::ptr tm)
+		sm_disk_base(index_store::ptr indexes,
+			     transaction_manager::ptr tm)
 			: tm_(tm),
-			  entries_per_block_((MD_BLOCK_SIZE - sizeof(bitmap_header)) * 4),
+			  indexes_(indexes),
 			  nr_blocks_(0),
 			  nr_allocated_(0),
 			  ref_counts_(tm_, ref_count_traits::ref_counter()) {
 		}
 
-		sm_disk_base(transaction_manager::ptr tm,
+		sm_disk_base(index_store::ptr indexes,
+			     transaction_manager::ptr tm,
 			     sm_root const &root)
 			: tm_(tm),
-			  entries_per_block_((MD_BLOCK_SIZE - sizeof(bitmap_header)) * 4),
+			  indexes_(indexes),
 			  nr_blocks_(root.nr_blocks_),
 			  nr_allocated_(root.nr_allocated_),
 			  ref_counts_(tm_, root.ref_count_root_, ref_count_traits::ref_counter()) {
@@ -276,18 +293,18 @@ namespace {
 		block_address new_block() {
 			// FIXME: silly to always start searching from the
 			// beginning.
-			block_address nr_indexes = div_up<block_address>(nr_blocks_, entries_per_block_);
+			block_address nr_indexes = div_up<block_address>(nr_blocks_, ENTRIES_PER_BLOCK);
 			for (block_address index = 0; index < nr_indexes; index++) {
 				index_entry ie = find_ie(index);
 
 				bitmap bm(tm_, ie);
 				optional<unsigned> maybe_b = bm.find_free(0, (index == nr_indexes - 1) ?
-							  nr_blocks_ % entries_per_block_ : entries_per_block_);
+							  nr_blocks_ % ENTRIES_PER_BLOCK : ENTRIES_PER_BLOCK);
 				if (maybe_b) {
 					block_address b = *maybe_b;
 					save_ie(index, bm.get_ie());
 					nr_allocated_++;
-					b = (index * entries_per_block_) + b;
+					b = (index * ENTRIES_PER_BLOCK) + b;
 					assert(get_count(b) == 1);
 					return b;
 				}
@@ -303,15 +320,17 @@ namespace {
 		virtual void extend(block_address extra_blocks) {
 			block_address nr_blocks = nr_blocks_ + extra_blocks;
 
-			block_address bitmap_count = div_up<block_address>(nr_blocks, entries_per_block_);
-			block_address old_bitmap_count = div_up<block_address>(nr_blocks_, entries_per_block_);
+			block_address bitmap_count = div_up<block_address>(nr_blocks, ENTRIES_PER_BLOCK);
+			block_address old_bitmap_count = div_up<block_address>(nr_blocks_, ENTRIES_PER_BLOCK);
+
+			indexes_->resize(bitmap_count);
 			for (block_address i = old_bitmap_count; i < bitmap_count; i++) {
 				write_ref wr = tm_->new_block(bitmap_validator());
 
 				index_entry ie;
 				ie.blocknr_ = wr.get_location();
 				ie.nr_free_ = i == (bitmap_count - 1) ?
-					(nr_blocks % entries_per_block_) : entries_per_block_;
+					(nr_blocks % ENTRIES_PER_BLOCK) : ENTRIES_PER_BLOCK;
 				ie.none_free_before_ = 0;
 
 				save_ie(i, ie);
@@ -323,6 +342,9 @@ namespace {
 		virtual void check(block_counter &counter) const {
 			ref_count_checker::ptr v(new ref_count_checker(counter));
 			ref_counts_.visit(v);
+
+			block_address nr_entries = div_up<block_address>(get_nr_blocks(), ENTRIES_PER_BLOCK);
+			indexes_->check(counter, nr_entries);
 		}
 
 		struct look_aside_iterator : public iterator {
@@ -343,16 +365,36 @@ namespace {
 
 		virtual void iterate(iterator &it) const {
 			look_aside_iterator wrapper(*this, it);
-			unsigned nr_indexes = div_up<block_address>(nr_blocks_, entries_per_block_);
+			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_;
+				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);
+				bm.iterate(i * ENTRIES_PER_BLOCK, hi, wrapper);
 			}
 		}
 
+		size_t root_size() {
+			return sizeof(sm_root_disk);
+		}
+
+		void copy_root(void *dest, size_t len) {
+			sm_root_disk d;
+			sm_root v;
+
+			if (len < sizeof(d))
+				throw runtime_error("root too small");
+
+			v.nr_blocks_ = sm_disk_base::get_nr_blocks();
+			v.nr_allocated_ = sm_disk_base::get_nr_allocated();
+			v.bitmap_root_ = get_index_store()->get_root();
+			v.ref_count_root_ = sm_disk_base::get_ref_count_root();
+
+			sm_root_traits::pack(v, d);
+			::memcpy(dest, &d, sizeof(d));
+		}
+
 	protected:
 		transaction_manager::ptr get_tm() const {
 			return tm_;
@@ -366,29 +408,38 @@ namespace {
 			return ref_counts_.get_root();
 		}
 
-		unsigned get_entries_per_block() const {
-			return entries_per_block_;
+		index_store::ptr get_index_store() const {
+			return indexes_;
 		}
 
 	private:
-		virtual index_entry find_ie(block_address b) const = 0;
-		virtual void save_ie(block_address b, struct index_entry ie) = 0;
-		virtual void commit_ies() = 0;
+		// FIXME: remove these, they're just for the transistion
+		index_entry find_ie(block_address b) const {
+			return indexes_->find_ie(b);
+		}
+
+		void save_ie(block_address b, struct index_entry ie) {
+			return indexes_->save_ie(b, ie);
+		}
+
+		void commit_ies() {
+			return indexes_->commit_ies();
+		}
 
 		ref_t lookup_bitmap(block_address b) const {
-			index_entry ie = find_ie(b / entries_per_block_);
+			index_entry ie = find_ie(b / ENTRIES_PER_BLOCK);
 			bitmap bm(tm_, ie);
-			return bm.lookup(b % entries_per_block_);
+			return bm.lookup(b % ENTRIES_PER_BLOCK);
 		}
 
 		void insert_bitmap(block_address b, unsigned n) {
 			if (n > 3)
 				throw runtime_error("bitmap can only hold 2 bit values");
 
-			index_entry ie = find_ie(b / entries_per_block_);
+			index_entry ie = find_ie(b / ENTRIES_PER_BLOCK);
 			bitmap bm(tm_, ie);
-			bm.insert(b % entries_per_block_, n);
-			save_ie(b / entries_per_block_, bm.get_ie());
+			bm.insert(b % ENTRIES_PER_BLOCK, n);
+			save_ie(b / ENTRIES_PER_BLOCK, bm.get_ie());
 		}
 
 		ref_t lookup_ref_count(block_address b) const {
@@ -410,7 +461,7 @@ namespace {
 		}
 
 		transaction_manager::ptr tm_;
-		uint32_t entries_per_block_;
+		index_store::ptr indexes_;
 		block_address nr_blocks_;
 		block_address nr_allocated_;
 
@@ -470,53 +521,26 @@ namespace {
 		set<block_address> seen_indexes_;
 	};
 
-	class sm_disk : public sm_disk_base {
+	class btree_index_store : public index_store {
 	public:
-		typedef boost::shared_ptr<sm_disk> ptr;
+		typedef boost::shared_ptr<btree_index_store> ptr;
 
-		sm_disk(transaction_manager::ptr tm)
-			: sm_disk_base(tm),
-			  bitmaps_(sm_disk_base::get_tm(), index_entry_traits::ref_counter()) {
+		btree_index_store(transaction_manager::ptr tm)
+			: tm_(tm),
+			  bitmaps_(tm, index_entry_traits::ref_counter()) {
 		}
 
-		sm_disk(transaction_manager::ptr tm,
-			sm_root const &root)
-			: sm_disk_base(tm, root),
-			  bitmaps_(sm_disk_base::get_tm(), root.bitmap_root_, index_entry_traits::ref_counter()) {
+		btree_index_store(transaction_manager::ptr tm,
+				  block_address root)
+			: tm_(tm),
+			  bitmaps_(tm, root, index_entry_traits::ref_counter()) {
 		}
 
-		size_t root_size() {
-			return sizeof(sm_root_disk);
+		virtual void resize(block_address nr_entries) {
+			// No op
 		}
 
-		void copy_root(void *dest, size_t len) {
-			sm_root_disk d;
-			sm_root v;
-
-			if (len < sizeof(d))
-				throw runtime_error("root too small");
-
-			v.nr_blocks_ = sm_disk_base::get_nr_blocks();
-			v.nr_allocated_ = sm_disk_base::get_nr_allocated();
-			v.bitmap_root_ = bitmaps_.get_root();
-			v.ref_count_root_ = sm_disk_base::get_ref_count_root();
-
-			sm_root_traits::pack(v, d);
-			::memcpy(dest, &d, sizeof(d));
-		}
-
-		void check(block_counter &counter) const {
-			sm_disk_base::check(counter);
-
-			bitmap_tree_validator::ptr v(new bitmap_tree_validator(counter));
-			bitmaps_.visit(v);
-
-			block_address nr_entries = div_up<block_address>(get_nr_blocks(), get_entries_per_block());
-			v->check_all_index_entries_present(nr_entries);
-		}
-
-	private:
-		index_entry find_ie(block_address ie_index) const {
+		virtual index_entry find_ie(block_address ie_index) const {
 			uint64_t key[1] = {ie_index};
 			optional<index_entry> mindex = bitmaps_.lookup(key);
 			if (!mindex)
@@ -525,90 +549,80 @@ namespace {
 			return *mindex;
 		}
 
-		void save_ie(block_address ie_index, struct index_entry ie) {
+		virtual void save_ie(block_address ie_index, struct index_entry ie) {
 			uint64_t key[1] = {ie_index};
 			bitmaps_.insert(key, ie);
 		}
 
-		void commit_ies() {
+		virtual void commit_ies() {
+			// No op
 		}
 
-		btree<1, index_entry_traits> bitmaps_;
-	};
-
-	class sm_metadata : public sm_disk_base {
-	public:
-		typedef boost::shared_ptr<sm_metadata> ptr;
-
-		sm_metadata(transaction_manager::ptr tm)
-			: sm_disk_base(tm),
-			  entries_(MAX_METADATA_BITMAPS) {
-			block_manager<>::write_ref wr = tm->new_block(index_validator());
-			bitmap_root_ = wr.get_location();
+		virtual index_store::ptr clone() const {
+			return index_store::ptr(new btree_index_store(tm_, bitmaps_.get_root()));
 		}
 
-		sm_metadata(transaction_manager::ptr tm,
-			    sm_root const &root)
-			: sm_disk_base(tm, root),
-			  bitmap_root_(root.bitmap_root_),
-			  entries_(MAX_METADATA_BITMAPS) {
-			load_ies();
+		virtual block_address get_root() const {
+			return bitmaps_.get_root();
 		}
 
-		size_t root_size() {
-			return sizeof(sm_root_disk);
-		}
-
-		// FIXME: common code
-		void copy_root(void *dest, size_t len) {
-			sm_root_disk d;
-			sm_root v;
-
-			if (len < sizeof(d))
-				throw runtime_error("root too small");
-
-			v.nr_blocks_ = sm_disk_base::get_nr_blocks();
-			v.nr_allocated_ = sm_disk_base::get_nr_allocated();
-			v.bitmap_root_ = bitmap_root_;
-			v.ref_count_root_ = sm_disk_base::get_ref_count_root();
-
-			sm_root_traits::pack(v, d);
-			::memcpy(dest, &d, sizeof(d));
-		}
-
-		void check(block_counter &counter) const {
-			sm_disk_base::check(counter);
-
-			counter.inc(bitmap_root_);
-			for (unsigned i = 0; i < entries_.size(); i++)
-				if (entries_[i].blocknr_ != 0) // superblock
-					counter.inc(entries_[i].blocknr_);
+		virtual void check(block_counter &counter, block_address nr_index_entries) const {
+			bitmap_tree_validator::ptr v(new bitmap_tree_validator(counter));
+			bitmaps_.visit(v);
+			v->check_all_index_entries_present(nr_index_entries);
 		}
 
 	private:
-		index_entry find_ie(block_address ie_index) const {
+		transaction_manager::ptr tm_;
+		btree<1, index_entry_traits> bitmaps_;
+	};
+
+	class sm_disk : public sm_disk_base {
+	public:
+		typedef boost::shared_ptr<sm_disk> ptr;
+
+		sm_disk(transaction_manager::ptr tm)
+			: sm_disk_base(index_store::ptr(new btree_index_store(tm)), tm) {
+		}
+
+		sm_disk(transaction_manager::ptr tm,
+			sm_root const &root)
+			: sm_disk_base(index_store::ptr(new btree_index_store(tm, root.bitmap_root_)), tm, root) {
+		}
+	};
+
+	class metadata_index_store : public index_store {
+	public:
+		typedef boost::shared_ptr<metadata_index_store> ptr;
+
+		metadata_index_store(transaction_manager::ptr tm)
+			: tm_(tm) {
+			block_manager<>::write_ref wr = tm_->new_block(index_validator());
+			bitmap_root_ = wr.get_location();
+		}
+
+		metadata_index_store(transaction_manager::ptr tm, block_address root, block_address nr_indexes)
+			: tm_(tm),
+			  bitmap_root_(root) {
+			resize(nr_indexes);
+			load_ies();
+		}
+
+		virtual void resize(block_address nr_indexes) {
+			entries_.resize(nr_indexes);
+		}
+
+		virtual index_entry find_ie(block_address ie_index) const {
 			return entries_[ie_index];
 		}
 
-		void save_ie(block_address ie_index, struct index_entry ie) {
+		virtual void save_ie(block_address ie_index, struct index_entry ie) {
 			entries_[ie_index] = ie;
 		}
 
-		void load_ies() {
-			block_manager<>::read_ref rr =
-				sm_disk_base::get_tm()->read_lock(bitmap_root_, index_validator());
-
-			metadata_index const *mdi = reinterpret_cast<metadata_index const *>(&rr.data());
-
-			unsigned nr_indexes = div_up<block_address>(sm_disk_base::get_nr_blocks(),
-								    sm_disk_base::get_entries_per_block());
-			for (unsigned i = 0; i < nr_indexes; i++)
-				index_entry_traits::unpack(*(mdi->index + i), entries_[i]);
-		}
-
-		void commit_ies() {
+		virtual void commit_ies() {
 			std::pair<block_manager<>::write_ref, bool> p =
-				sm_disk_base::get_tm()->shadow(bitmap_root_, index_validator());
+				tm_->shadow(bitmap_root_, index_validator());
 
 			bitmap_root_ = p.first.get_location();
 			metadata_index *mdi = reinterpret_cast<metadata_index *>(&p.first.data());
@@ -617,9 +631,55 @@ namespace {
 				index_entry_traits::pack(entries_[i], mdi->index[i]);
 		}
 
+		virtual index_store::ptr clone() const {
+			return index_store::ptr(new metadata_index_store(tm_, bitmap_root_, entries_.size()));
+		}
+
+		virtual block_address get_root() const {
+			return bitmap_root_;
+		}
+
+		virtual void check(block_counter &counter, block_address nr_index_entries) const {
+			counter.inc(bitmap_root_);
+			for (unsigned i = 0; i < entries_.size(); i++)
+				if (entries_[i].blocknr_ != 0) // superblock
+					counter.inc(entries_[i].blocknr_);
+		}
+
+	private:
+		void load_ies() {
+			block_manager<>::read_ref rr =
+				tm_->read_lock(bitmap_root_, index_validator());
+
+			metadata_index const *mdi = reinterpret_cast<metadata_index const *>(&rr.data());
+			for (unsigned i = 0; i < entries_.size(); i++)
+				index_entry_traits::unpack(*(mdi->index + i), entries_[i]);
+		}
+
+		transaction_manager::ptr tm_;
 		block_address bitmap_root_;
 		std::vector<index_entry> entries_;
 	};
+
+	class sm_metadata : public sm_disk_base {
+	public:
+		typedef boost::shared_ptr<sm_metadata> ptr;
+
+		sm_metadata(transaction_manager::ptr tm)
+			: sm_disk_base(index_store::ptr(new metadata_index_store(tm)), tm) {
+		}
+
+		sm_metadata(transaction_manager::ptr tm,
+			    sm_root const &root)
+			: sm_disk_base(mk_index_store(tm, root.bitmap_root_, root.nr_blocks_), tm, root) {
+		}
+
+	private:
+		index_store::ptr mk_index_store(transaction_manager::ptr tm, block_address root, block_address nr_blocks) const {
+			block_address nr_indexes = div_up<block_address>(nr_blocks, ENTRIES_PER_BLOCK);
+			return index_store::ptr(new metadata_index_store(tm, root, nr_indexes));
+		}
+	};
 }
 
 //----------------------------------------------------------------
diff --git a/space_map_recursive.cc b/space_map_recursive.cc
index 099f2e5..a68597d 100644
--- a/space_map_recursive.cc
+++ b/space_map_recursive.cc
@@ -36,7 +36,7 @@ namespace {
 	public:
 		sm_recursive(checked_space_map::ptr sm)
 			: sm_(sm),
-			  recursing_(false) {
+			  depth_(0) {
 		}
 
 		virtual block_address get_nr_blocks() const {
@@ -54,7 +54,7 @@ namespace {
 		}
 
 		virtual void set_count(block_address b, ref_t c) {
-			if (recursing_)
+			if (depth_)
 				add_op(block_op(block_op::SET, b, c));
 			else {
 				recursing_lock lock(*this);
@@ -68,7 +68,7 @@ namespace {
 		}
 
 		virtual void inc(block_address b) {
-			if (recursing_)
+			if (depth_)
 				add_op(block_op(block_op::INC, b));
 			else {
 				recursing_lock lock(*this);
@@ -77,7 +77,7 @@ namespace {
 		}
 
 		virtual void dec(block_address b) {
-			if (recursing_)
+			if (depth_)
 				add_op(block_op(block_op::DEC, b));
 			else {
 				recursing_lock lock(*this);
@@ -86,13 +86,15 @@ namespace {
 		}
 
 		virtual block_address new_block() {
-			cant_recurse("new_block");
+			// new_block can recurse, because we know it's
+			// looking up entries in the _previous_
+			// transaction.
 			recursing_lock lock(*this);
 			return sm_->new_block();
 		}
 
 		virtual bool count_possibly_greater_than_one(block_address b) const {
-			if (recursing_)
+			if (depth_)
 				return true;
 
 			else {
@@ -107,6 +109,10 @@ namespace {
 			return sm_->extend(extra_blocks);
 		}
 
+		virtual void iterate(iterator &it) const {
+			sm_->iterate(it);
+		}
+
 		virtual size_t root_size() {
 			cant_recurse("root_size");
 			recursing_const_lock lock(*this);
@@ -119,7 +125,7 @@ namespace {
 			return sm_->copy_root(dest, len);
 		}
 
-		virtual void check(block_counter &counter) const {
+		virtual void check(persistent_data::block_counter &counter) const {
 			cant_recurse("check");
 			recursing_const_lock lock(*this);
 			return sm_->check(counter);
@@ -158,23 +164,19 @@ namespace {
 		}
 
 		void cant_recurse(string const &method) const {
-			if (recursing_)
+			if (depth_)
 				throw runtime_error("recursive '" + method + "' not supported");
 		}
 
-		void set_recursing() const {
-			recursing_ = true;
-		}
-
 		struct recursing_lock {
 			recursing_lock(sm_recursive &smr)
 				: smr_(smr) {
-				smr_.recursing_ = true;
+				smr_.depth_++;
 			}
 
 			~recursing_lock() {
-				smr_.flush_ops();
-				smr_.recursing_ = false;
+				if (!--smr_.depth_)
+					smr_.flush_ops();
 			}
 
 		private:
@@ -184,11 +186,11 @@ namespace {
 		struct recursing_const_lock {
 			recursing_const_lock(sm_recursive const &smr)
 				: smr_(smr) {
-				smr_.recursing_ = true;
+				smr_.depth_++;
 			}
 
 			~recursing_const_lock() {
-				smr_.recursing_ = false;
+				smr_.depth_--;
 			}
 
 		private:
@@ -196,7 +198,7 @@ namespace {
 		};
 
 		checked_space_map::ptr sm_;
-		mutable bool recursing_;
+		mutable int depth_;
 
 		enum op {
 			BOP_INC,