diff --git a/caching/cache_check.cc b/caching/cache_check.cc
index 33d041b..b82b4f7 100644
--- a/caching/cache_check.cc
+++ b/caching/cache_check.cc
@@ -248,7 +248,12 @@ namespace {
 			{
 				nested_output::nest _ = out.push();
 				mapping_array ma(*tm, mapping_array::ref_counter(), sb.mapping_root, sb.cache_blocks);
-				check_mapping_array(ma, mapping_rep);
+				check_mapping_array(ma, mapping_rep, sb.version);
+			}
+
+			if (sb.version >= 2) {
+				persistent_data::bitset dirty(*tm, *sb.dirty_root, sb.cache_blocks);
+				// FIXME: is there no bitset checker?
 			}
 		}
 
@@ -275,6 +280,7 @@ namespace {
 				{
 					nested_output::nest _ = out.push();
 					persistent_data::bitset discards(*tm, sb.discard_root, sb.discard_nr_blocks);
+					// FIXME: is there no bitset checker?
 				}
 			}
 		}
diff --git a/caching/cache_restore.cc b/caching/cache_restore.cc
index 6eb42b4..37be5c7 100644
--- a/caching/cache_restore.cc
+++ b/caching/cache_restore.cc
@@ -43,29 +43,33 @@ namespace {
 	struct flags {
 		flags()
 			: metadata_version(1),
-			  override_metadata_version(false),
 			  clean_shutdown(true),
-			  quiet(false) {
+			  quiet(false),
+			  override_metadata_version(false),
+			  override_version(1) {
 		}
 
 		optional<string> input;
 		optional<string> output;
 
 		uint32_t metadata_version;
-		bool override_metadata_version;
 		bool clean_shutdown;
 		bool quiet;
+
+		bool override_metadata_version;
+		unsigned override_version;
 	};
 
 	int restore(flags const &fs) {
 		try {
 			block_manager<>::ptr bm = open_bm(*fs.output, block_manager<>::READ_WRITE);
 			metadata::ptr md(new metadata(bm, metadata::CREATE));
-			emitter::ptr restorer = create_restore_emitter(md, fs.clean_shutdown);
+			emitter::ptr restorer = create_restore_emitter(md, fs.clean_shutdown,
+								       fs.metadata_version);
 
 			if (fs.override_metadata_version) {
 				cerr << "overriding" << endl;
-				md->sb_.version = fs.metadata_version;
+				md->sb_.version = fs.override_version;
 			}
 
 			check_file_exists(*fs.input);
@@ -95,12 +99,13 @@ cache_restore_cmd::usage(std::ostream &out) const
 {
 	out << "Usage: " << get_name() << " [options]" << endl
 	    << "Options:" << endl
-	    << "  {-h|--help}" << endl
-	    << "  {-i|--input} <input xml file>" << endl
-	    << "  {-o|--output} <output device or file>" << endl
-	    << "  {-q|--quiet}" << endl
-	    << "  {-V|--version}" << endl
-	    << endl
+	    << "  {-h|--help}\n"
+	    << "  {-i|--input} <input xml file>\n"
+	    << "  {-o|--output} <output device or file>\n"
+	    << "  {-q|--quiet}\n"
+	    << "  {--metadata-version} <1 or 2>\n"
+	    << "  {-V|--version}\n"
+	    << "\n"
 	    << "  {--debug-override-metadata-version} <integer>" << endl
 	    << "  {--omit-clean-shutdown}" << endl;
 }
@@ -114,6 +119,7 @@ cache_restore_cmd::run(int argc, char **argv)
 	option const long_opts[] = {
 		{ "debug-override-metadata-version", required_argument, NULL, 0 },
 		{ "omit-clean-shutdown", no_argument, NULL, 1 },
+		{ "metadata-version", required_argument, NULL, 2 },
 		{ "help", no_argument, NULL, 'h'},
 		{ "input", required_argument, NULL, 'i' },
 		{ "output", required_argument, NULL, 'o'},
@@ -125,14 +131,23 @@ cache_restore_cmd::run(int argc, char **argv)
 	while ((c = getopt_long(argc, argv, short_opts, long_opts, NULL)) != -1) {
 		switch(c) {
 		case 0:
-			fs.metadata_version = lexical_cast<uint32_t>(optarg);
 			fs.override_metadata_version = true;
+			fs.override_version = lexical_cast<uint32_t>(optarg);
 			break;
 
 		case 1:
 			fs.clean_shutdown = false;
 			break;
 
+		case 2:
+			fs.metadata_version = lexical_cast<uint32_t>(optarg);
+			if ((fs.metadata_version < MIN_METADATA_VERSION) ||
+			    (fs.metadata_version > MAX_METADATA_VERSION))  {
+				cerr << "Bad metadata version\n\n";
+				return 1;
+			}
+			break;
+
 		case 'h':
 			usage(cout);
 			return 0;
diff --git a/caching/mapping_array.cc b/caching/mapping_array.cc
index c6af4ef..0ecf110 100644
--- a/caching/mapping_array.cc
+++ b/caching/mapping_array.cc
@@ -60,8 +60,9 @@ invalid_mapping::visit(damage_visitor &v) const
 namespace {
 	class check_mapping_visitor : public mapping_visitor {
 	public:
-		check_mapping_visitor(damage_visitor &visitor)
-		: visitor_(visitor) {
+		check_mapping_visitor(damage_visitor &visitor, unsigned metadata_version)
+		: visitor_(visitor),
+		  allowed_flags_(metadata_version == 1 ? (M_VALID | M_DIRTY) : M_VALID) {
 		}
 
 		virtual void visit(block_address cblock, mapping const &m) {
@@ -90,12 +91,13 @@ namespace {
 			seen_oblocks_.insert(m.oblock_);
 		}
 
-		static bool unknown_flags(mapping const &m) {
-			return (m.flags_ & ~(M_VALID | M_DIRTY));
+		bool unknown_flags(mapping const &m) {
+			return m.flags_ & ~allowed_flags_;
 		}
 
 		damage_visitor &visitor_;
 		set<block_address> seen_oblocks_;
+		uint32_t allowed_flags_;
 	};
 
 	class ll_damage_visitor {
@@ -123,9 +125,9 @@ caching::walk_mapping_array(mapping_array const &array,
 }
 
 void
-caching::check_mapping_array(mapping_array const &array, damage_visitor &visitor)
+caching::check_mapping_array(mapping_array const &array, damage_visitor &visitor, unsigned metadata_version)
 {
-	check_mapping_visitor mv(visitor);
+	check_mapping_visitor mv(visitor, metadata_version);
 	walk_mapping_array(array, mv, visitor);
 }
 
diff --git a/caching/mapping_array.h b/caching/mapping_array.h
index 00d58d8..270400c 100644
--- a/caching/mapping_array.h
+++ b/caching/mapping_array.h
@@ -91,7 +91,8 @@ namespace caching {
 				mapping_array_damage::damage_visitor &dv);
 
 	void check_mapping_array(mapping_array const &array,
-				 mapping_array_damage::damage_visitor &visitor);
+				 mapping_array_damage::damage_visitor &visitor,
+				 unsigned metadata_version);
 }
 
 //----------------------------------------------------------------
diff --git a/caching/metadata.cc b/caching/metadata.cc
index 2368a3f..7d8bfae 100644
--- a/caching/metadata.cc
+++ b/caching/metadata.cc
@@ -3,6 +3,7 @@
 #include "persistent-data/space-maps/core.h"
 
 using namespace caching;
+namespace pd = persistent_data;
 
 //----------------------------------------------------------------
 
@@ -30,15 +31,11 @@ namespace {
 
 //----------------------------------------------------------------
 
-metadata::metadata(block_manager<>::ptr bm, open_type ot)
+metadata::metadata(block_manager<>::ptr bm, open_type ot, unsigned metadata_version)
 {
 	switch (ot) {
 	case CREATE:
-		create_metadata(bm);
-		break;
-
-	case OPEN:
-		open_metadata(bm);
+		create_metadata(bm, metadata_version);
 		break;
 
 	default:
@@ -46,6 +43,11 @@ metadata::metadata(block_manager<>::ptr bm, open_type ot)
 	}
 }
 
+metadata::metadata(block_manager<>::ptr bm)
+{
+	open_metadata(bm);
+}
+
 void
 metadata::commit(bool clean_shutdown)
 {
@@ -65,7 +67,7 @@ metadata::setup_hint_array(size_t width)
 }
 
 void
-metadata::create_metadata(block_manager<>::ptr bm)
+metadata::create_metadata(block_manager<>::ptr bm, unsigned metadata_version)
 {
 	tm_ = open_tm(bm);
 
@@ -79,7 +81,10 @@ metadata::create_metadata(block_manager<>::ptr bm)
 	// We can't instantiate the hint array yet, since we don't know the
 	// hint width.
 
-	discard_bits_ = persistent_data::bitset::ptr(new persistent_data::bitset(*tm_));
+	discard_bits_ = pd::bitset::ptr(new pd::bitset(*tm_));
+
+	if (metadata_version >= 2)
+		dirty_bits_ = pd::bitset::ptr(new pd::bitset(*tm_));
 }
 
 void
@@ -100,8 +105,12 @@ metadata::open_metadata(block_manager<>::ptr bm)
 				       sb_.hint_root, sb_.cache_blocks));
 
 	if (sb_.discard_root)
-		discard_bits_ = persistent_data::bitset::ptr(
-			new persistent_data::bitset(*tm_, sb_.discard_root, sb_.discard_nr_blocks));
+		discard_bits_ = pd::bitset::ptr(
+			new pd::bitset(*tm_, sb_.discard_root, sb_.discard_nr_blocks));
+
+	if (sb_.version >= 2)
+		dirty_bits_ = pd::bitset::ptr(
+			new pd::bitset(*tm_, *sb_.dirty_root, sb_.cache_blocks));
 }
 
 void
diff --git a/caching/metadata.h b/caching/metadata.h
index 46c2b93..268b2f5 100644
--- a/caching/metadata.h
+++ b/caching/metadata.h
@@ -27,7 +27,8 @@ namespace caching {
 		typedef block_manager<>::write_ref write_ref;
 		typedef boost::shared_ptr<metadata> ptr;
 
-		metadata(block_manager<>::ptr bm, open_type ot);
+		metadata(block_manager<>::ptr bm, open_type ot, unsigned metadata_version = 2); // Create only
+		metadata(block_manager<>::ptr bm);
 
 		void commit(bool clean_shutdown = true);
 		void setup_hint_array(size_t width);
@@ -40,11 +41,12 @@ namespace caching {
 		mapping_array::ptr mappings_;
 		hint_array::ptr hints_;
 		persistent_data::bitset::ptr discard_bits_;
+		persistent_data::bitset::ptr dirty_bits_;
 
 	private:
 		void init_superblock();
 
-		void create_metadata(block_manager<>::ptr bm);
+		void create_metadata(block_manager<>::ptr bm, unsigned metadata_version);
 		void open_metadata(block_manager<>::ptr bm);
 
 		void commit_space_map();
diff --git a/caching/restore_emitter.cc b/caching/restore_emitter.cc
index 12da592..34c5020 100644
--- a/caching/restore_emitter.cc
+++ b/caching/restore_emitter.cc
@@ -11,10 +11,11 @@ using namespace superblock_damage;
 namespace {
 	class restorer : public emitter {
 	public:
-		restorer(metadata::ptr md, bool clean_shutdown)
+		restorer(metadata::ptr md, bool clean_shutdown, unsigned metadata_version)
 			: in_superblock_(false),
 			  md_(md),
-			  clean_shutdown_(clean_shutdown) {
+			  clean_shutdown_(clean_shutdown),
+			  metadata_version_(metadata_version) {
 		}
 
 		virtual void begin_superblock(std::string const &uuid,
@@ -24,6 +25,7 @@ namespace {
 					      size_t hint_width) {
 
 			superblock &sb = md_->sb_;
+			sb.version = metadata_version_;
 			strncpy((char *) sb.policy_name, policy.c_str(), sizeof(sb.policy_name));
 			memset(sb.policy_version, 0, sizeof(sb.policy_version)); // FIXME: should come from xml
 			sb.policy_hint_size = hint_width;
@@ -37,6 +39,10 @@ namespace {
 			unmapped_value.flags_ = 0;
 			md_->mappings_->grow(nr_cache_blocks, unmapped_value);
 
+			if (metadata_version_ > 1)
+				// make everything dirty by default
+				md_->dirty_bits_->grow(nr_cache_blocks, true);
+
 			vector<unsigned char> hint_value(hint_width, '\0');
 			md_->hints_->grow(nr_cache_blocks, hint_value);
 		}
@@ -60,8 +66,11 @@ namespace {
 			m.oblock_ = oblock;
 			m.flags_ = M_VALID;
 
-			if (dirty)
-				m.flags_ = m.flags_ | M_DIRTY;
+			if (metadata_version_ == 1) {
+				if (dirty)
+					m.flags_ = m.flags_ | M_DIRTY;
+			} else
+				md_->dirty_bits_->set(cblock, dirty);
 
 			md_->mappings_->set(cblock, m);
 		}
@@ -98,15 +107,17 @@ namespace {
 		bool in_superblock_;
 		metadata::ptr md_;
 		bool clean_shutdown_;
+		unsigned metadata_version_;
 	};
 }
 
 //----------------------------------------------------------------
 
 emitter::ptr
-caching::create_restore_emitter(metadata::ptr md, bool clean_shutdown)
+caching::create_restore_emitter(metadata::ptr md, unsigned metadata_version,
+				bool clean_shutdown)
 {
-	return emitter::ptr(new restorer(md, clean_shutdown));
+	return emitter::ptr(new restorer(md, clean_shutdown, metadata_version));
 }
 
 //----------------------------------------------------------------
diff --git a/caching/restore_emitter.h b/caching/restore_emitter.h
index 9282895..5c077af 100644
--- a/caching/restore_emitter.h
+++ b/caching/restore_emitter.h
@@ -7,7 +7,9 @@
 //----------------------------------------------------------------
 
 namespace caching {
-	emitter::ptr create_restore_emitter(metadata::ptr md, bool clean_shutdown = true);
+	emitter::ptr create_restore_emitter(metadata::ptr md,
+					    unsigned metadata_version,
+					    bool clean_shutdown = true);
 }
 
 //----------------------------------------------------------------
diff --git a/caching/superblock.cc b/caching/superblock.cc
index 3270b3f..0a2d8bb 100644
--- a/caching/superblock.cc
+++ b/caching/superblock.cc
@@ -45,6 +45,8 @@ namespace {
 		le32 write_misses;
 
 		le32 policy_version[CACHE_POLICY_VERSION_SIZE];
+
+		le64 dirty_root; // format 2 only
 	} __attribute__ ((packed));
 
 	struct superblock_traits {
@@ -57,7 +59,7 @@ namespace {
 
 	uint32_t const SUPERBLOCK_MAGIC = 06142003;
 	uint32_t const VERSION_BEGIN = 1;
-	uint32_t const VERSION_END = 2;
+	uint32_t const VERSION_END = 3;
 }
 
 //----------------------------------------------------------------
@@ -195,6 +197,9 @@ superblock_traits::unpack(superblock_disk const &disk, superblock &core)
 	core.read_misses = to_cpu<uint32_t>(disk.read_misses);
 	core.write_hits = to_cpu<uint32_t>(disk.write_hits);
 	core.write_misses = to_cpu<uint32_t>(disk.write_misses);
+
+	if (core.version >= 2)
+		core.dirty_root = to_cpu<uint64_t>(disk.dirty_root);
 }
 
 void
diff --git a/caching/superblock.h b/caching/superblock.h
index fdac263..2366bbc 100644
--- a/caching/superblock.h
+++ b/caching/superblock.h
@@ -4,6 +4,7 @@
 #include "base/endian_utils.h"
 #include "persistent-data/data-structures/btree.h"
 
+#include <boost/optional.hpp>
 #include <set>
 
 //----------------------------------------------------------------
@@ -11,6 +12,8 @@
 namespace caching {
 	typedef unsigned char __u8;
 
+	unsigned const MIN_METADATA_VERSION = 1;
+	unsigned const MAX_METADATA_VERSION = 2;
 	unsigned const SPACE_MAP_ROOT_SIZE = 128;
 	unsigned const CACHE_POLICY_NAME_SIZE = 16;
 	unsigned const CACHE_POLICY_VERSION_SIZE = 3;
@@ -78,6 +81,8 @@ namespace caching {
 		uint32_t read_misses;
 		uint32_t write_hits;
 		uint32_t write_misses;
+
+		boost::optional<uint64_t> dirty_root;
 	};
 
 	enum incompat_bits {