diff --git a/caching/hint_array.cc b/caching/hint_array.cc
index 315ed42..8e9f275 100644
--- a/caching/hint_array.cc
+++ b/caching/hint_array.cc
@@ -234,14 +234,14 @@ missing_hints::visit(damage_visitor &v) const
 //----------------------------------------------------------------
 
 hint_array::hint_array(tm_ptr tm, unsigned width)
-	: width_(width),
+	: width_(check_width(width)),
 	  impl_(mk_array(tm, width))
 {
 }
 
 hint_array::hint_array(typename hint_array::tm_ptr tm, unsigned width,
 		       block_address root, unsigned nr_entries)
-	: width_(width),
+	: width_(check_width(width)),
 	  impl_(mk_array(tm, width, root, nr_entries))
 {
 }
@@ -283,4 +283,16 @@ hint_array::check(hint_array_damage::damage_visitor &visitor)
 	walk(vv, visitor);
 }
 
+uint32_t
+hint_array::check_width(uint32_t width)
+{
+	if (width % 4 || width == 0 || width > 128) {
+		ostringstream msg;
+		msg << "invalid hint width: " << width;
+		throw runtime_error(msg.str());
+	}
+
+	return width;
+}
+
 //----------------------------------------------------------------
diff --git a/caching/hint_array.h b/caching/hint_array.h
index fccf7b3..dfd3bcd 100644
--- a/caching/hint_array.h
+++ b/caching/hint_array.h
@@ -75,6 +75,8 @@ namespace caching {
 		void check(hint_array_damage::damage_visitor &visitor);
 
 	private:
+		static uint32_t check_width(uint32_t width);
+
 		unsigned width_;
 		boost::shared_ptr<persistent_data::array_base> impl_;
 	};
diff --git a/caching/superblock.cc b/caching/superblock.cc
index 46c2e04..30c733c 100644
--- a/caching/superblock.cc
+++ b/caching/superblock.cc
@@ -55,7 +55,7 @@ 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;
 }
 
 //----------------------------------------------------------------
@@ -109,7 +109,7 @@ superblock::superblock()
 	: csum(0),
 	  blocknr(SUPERBLOCK_LOCATION),
 	  magic(SUPERBLOCK_MAGIC),
-	  version(VERSION_BEGIN),
+	  version(VERSION_END - 1u),
 	  policy_hint_size(0),
 	  mapping_root(0),
 	  hint_root(0),
@@ -152,7 +152,7 @@ superblock_traits::unpack(superblock_disk const &disk, superblock &core)
 	for (unsigned i = 0; i < CACHE_POLICY_VERSION_SIZE; i++)
 		core.policy_version[i] = to_cpu<uint32_t>(disk.policy_version[i]);
 
-	core.policy_hint_size = to_cpu<uint32_t>(disk.policy_hint_size);
+	core.policy_hint_size = core.version == 1 ? 4 : to_cpu<uint32_t>(disk.policy_hint_size);
 
 	::memcpy(core.metadata_space_map_root,
 		 disk.metadata_space_map_root,
@@ -194,7 +194,6 @@ superblock_traits::pack(superblock const &core, superblock_disk &disk)
 
 	for (unsigned i = 0; i < CACHE_POLICY_VERSION_SIZE; i++)
 		disk.policy_version[i] = to_disk<le32>(core.policy_version[i]);
-
 	disk.policy_hint_size = to_disk<le32>(core.policy_hint_size);
 
 	::memcpy(disk.metadata_space_map_root,