diff --git a/src/bees-context.cc b/src/bees-context.cc
index 194c124..d4c533e 100644
--- a/src/bees-context.cc
+++ b/src/bees-context.cc
@@ -182,7 +182,7 @@ BeesContext::home_fd()
 }
 
 bool
-BeesContext::is_root_ro(uint64_t root)
+BeesContext::is_root_ro(uint64_t const root)
 {
 	return roots()->is_root_ro(root);
 }
diff --git a/src/bees-roots.cc b/src/bees-roots.cc
index 8477570..2eb143c 100644
--- a/src/bees-roots.cc
+++ b/src/bees-roots.cc
@@ -4,6 +4,7 @@
 #include "crucible/cache.h"
 #include "crucible/ntoa.h"
 #include "crucible/string.h"
+#include "crucible/table.h"
 #include "crucible/task.h"
 
 #include <algorithm>
@@ -13,6 +14,43 @@
 using namespace crucible;
 using namespace std;
 
+static
+string
+pretty_seconds(uint64_t t)
+{
+	struct unit {
+		uint64_t m_count;
+		const char *m_suffix;
+	};
+	static const unit units[] = {
+		{ .m_count = 365 * 24 * 60 * 60, .m_suffix = "y" },
+		{ .m_count =   7 * 24 * 60 * 60, .m_suffix = "w" },
+		{ .m_count =   1 * 24 * 60 * 60, .m_suffix = "d" },
+		{ .m_count =   1 *  1 * 60 * 60, .m_suffix = "h" },
+		{ .m_count =   1 *  1 *  1 * 60, .m_suffix = "m" },
+		{ .m_count =   1 *  1 *  1 *  1, .m_suffix = "s" },
+	};
+
+	ostringstream oss;
+	uint64_t prev_unit_limit = numeric_limits<uint64_t>::max();
+	size_t precision_count = 0;
+	for (const auto &i : units) {
+		const auto this_t = t % prev_unit_limit;
+		if (this_t >= i.m_count) {
+			if (precision_count) {
+				oss << " ";
+			}
+			++precision_count;
+			oss << this_t / i.m_count << i.m_suffix;
+		}
+		prev_unit_limit = i.m_count;
+		if (precision_count > 1) {
+			break;
+		}
+	}
+	return oss.str();
+}
+
 string
 format_time(time_t t)
 {
@@ -50,19 +88,34 @@ BeesCrawlState::operator<(const BeesCrawlState &that) const
 		< tie(that.m_min_transid, that.m_max_transid, that.m_objectid, that.m_offset, that.m_root);
 }
 
-class BeesScanMode {
+static
+bool
+is_subvol_tree(uint64_t objectid)
+{
+	return (objectid == BTRFS_FS_TREE_OBJECTID) || (objectid >= BTRFS_FIRST_FREE_OBJECTID);
+}
+
+class BeesScanMode : public enable_shared_from_this<BeesScanMode> {
 protected:
+	shared_ptr<BeesContext>	m_ctx;
 	shared_ptr<BeesRoots>	m_roots;
 	bool crawl_batch(const shared_ptr<BeesCrawl>& crawl);
 public:
 	virtual ~BeesScanMode() {}
-	BeesScanMode(const shared_ptr<BeesRoots>& roots) : m_roots(roots) {}
+	BeesScanMode(const shared_ptr<BeesRoots>& roots, const shared_ptr<BeesContext>& ctx);
+	virtual bool do_scan() = 0;
 	virtual bool scan() = 0;
 	using CrawlMap = decltype(BeesRoots::m_root_crawl_map);
 	virtual void next_transid(const CrawlMap &crawl_map) = 0;
 	virtual const char *ntoa() const = 0;
 };
 
+BeesScanMode::BeesScanMode(const shared_ptr<BeesRoots>& roots, const shared_ptr<BeesContext>& ctx) :
+	m_ctx(ctx),
+	m_roots(roots)
+{
+}
+
 bool
 BeesScanMode::crawl_batch(const shared_ptr<BeesCrawl>& crawl)
 {
@@ -79,6 +132,7 @@ class BeesScanModeLockstep : public BeesScanMode {
 public:
 	using BeesScanMode::BeesScanMode;
 	~BeesScanModeLockstep() override {}
+	bool do_scan() override;
 	bool scan() override;
 	void next_transid(const CrawlMap &crawl_map) override;
 	const char *ntoa() const override;
@@ -90,6 +144,12 @@ BeesScanModeLockstep::ntoa() const
 	return "LOCKSTEP";
 }
 
+bool
+BeesScanModeLockstep::do_scan()
+{
+	return true;
+}
+
 bool
 BeesScanModeLockstep::scan()
 {
@@ -124,6 +184,7 @@ BeesScanModeLockstep::next_transid(const CrawlMap &crawl_map)
 {
 	auto new_map = make_shared<Map>();
 	for (const auto &i : crawl_map) {
+		if (!is_subvol_tree(i.first)) continue;
 		const auto this_crawl = i.second;
 		const auto this_range = this_crawl->peek_front();
 		if (this_range) {
@@ -146,6 +207,7 @@ class BeesScanModeIndependent : public BeesScanMode {
 public:
 	using BeesScanMode::BeesScanMode;
 	~BeesScanModeIndependent() override {}
+	bool do_scan() override;
 	bool scan() override;
 	void next_transid(const CrawlMap &crawl_map) override;
 	const char *ntoa() const override;
@@ -157,6 +219,12 @@ BeesScanModeIndependent::ntoa() const
 	return "INDEPENDENT";
 }
 
+bool
+BeesScanModeIndependent::do_scan()
+{
+	return true;
+}
+
 bool
 BeesScanModeIndependent::scan()
 {
@@ -185,6 +253,7 @@ BeesScanModeIndependent::next_transid(const CrawlMap &crawl_map)
 {
 	auto new_subvols = make_shared<List>();
 	for (const auto &i : crawl_map) {
+		if (!is_subvol_tree(i.first)) continue;
 		const auto this_crawl = i.second;
 		const auto this_range = this_crawl->peek_front();
 		if (this_range) {
@@ -206,6 +275,7 @@ class BeesScanModeSequential : public BeesScanMode {
 public:
 	using BeesScanMode::BeesScanMode;
 	~BeesScanModeSequential() override {}
+	bool do_scan() override;
 	bool scan() override;
 	void next_transid(const CrawlMap &crawl_map) override;
 	const char *ntoa() const override;
@@ -217,6 +287,12 @@ BeesScanModeSequential::ntoa() const
 	return "SEQUENTIAL";
 }
 
+bool
+BeesScanModeSequential::do_scan()
+{
+	return true;
+}
+
 bool
 BeesScanModeSequential::scan()
 {
@@ -245,6 +321,7 @@ BeesScanModeSequential::next_transid(const CrawlMap &crawl_map)
 {
 	auto new_map = make_shared<Map>();
 	for (const auto &i : crawl_map) {
+		if (!is_subvol_tree(i.first)) continue;
 		const auto this_crawl = i.second;
 		const auto this_range = this_crawl->peek_front();
 		if (this_range) {
@@ -273,6 +350,7 @@ class BeesScanModeRecent : public BeesScanMode {
 public:
 	using BeesScanMode::BeesScanMode;
 	~BeesScanModeRecent() override {}
+	bool do_scan() override;
 	bool scan() override;
 	void next_transid(const CrawlMap &crawl_map) override;
 	const char *ntoa() const override;
@@ -284,6 +362,12 @@ BeesScanModeRecent::ntoa() const
 	return "RECENT";
 }
 
+bool
+BeesScanModeRecent::do_scan()
+{
+	return true;
+}
+
 bool
 BeesScanModeRecent::scan()
 {
@@ -318,6 +402,7 @@ BeesScanModeRecent::next_transid(const CrawlMap &crawl_map)
 	auto new_map = make_shared<Map>();
 	auto &sorted = *new_map;
 	for (const auto &i : crawl_map) {
+		if (!is_subvol_tree(i.first)) continue;
 		const auto this_crawl = i.second;
 		const auto this_range = this_crawl->peek_front();
 		if (this_range) {
@@ -336,6 +421,607 @@ BeesScanModeRecent::next_transid(const CrawlMap &crawl_map)
 	swap(m_sorted, new_map);
 }
 
+/// Scan the extent tree and submit each extent's references in a single batch.
+class BeesScanModeExtent : public BeesScanMode {
+	mutex m_mutex;
+	mutex m_insert_root_mutex;
+	CrawlMap m_crawl_map;
+	map<uint64_t, Task> m_task_map;
+	struct MagicCrawl {
+		uint64_t m_min_size;
+		uint64_t m_max_size;
+	};
+friend ostream& operator<<(ostream &os, const BeesScanModeExtent::MagicCrawl& magic);
+	using MagicCrawlMap = map<uint64_t, BeesScanModeExtent::MagicCrawl>;
+	static MagicCrawlMap s_magic_crawl_map;
+	struct ExtentRef {
+		uint64_t m_root;
+		uint64_t m_inum;
+		uint64_t m_offset;
+		uint64_t m_length;
+		ProgressTracker<BeesCrawlState>::ProgressHolder m_hold;
+		Timer m_age;
+	};
+friend ostream& operator<<(ostream &os, const BeesScanModeExtent::ExtentRef& todo);
+
+	void init_tasks();
+	void run_tasks();
+	void map_next_extent(uint64_t subvol);
+	void crawl_one_extent(const ExtentRef &bior);
+	void create_extent_map(const uint64_t bytenr, const ProgressTracker<BeesCrawlState>::ProgressHolder& m_hold, uint64_t len);
+
+public:
+	BeesScanModeExtent(const shared_ptr<BeesRoots>& roots, const shared_ptr<BeesContext>& ctx);
+	~BeesScanModeExtent() override {}
+	bool do_scan() override;
+	bool scan() override;
+	void next_transid(const CrawlMap &crawl_map) override;
+	const char *ntoa() const override;
+};
+
+ostream &
+operator<<(ostream &os, const BeesScanModeExtent::MagicCrawl& magic)
+{
+	return os << "[" << pretty(magic.m_min_size) << ".." << pretty(magic.m_max_size) << "]";
+}
+
+ostream &
+operator<<(ostream &os, const BeesScanModeExtent::ExtentRef& todo)
+{
+	return os << todo.m_root << ":" << todo.m_inum << " " << to_hex(todo.m_offset) << "+" << pretty(todo.m_length) << " age " << todo.m_age;
+}
+
+map<uint64_t, BeesScanModeExtent::MagicCrawl> BeesScanModeExtent::s_magic_crawl_map {
+	{
+		BTRFS_FIRST_FREE_OBJECTID - 6,
+		(BeesScanModeExtent::MagicCrawl) {
+			.m_min_size = 32 * 1024 * 1024 + 1,
+			.m_max_size = numeric_limits<uint64_t>::max(),
+		},
+	},
+	{
+		BTRFS_FIRST_FREE_OBJECTID - 5,
+		(BeesScanModeExtent::MagicCrawl) {
+			.m_min_size = 8 * 1024 * 1024 + 1,
+			.m_max_size = 32 * 1024 * 1024,
+		},
+	},
+	{
+		BTRFS_FIRST_FREE_OBJECTID - 4,
+		(BeesScanModeExtent::MagicCrawl) {
+			.m_min_size = 2 * 1024 * 1024 + 1,
+			.m_max_size = 8 * 1024 * 1024,
+		},
+	},
+	{
+		BTRFS_FIRST_FREE_OBJECTID - 3,
+		(BeesScanModeExtent::MagicCrawl) {
+			.m_min_size = 512 * 1024 + 1,
+			.m_max_size = 2 * 1024 * 1024,
+		},
+	},
+	{
+		BTRFS_FIRST_FREE_OBJECTID - 2,
+		(BeesScanModeExtent::MagicCrawl) {
+			.m_min_size = 128 * 1024 + 1,
+			.m_max_size = 512 * 1024,
+		},
+	},
+	{
+		BTRFS_FIRST_FREE_OBJECTID - 1,
+		(BeesScanModeExtent::MagicCrawl) {
+			.m_min_size = 0,
+			.m_max_size = 128 * 1024,
+		},
+	},
+};
+
+BeesScanModeExtent::BeesScanModeExtent(const shared_ptr<BeesRoots>& roots, const shared_ptr<BeesContext>& ctx) :
+	BeesScanMode(roots, ctx)
+{
+}
+
+const char *
+BeesScanModeExtent::ntoa() const
+{
+	return "EXTENT";
+}
+
+static
+bool
+should_throttle()
+{
+	static bool s_throttled = false;
+	// If there's too many entries in the queue, stop adding new ones until workers catch up
+	// If there's not too many entries in the queue, restart the scan task
+	const auto instance_count = Task::instance_count();
+	const auto instance_limit = BEES_MAX_EXTENT_REF_COUNT;
+	const bool queue_empty = s_throttled && instance_count < instance_limit;
+	const bool queue_full = !s_throttled && instance_count > instance_limit;
+	if (queue_full) {
+		BEESLOGDEBUG("Throttling crawl at " << instance_count << " tasks");
+		s_throttled = true;
+		BEESCOUNT(crawl_throttled);
+	}
+	if (queue_empty) {
+		BEESLOGDEBUG("Unthrottling crawl at " << instance_count << " tasks");
+		s_throttled = false;
+		BEESCOUNT(crawl_unthrottled);
+	}
+	return s_throttled;
+}
+
+void
+BeesScanModeExtent::crawl_one_extent(const BeesScanModeExtent::ExtentRef &bior)
+{
+	auto inode_mutex = m_ctx->get_inode_mutex(bior.m_inum);
+	auto inode_lock = inode_mutex->try_lock(Task::current_task());
+	if (!inode_lock) {
+		BEESCOUNT(extent_deferred_inode);
+		return;
+	}
+
+	BEESNOTE("scanning root " << bior.m_root << " ino " << bior.m_inum << " offset " << to_hex(bior.m_offset) << " length " << pretty(bior.m_length));
+	BEESTRACE("scanning root " << bior.m_root << " ino " << bior.m_inum << " offset " << to_hex(bior.m_offset) << " length " << pretty(bior.m_length));
+
+	BeesFileRange bfr(
+		BeesFileId(bior.m_root, bior.m_inum),
+		bior.m_offset,
+		bior.m_offset + bior.m_length
+	);
+
+	BEESCOUNT(extent_forward);
+	m_ctx->scan_forward(bfr);
+}
+
+void
+BeesScanModeExtent::create_extent_map(const uint64_t bytenr, const ProgressTracker<BeesCrawlState>::ProgressHolder& hold, const uint64_t len)
+{
+	BEESNOTE("Creating extent map for " << to_hex(bytenr) << " with LOGICAL_INO");
+	BEESTRACE("Creating extent map for " << to_hex(bytenr) << " with LOGICAL_INO");
+
+	const auto log_ino_ptr = m_ctx->logical_ino(bytenr, true);
+	auto &log_ino = *log_ino_ptr;
+
+	{
+		BEESNOTE("waiting to create extent map for " << to_hex(bytenr) << " with LOGICAL_INO");
+		const auto lock = MultiLocker::get_lock("logical_ino");
+
+		BEESNOTE("Resolving bytenr " << to_hex(bytenr) << " refs " << log_ino.m_iors.size());
+		BEESTOOLONG("Resolving bytenr " << to_hex(bytenr) << " refs " << log_ino.m_iors.size());
+
+		// Time how long this takes
+		Timer resolve_timer;
+
+		if (log_ino.do_ioctl_nothrow(m_ctx->root_fd())) {
+			BEESCOUNT(extent_ok);
+		} else {
+			BEESCOUNT(extent_fail);
+		}
+
+		BEESCOUNTADD(extent_ms, resolve_timer.age() * 1000);
+	}
+
+        const size_t rv_count = log_ino.m_iors.size();
+
+	// BEESLOGDEBUG("Inserting " << rv_count << " extent refs from " << to_hex(bytenr) << ", " << Task::instance_count() << " tasks");
+	BEESNOTE("Inserting " << rv_count << " extent refs from " << to_hex(bytenr));
+	BEESTRACE("Inserting " << rv_count << " extent refs from " << to_hex(bytenr));
+
+	// Avoid performance problems - pretend resolve failed if there are too many refs
+        if (rv_count == 0) {
+		BEESLOGDEBUG("LOGICAL_INO returned 0 refs for " << len << " bytes (" << pretty(len) << ") at " << to_hex(bytenr));
+		BEESCOUNT(extent_zero);
+		return;
+        } else if (rv_count >= BEES_MAX_EXTENT_REF_COUNT) {
+		// If we find any duplicates when there are BEES_MAX_EXTENT_REF_COUNT references, then
+		// we'll end up with some extent with at least BEES_MAX_EXTENT_REF_COUNT + 1 references.
+		// That's too many, so don't let that happen.
+                BEESLOGINFO("bytenr " << to_hex(bytenr) << " refs " << rv_count << " overflows configured ref limit " << BEES_MAX_EXTENT_REF_COUNT);
+                BEESCOUNT(extent_overflow);
+		return;
+	}
+
+	BtrfsExtentDataFetcher bedf(m_ctx->root_fd());
+
+	// Collect extent ref tasks as a series of stand-alone events
+	// chained after the first task created, then run the first one.
+	// This prevents other threads from starting to process an
+	// extent until we have all of its refs in the queue.
+	Task first_task;
+	for (const auto &i : log_ino.m_iors) {
+		catch_all([&](){
+			BEESTRACE("mapping extent " << to_hex(bytenr) << " ref at root " << i.m_root << " ino " << i.m_inum << " offset " << to_hex(i.m_offset));
+			BEESNOTE("mapping extent " << to_hex(bytenr) << " ref at root " << i.m_root << " ino " << i.m_inum << " offset " << to_hex(i.m_offset));
+
+			bedf.tree(i.m_root);
+			bedf.objectid(i.m_inum);
+			const auto bti = bedf.at(i.m_offset);
+			if (!bti) {
+				BEESLOGDEBUG("No ref for extent " << to_hex(bytenr) << " at root " << i.m_root << " ino " << i.m_inum << " offset " << to_hex(i.m_offset));
+				BEESCOUNT(extent_ref_missing);
+				return;
+			}
+
+			const auto bec = dynamic_pointer_cast<BeesScanModeExtent>(shared_from_this());
+			const auto length = bti.file_extent_logical_bytes();
+			const ExtentRef extref = {
+				.m_root = i.m_root,
+				.m_inum = i.m_inum,
+				.m_offset = i.m_offset,
+				.m_length = length,
+				.m_hold = hold,
+			};
+			ostringstream oss;
+			oss << "R" << i.m_root << "_" << i.m_inum << "_" << pretty(i.m_offset) << "_" << pretty(length);
+			Task crawl_one(oss.str(), [bec, extref]() {
+				bec->crawl_one_extent(extref);
+			});
+			if (!first_task) {
+				first_task = crawl_one;
+			} else {
+				first_task.append(crawl_one);
+			}
+			BEESCOUNT(extent_ref_ok);
+		});
+	}
+	if (first_task) {
+		// first_task.append(Task::current_task());
+		first_task.run();
+	}
+
+	// Go back for more tasks if the current task isn't already appended
+	Task::current_task().run();
+
+	BEESCOUNT(extent_mapped);
+}
+
+void
+BeesScanModeExtent::init_tasks()
+{
+	// Make sure all the magic crawlers are inserted in m_crawl_map,
+	// and each one has a Task
+	unique_lock<mutex> lock_insert_root(m_insert_root_mutex);
+	unique_lock<mutex> lock(m_mutex);
+	for (const auto &i : s_magic_crawl_map) {
+		const auto subvol = i.first;
+		const auto &magic = i.second;
+		auto found = m_crawl_map.find(subvol);
+		if (found == m_crawl_map.end()) {
+			lock.unlock();
+			BeesCrawlState new_bcs;
+			new_bcs.m_root = subvol;
+			new_bcs.m_min_transid = m_roots->transid_min();
+			new_bcs.m_max_transid = m_roots->transid_max();
+			const auto this_crawl = m_roots->insert_root(new_bcs);
+			lock.lock();
+			m_crawl_map.insert(make_pair(subvol, this_crawl));
+			BEESCOUNT(crawl_create);
+		}
+		auto task_found = m_task_map.find(subvol);
+		if (task_found == m_task_map.end()) {
+			ostringstream oss;
+			oss << "extent_" << subvol << "_" << pretty(magic.m_min_size & ~BLOCK_MASK_CLONE)
+				<< "_" << pretty(magic.m_max_size);
+			const auto bec = dynamic_pointer_cast<BeesScanModeExtent>(shared_from_this());
+			m_task_map.insert(make_pair(subvol, Task(oss.str(), [bec, subvol]() {
+				bec->map_next_extent(subvol);
+			})));
+		}
+	}
+}
+
+void
+BeesScanModeExtent::run_tasks()
+{
+	if (should_throttle()) return;
+
+	unique_lock<mutex> lock(m_mutex);
+	// Good to go, start everything running
+	for (const auto &i : m_task_map) {
+		i.second.run();
+	}
+}
+
+void
+BeesScanModeExtent::map_next_extent(uint64_t const subvol)
+{
+	BEESTRACE("map_next_extent " << subvol);
+
+	if (should_throttle()) return;
+
+	size_t discard_count = 0;
+	size_t gen_low_count = 0;
+	size_t gen_high_count = 0;
+	size_t loop_count = 0;
+	size_t init_s_calls = BtrfsIoctlSearchKey::s_calls;
+	size_t init_s_loops = BtrfsIoctlSearchKey::s_loops;
+	Timer crawl_time;
+
+	unique_lock<mutex> lock(m_mutex);
+	auto found = m_crawl_map.find(subvol);
+	THROW_CHECK0(runtime_error, found != m_crawl_map.end());
+	CrawlMap::mapped_type this_crawl = found->second;
+	lock.unlock();
+	THROW_CHECK0(runtime_error, this_crawl);
+
+	BtrfsExtentItemFetcher beif(m_ctx->root_fd());
+	beif.scale_size(BLOCK_SIZE_SUMS);
+
+	// Get the next item from the crawler
+	while (true) {
+		++loop_count;
+
+		BEESTRACE("get_state_end");
+		const auto this_state = this_crawl->get_state_end();
+		BEESNOTE("Crawling extent " << this_state);
+		BEESTRACE("Crawling extent " << this_state);
+		const auto this_range = this_crawl->pop_front();
+		BEESTRACE("this_range check");
+
+		// Ran out of data in this subvol, wait for next_transid to refill it
+		if (!this_range) {
+			break;
+		}
+
+		// Check extent length against size range
+		const auto &subvol_magic = s_magic_crawl_map.at(subvol);
+		const uint64_t lower_size_bound = subvol_magic.m_min_size;
+		const uint64_t upper_size_bound = subvol_magic.m_max_size;
+		const uint64_t this_range_size = this_range.size();
+
+		// If this extent is out of range, move on to the next
+		if (this_range_size < lower_size_bound || this_range_size > upper_size_bound) {
+			// Advance the begin point in case we get into trouble later on
+			this_crawl->hold_state(this_state);
+			BEESCOUNT(crawl_discard);
+			++discard_count;
+
+			// Skip the skipping until we get the issues sorted out
+			continue;
+
+			// Skip ahead over any below-min-size extents
+			BEESTRACE("min_size " << pretty(lower_size_bound) << " > scale_size " << pretty(beif.scale_size()));
+			const auto lsb_rounded = lower_size_bound & ~(beif.scale_size() - 1);
+			// Don't bother doing backward searches when skipping less than 128K,
+			// the search will cost more than reading 32 consecutive extent records
+			// FIXME: need to make this aware of block group boundaries so it doesn't
+			// blow 5 CPU seconds scanning metadata
+			if (lsb_rounded >= 128 * 1024) {
+				const auto lsb_rounded = lower_size_bound & ~(beif.scale_size() - 1);
+				const auto objectid = this_range.end() + lsb_rounded - beif.scale_size();
+				BEESTRACE("objectid = " << this_state.m_objectid << ", adjusted to " << objectid);
+				BEESTOOLONG("subvol " << subvol << " skipping forward " << pretty(lsb_rounded) << " from " << to_hex(this_state.m_objectid) << " to " << to_hex(objectid));
+				BEESNOTE("subvol " << subvol << " skipping forward " << pretty(lsb_rounded) << " from " << to_hex(this_state.m_objectid) << " to " << to_hex(objectid));
+				const auto bti = beif.rlower_bound(objectid);
+				auto mutable_state = this_state;
+				mutable_state.m_objectid = bti.objectid();
+				if (mutable_state.m_objectid <= this_state.m_objectid) {
+					// BEESLOGDEBUG("skip failed: this_state " << this_state << ", mutable_state " << mutable_state);
+					// No extent found between end and search position, skip ahead to search position
+					mutable_state.m_objectid = objectid;
+					BEESCOUNT(crawl_skip_fail);
+				} else {
+					const auto discard_hold = this_crawl->hold_state(mutable_state);
+					BEESCOUNT(crawl_skip_ok);
+				}
+			}
+			continue;
+		}
+
+		const auto bytenr = this_range.fid().ino();
+
+		// Check extent item generation is in range
+		// FIXME: we already had this in crawl state, and we threw it away
+		const auto bti = beif.at(bytenr);
+		const auto gen = bti.extent_generation();
+		if (gen < this_state.m_min_transid) {
+			BEESCOUNT(crawl_gen_low);
+			++gen_low_count;
+			continue;
+		}
+		if (gen > this_state.m_max_transid) {
+			BEESCOUNT(crawl_gen_high);
+			++gen_high_count;
+			continue;
+		}
+
+		// Map this extent here to regulate task creation
+		create_extent_map(bytenr, this_crawl->hold_state(this_state), bti.offset());
+
+		BEESCOUNT(crawl_extent);
+		const auto search_calls = BtrfsIoctlSearchKey::s_calls - init_s_calls;
+		const auto search_loops = BtrfsIoctlSearchKey::s_loops - init_s_loops;
+		if (crawl_time.age() > 1) {
+			BEESLOGDEBUG("loop_count " << loop_count << " discard_count " << discard_count
+				<< " gen_low_count " << gen_low_count << " gen_high_count " << gen_high_count
+				<< " search_calls " << search_calls << " search_loops " << search_loops
+				<< " time " << crawl_time << " subvol " << subvol);
+		}
+
+		// We did something!  Get in line to run again
+		Task::current_task().run();
+		return;
+	}
+
+	// All crawls done
+	BEESCOUNT(crawl_done);
+}
+
+bool
+BeesScanModeExtent::do_scan()
+{
+	return false;
+}
+
+bool
+BeesScanModeExtent::scan()
+{
+	// This is now driven directly from next_transid
+	return false;
+}
+
+void
+BeesScanModeExtent::next_transid(const CrawlMap &crawl_map)
+{
+	BEESTRACE("Extent next_transid");
+
+	// Do the important parts first, the rest can return early or die with an exception
+
+	// Can't set this up in the constructor because shared_from_this is a method on a
+	// virtual base.  So we do it here.
+	init_tasks();
+
+	// insert_root does this for non-magic subvols, we have to do it ourselves
+	for (const auto &i : s_magic_crawl_map) {
+		const auto subvol = i.first;
+		const auto found = crawl_map.find(subvol);
+		if (found != crawl_map.end()) {
+			found->second->deferred(false);
+		}
+	}
+
+	// Kick off tasks if they aren't already running
+	run_tasks();
+
+	// Swap in the new crawl map with freshly undeferred crawlers
+	auto crawl_map_copy = crawl_map;
+	unique_lock<mutex> lock(m_mutex);
+	swap(m_crawl_map, crawl_map_copy);
+	lock.unlock();
+
+	// Estimate progress by building a map of where the extent bytenrs are (they are sparse,
+	// no extents exist between block groups), and report the position within that map.
+
+	BtrfsTreeOffsetFetcher btf(m_ctx->root_fd());
+        btf.tree(BTRFS_CHUNK_TREE_OBJECTID);
+        btf.objectid(BTRFS_FIRST_CHUNK_TREE_OBJECTID);
+        btf.type(BTRFS_CHUNK_ITEM_KEY);
+	uint64_t fs_size = 0;
+	uint64_t last_bgaddr = 0;
+	struct bg_info {
+		uint64_t first_bytenr;
+		uint64_t first_total;
+	};
+	map<uint64_t, bg_info> bg_info_map;
+	while (true) {
+		const auto bti = btf.lower_bound(last_bgaddr);
+		if (!bti) {
+			break;
+		}
+		const auto offset = bti.offset();
+		const auto chunk_length = bti.chunk_length();
+		THROW_CHECK0(runtime_error, offset > 0);
+		THROW_CHECK0(runtime_error, chunk_length > 0);
+		last_bgaddr = offset + chunk_length;
+		bg_info_map[last_bgaddr] = (bg_info) {
+			.first_bytenr = offset,
+			.first_total = fs_size,
+		};
+		if (bti.chunk_type() & (BTRFS_BLOCK_GROUP_METADATA | BTRFS_BLOCK_GROUP_SYSTEM)) {
+			continue;
+		}
+		fs_size += chunk_length;
+	}
+
+	if (!fs_size) {
+		BEESLOGDEBUG("PROGRESS: no data block groups found in filesystem");
+		BEESCOUNT(progress_no_data_bg);
+		return;
+	}
+
+	// Report on progress using extent bytenr map
+	Table::Table eta;
+	for (const auto &i : s_magic_crawl_map) {
+		const auto &subvol = i.first;
+		const auto &magic = i.second;
+
+		const auto found = crawl_map.find(subvol);
+		if (found == crawl_map.end()) {
+			// BEESLOGDEBUG("PROGRESS: crawler not yet created for " << magic);
+			BEESCOUNT(progress_not_created);
+			continue;
+		}
+
+		const auto this_crawl = found->second;
+		THROW_CHECK1(runtime_error, subvol, this_crawl);
+
+		const auto this_range = this_crawl->peek_front();
+		if (!this_range) {
+			BEESLOGDEBUG("PROGRESS: completed crawl " << magic);
+			BEESCOUNT(progress_complete);
+			continue;
+		}
+
+		const auto bytenr = this_range.fid().ino();
+		const auto bg_found = bg_info_map.lower_bound(bytenr);
+		if (bg_found == bg_info_map.end()) {
+			BEESLOGDEBUG("PROGRESS: bytenr " << to_hex(bytenr) << " not found in a block group for " << magic);
+			BEESCOUNT(progress_not_found);
+			continue;
+		}
+		const auto &bi = bg_found->second;
+		const auto bi_last_bytenr = bg_found->first;
+		if (bytenr > bi_last_bytenr || bytenr < bi.first_bytenr) {
+			// This can happen if the crawler happens to be in a metadata block group,
+			// or if a block group was deleted under us.
+			BEESLOGDEBUG("PROGRESS: bytenr " << to_hex(bytenr) << " out of range for block group " << to_hex(bi.first_bytenr) << ".." << to_hex(bg_found->first) << " (block group deleted?) for " << magic);
+			BEESCOUNT(progress_out_of_bg);
+		}
+		const auto bytenr_offset = min(bi_last_bytenr, max(bytenr, bi.first_bytenr)) - bi.first_bytenr + bi.first_total;
+		const auto bytenr_percent = bytenr_offset / (0.01 * fs_size);
+		const auto this_state = this_crawl->get_state_end();
+		const auto now = time(NULL);
+		const auto time_so_far = now - min(now, this_state.m_started);
+		string eta_stamp = "-";
+		string eta_pretty = "-";
+		if (time_so_far > 1 && bytenr_percent > 0) {
+			const time_t eta_duration = time_so_far / (bytenr_percent / 100);
+			const time_t eta_time = eta_duration + now;
+			struct tm ltm = { 0 };
+			DIE_IF_ZERO(localtime_r(&eta_time, &ltm));
+
+			char buf[1024] = { 0 };
+			DIE_IF_ZERO(strftime(buf, sizeof(buf), "%Y-%m-%d %H:%M:%S", &ltm));
+			eta_stamp = string(buf);
+			eta_pretty = pretty_seconds(eta_duration);
+		}
+		eta.insert_row(Table::endpos, vector<Table::Content> {
+			Table::Text(pretty(bytenr_offset)),
+			Table::Text(pretty(fs_size)),
+			Table::Text(astringprintf("%.4f%%", bytenr_percent)),
+			Table::Number(subvol),
+			Table::Text(pretty(magic.m_min_size & ~BLOCK_MASK_CLONE)),
+			Table::Text(pretty(magic.m_max_size)),
+			Table::Number(this_state.m_min_transid),
+			Table::Number(this_state.m_max_transid),
+			Table::Text(eta_pretty),
+			Table::Text(eta_stamp),
+		});
+		BEESCOUNT(progress_ok);
+	}
+	ostringstream oss;
+	eta.left("PROGRESS: ");
+	eta.mid(" ");
+	eta.right("");
+	eta.insert_row(0, vector<Table::Content> {
+		Table::Text("done"),
+		Table::Text("total"),
+		Table::Text("%done"),
+		Table::Text("sub"),
+		Table::Text("szmn"),
+		Table::Text("szmx"),
+		Table::Text("transid"),
+		Table::Number(m_roots->transid_max()),
+		Table::Text("remain"),
+		Table::Text("ETA"),
+	});
+	const auto dash_fill = Table::Fill('-');
+	eta.insert_row(1, vector<Table::Content>(eta.cols().size(), dash_fill));
+	oss << eta;
+	BEESLOGDEBUG(oss.str());
+}
+
 void
 BeesRoots::set_scan_mode(ScanMode mode)
 {
@@ -343,19 +1029,23 @@ BeesRoots::set_scan_mode(ScanMode mode)
 	unique_lock<mutex> lock(m_mutex);
 	switch (mode) {
 		case SCAN_MODE_LOCKSTEP: {
-			m_scanner = make_shared<BeesScanModeLockstep>(shared_from_this());
+			m_scanner = make_shared<BeesScanModeLockstep>(shared_from_this(), m_ctx);
 			break;
 		}
 		case SCAN_MODE_INDEPENDENT: {
-			m_scanner = make_shared<BeesScanModeIndependent>(shared_from_this());
+			m_scanner = make_shared<BeesScanModeIndependent>(shared_from_this(), m_ctx);
 			break;
 		}
 		case SCAN_MODE_SEQUENTIAL: {
-			m_scanner = make_shared<BeesScanModeSequential>(shared_from_this());
+			m_scanner = make_shared<BeesScanModeSequential>(shared_from_this(), m_ctx);
 			break;
 		}
 		case SCAN_MODE_RECENT: {
-			m_scanner = make_shared<BeesScanModeRecent>(shared_from_this());
+			m_scanner = make_shared<BeesScanModeRecent>(shared_from_this(), m_ctx);
+			break;
+		}
+		case SCAN_MODE_EXTENT: {
+			m_scanner = make_shared<BeesScanModeExtent>(shared_from_this(), m_ctx);
 			break;
 		}
 		case SCAN_MODE_COUNT:
@@ -714,6 +1404,10 @@ BeesRoots::crawl_roots()
 	BEESNOTE("Scanning roots in " << hold_scanner->ntoa() << " mode");
 	BEESTRACE("scanning roots in " << hold_scanner->ntoa() << " mode");
 
+	// Clumsy adapter for legacy scan modes
+	if (!hold_scanner->do_scan()) {
+		return false;
+	}
 	if (hold_scanner->scan()) {
 		return true;
 	}
@@ -816,7 +1510,7 @@ BeesRoots::writeback_thread()
 	}
 }
 
-void
+shared_ptr<BeesCrawl>
 BeesRoots::insert_root(const BeesCrawlState &new_bcs)
 {
 	unique_lock<mutex> lock(m_mutex);
@@ -826,9 +1520,10 @@ BeesRoots::insert_root(const BeesCrawlState &new_bcs)
 		m_root_crawl_map.insert(new_pair);
 		++m_crawl_dirty;
 	}
-	auto found = m_root_crawl_map.find(new_bcs.m_root);
+	const auto found = m_root_crawl_map.find(new_bcs.m_root);
 	THROW_CHECK0(runtime_error, found != m_root_crawl_map.end());
 	found->second->deferred(false);
+	return found->second;
 }
 
 void
@@ -845,8 +1540,10 @@ BeesRoots::insert_new_crawl()
 	unique_lock<mutex> lock(m_mutex);
 	set<uint64_t> excess_roots;
 	for (const auto &i : m_root_crawl_map) {
-		BEESTRACE("excess_roots.insert(" << i.first << ")");
-		excess_roots.insert(i.first);
+		if (is_subvol_tree(i.first)) {
+			BEESTRACE("excess_roots.insert(" << i.first << ")");
+			excess_roots.insert(i.first);
+		}
 	}
 	lock.unlock();
 
@@ -1104,6 +1801,11 @@ BeesRoots::is_root_ro(uint64_t root)
 		return false;
 	}
 
+	// If it's not a subvol tree, it's rw
+	if (!is_subvol_tree(root)) {
+		return false;
+	}
+
 	BEESTRACE("checking subvol flags on root " << root);
 
 	BtrfsRootFetcher root_fetcher(m_ctx->root_fd());
@@ -1314,9 +2016,14 @@ BeesCrawl::BeesCrawl(shared_ptr<BeesContext> ctx, BeesCrawlState initial_state)
 	m_state(initial_state),
 	m_btof(ctx->root_fd())
 {
-	m_btof.scale_size(1);
-	m_btof.tree(initial_state.m_root);
-	m_btof.type(BTRFS_EXTENT_DATA_KEY);
+	if (is_subvol_tree(initial_state.m_root)) {
+		m_btof.tree(initial_state.m_root);
+		m_btof.scale_size(1);
+		m_btof.type(BTRFS_EXTENT_DATA_KEY);
+	} else {
+		m_btof.tree(BTRFS_EXTENT_TREE_OBJECTID);
+		m_btof.type(BTRFS_EXTENT_ITEM_KEY);
+	}
 }
 
 bool
@@ -1410,6 +2117,13 @@ BeesCrawl::fetch_extents()
 	m_btof.transid(old_state.m_min_transid);
 	if (catch_all([&]() {
 		m_next_extent_data = m_btof.lower_bound(old_state.m_objectid);
+		if (m_btof.tree() == BTRFS_EXTENT_TREE_OBJECTID) {
+			// Skip over TREE_BLOCK extent items, they don't have files
+			while (!!m_next_extent_data && (m_next_extent_data.extent_flags() & BTRFS_EXTENT_FLAG_TREE_BLOCK)) {
+				BEESCOUNT(crawl_tree_block);
+				m_next_extent_data = m_btof.next(m_next_extent_data.objectid());
+			}
+		}
 	})) {
 		// Whoops that didn't work.  Stop scanning this subvol, move on to the next.
 		m_deferred = true;
@@ -1421,7 +2135,7 @@ BeesCrawl::fetch_extents()
 		return restart_crawl();
 	}
 	auto new_state = old_state;
-	new_state.m_objectid = max(m_next_extent_data.objectid() + 1, m_next_extent_data.objectid());
+	new_state.m_objectid = max(m_next_extent_data.objectid() + m_btof.scale_size(), m_next_extent_data.objectid());
 	new_state.m_offset = 0;
 	set_state(new_state);
 	return true;
@@ -1446,11 +2160,19 @@ BeesCrawl::bti_to_bfr(const BtrfsTreeItem &bti) const
 	if (!bti) {
 		return BeesFileRange();
 	}
-	return BeesFileRange(
-		BeesFileId(get_state_end().m_root, bti.objectid()),
-		bti.offset(),
-		bti.offset() + bti.file_extent_logical_bytes()
-	);
+	if (bti.type() == BTRFS_EXTENT_ITEM_KEY) {
+		return BeesFileRange(
+			BeesFileId(get_state_end().m_root, bti.objectid()),
+			bti.objectid(),
+			bti.objectid() + bti.offset()
+		);
+	} else {
+		return BeesFileRange(
+			BeesFileId(get_state_end().m_root, bti.objectid()),
+			bti.offset(),
+			bti.offset() + bti.file_extent_logical_bytes()
+		);
+	}
 }
 
 BeesFileRange
diff --git a/src/bees.h b/src/bees.h
index 9438188..84a4084 100644
--- a/src/bees.h
+++ b/src/bees.h
@@ -561,11 +561,8 @@ class BeesRoots : public enable_shared_from_this<BeesRoots> {
 	bool					m_stop_requested = false;
 
 	void insert_new_crawl();
-	void insert_root(const BeesCrawlState &bcs);
 	Fd open_root_nocache(uint64_t root);
 	Fd open_root_ino_nocache(uint64_t root, uint64_t ino);
-	uint64_t transid_min();
-	uint64_t transid_max();
 	uint64_t transid_max_nocache();
 	void state_load();
 	ostream &state_to_stream(ostream &os);
@@ -582,6 +579,9 @@ class BeesRoots : public enable_shared_from_this<BeesRoots> {
 	bool crawl_batch(shared_ptr<BeesCrawl> crawl);
 	void clear_caches();
 
+friend class BeesScanModeExtent;
+	shared_ptr<BeesCrawl> insert_root(const BeesCrawlState &bcs);
+
 friend class BeesCrawl;
 friend class BeesFdCache;
 friend class BeesScanMode;
@@ -600,17 +600,20 @@ public:
 	Fd open_root_ino(const BeesFileId &bfi) { return open_root_ino(bfi.root(), bfi.ino()); }
 	bool is_root_ro(uint64_t root);
 
-	// TODO:  do extent-tree scans instead
 	enum ScanMode {
 		SCAN_MODE_LOCKSTEP,
 		SCAN_MODE_INDEPENDENT,
 		SCAN_MODE_SEQUENTIAL,
 		SCAN_MODE_RECENT,
+		SCAN_MODE_EXTENT,
 		SCAN_MODE_COUNT, // must be last
 	};
 
 	void set_scan_mode(ScanMode new_mode);
 	void set_workaround_btrfs_send(bool do_avoid);
+
+	uint64_t transid_min();
+	uint64_t transid_max();
 };
 
 struct BeesHash {