diff --git a/include/crucible/fs.h b/include/crucible/fs.h
index 73defe4..c0bfef5 100644
--- a/include/crucible/fs.h
+++ b/include/crucible/fs.h
@@ -156,7 +156,7 @@ namespace crucible {
 	ostream & operator<<(ostream &os, const BtrfsIoctlSearchHeader &hdr);
 
 	struct BtrfsIoctlSearchKey : public btrfs_ioctl_search_key {
-		BtrfsIoctlSearchKey(size_t buf_size = 16 * 1024);
+		BtrfsIoctlSearchKey(size_t buf_size = 4096);
 		virtual bool do_ioctl_nothrow(int fd);
 		virtual void do_ioctl(int fd);
 
diff --git a/lib/extentwalker.cc b/lib/extentwalker.cc
index dd7dacd..999f097 100644
--- a/lib/extentwalker.cc
+++ b/lib/extentwalker.cc
@@ -468,7 +468,7 @@ namespace crucible {
 	BtrfsExtentWalker::Vec
 	BtrfsExtentWalker::get_extent_map(off_t pos)
 	{
-		BtrfsIoctlSearchKey sk;
+		BtrfsIoctlSearchKey sk(sc_extent_fetch_max * (sizeof(btrfs_file_extent_item) + sizeof(btrfs_ioctl_search_header)));
 		if (!m_root_fd) {
 			m_root_fd = m_fd;
 		}
diff --git a/lib/fs.cc b/lib/fs.cc
index 7078fe3..16e5649 100644
--- a/lib/fs.cc
+++ b/lib/fs.cc
@@ -711,7 +711,9 @@ namespace crucible {
 	BtrfsIoctlSearchKey::do_ioctl_nothrow(int fd)
 	{
 		vector<char> ioctl_arg = vector_copy_struct<btrfs_ioctl_search_key>(this);
-		ioctl_arg.resize(sizeof(btrfs_ioctl_search_args_v2) + m_buf_size, 0);
+		// Normally we like to be paranoid and fill empty bytes with zero,
+		// but these buffers can be huge.  80% of a 4GHz CPU huge.
+		ioctl_arg.resize(sizeof(btrfs_ioctl_search_args_v2) + m_buf_size);
 		btrfs_ioctl_search_args_v2 *ioctl_ptr = reinterpret_cast<btrfs_ioctl_search_args_v2 *>(ioctl_arg.data());
 
 		ioctl_ptr->buf_size = m_buf_size;
diff --git a/src/bees-roots.cc b/src/bees-roots.cc
index 251374c..4bb19eb 100644
--- a/src/bees-roots.cc
+++ b/src/bees-roots.cc
@@ -644,7 +644,7 @@ BeesCrawl::fetch_extents()
 
 	Timer crawl_timer;
 
-	BtrfsIoctlSearchKey sk;
+	BtrfsIoctlSearchKey sk(BEES_MAX_CRAWL_SIZE * (sizeof(btrfs_file_extent_item) + sizeof(btrfs_ioctl_search_header)));
 	sk.tree_id = old_state.m_root;
 	sk.min_objectid = old_state.m_objectid;
 	sk.min_type = sk.max_type = BTRFS_EXTENT_DATA_KEY;