Skinny Metadata is a feature added to btrfs in Linux v3.10. The primary motivation is that the overhead imposed by the generic
EXTENT_ITEM record is too great at large scale.
Even in the earliest back reference traversal code, the
key field of the
btrfs_tree_block_info was unused, leaving only the
level field. Given that all tree blocks on a btrfs file system are of identical size, placing the extent size in the
offset field of the key was redundant.
The skinny metadata feature introduces a new
METADATA_ITEM  item type. The
offset field of the key represents the level of the tree that contains the extent. The
btrfs_tree_block_info structure is completely removed from the item, saving 18 bytes (up to approximately 30% smaller) per metadata extent record of which there is one per metadata block on the entire file system.