It tries to recover all inodes on an XFS filesystem marked as deleted. It's rather dumb, it just looks for the magic string IN\0\0\3\2\0\0 and considers those as deleted inodes. Then, it tries to make sense of the extents stored in the inode (which XFS does not delete) and collect the data blocks of the file. That file is then stored on another filesystem in a subdirectory, by default xfs_undeleted relative to the current directory. ianka/xfs_undelete