The file is divided into 4KiB blocks, where each block is either used or unused. If it is used, it contains a B-tree node, with the directory entries and block

The file is divided into 4KiB blocks, where each block is either used or unused. If it is used, it contains a B-tree node, with the directory entries and block number of child nodes. If it is unused, it is chained into a doubly linked free list so that new nodes can be allocated. Basically memory management and B-tree are handled simultaneously in the I/O process.