Fuse-xfs

static void xfs_lookup(fuse_req_t req, fuse_ino_t parent, const char *name) { struct xfs_inode *ip = xfs_iget(parent); xfs_dirent_t *de = xfs_dir_lookup(ip, name); fuse_reply_entry(req, &(struct fuse_entry_param){ .ino = de->inumber, .generation = ip->i_generation, .attr_timeout = 1.0, .entry_timeout = 1.0 }); } XFS divides the disk into equal-sized Allocation Groups. In fuse-xfs , each AG is a mmap() of a region in a backing file ( /var/lib/fuse-xfs/ag0.bin ). Reads and writes become pointer dereferences.

Want to understand delayed allocation? Step through xfs_iomap_write_delay() in userspace with printfs . Curious about AG btree splits? Corrupt an AG by writing random bytes and watch fuse-xfs segfault at the exact line of code where validation fails. fuse-xfs

Why? Because XFS inodes have a generation number (to handle inode reuse), and the low-level API lets us pass that back to the kernel’s dcache. Want to understand delayed allocation

This is where the kernel-to-userspace shift gets interesting. In the kernel, XFS uses xfs_buf_t with b_ops for verification. In fuse-xfs , we just cast: Corrupt an AG by writing random bytes and