You can’t. Not easily. The kernel is a fortress, and filesystems are its moat. Enter (Filesystem in USErspace). It’s the drawbridge. But FUSE has a reputation: it’s slow, it’s “toy” grade, and it lacks the low-level power of ext4 or xfs .
But fuse-xfs isn’t a port. It’s a reconstruction . fuse-xfs
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. You can’t
So go ahead. Write your own fuse-ext4 . Or fuse-zfs . Or fuse-ntfs . Mount your system’s root partition read-only and watch every lookup and read call pass through your printf . You’ll never look at df -h the same way again. Enter (Filesystem in USErspace)
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.
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.