Fuse-xfs

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.

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.

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 . fuse-xfs

So when I decided to write fuse-xfs —a userspace implementation of the —I wasn’t trying to build a production storage engine. I was trying to answer a single question: Can we take the soul of XFS (its allocation groups, B+tree extents, and delayed allocation) and lift it into userspace without losing its identity? Here’s what I learned. The Heresy: Userspace XFS XFS, designed by SGI in the ’90s, is a kernel beast . It assumes it owns the hardware. It assumes it can reorder writes, bypass the page cache when needed, and manipulate memory directly via kmem_cache . Porting that to userspace is not just difficult—it’s borderline heretical.

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. So go ahead

And when someone asks, “Why would you run a filesystem in userspace?” — you’ll know the answer.

But fuse-xfs isn’t a port. It’s a reconstruction . Or fuse-ntfs

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: