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.

There’s a moment in every systems programmer’s life where they stare at a kernel panic, a corrupted superblock, or an unreachable inode, and think: “I wish I could just put a breakpoint inside the filesystem.” fuse-xfs

But fuse-xfs isn’t a port. It’s a reconstruction . So go ahead

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. Or fuse-ntfs

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.

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:

The solution? . When fuse-xfs opens a file, it walks the entire B+tree and caches the extent list in a flat array. Memory-heavy? Yes. But it turns a 10ms seek into a 50µs array walk. 4. Writing: The Journaling Shim XFS’s journal (the “log”) is complex. It supports rolling transactions, buffer pinning, and tail pushing. fuse-xfs implements a naïve log : each write transaction is appended to a journal.bin file. On mount, we replay by applying every logged operation in order.