Fuse-xfs May 2026

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.

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.”

struct xfs_agf *agf = (struct xfs_agf *)(ag->map + XFS_AGF_OFFSET); if (be32_to_cpu(agf->agf_magicnum) != XFS_AGF_MAGIC) return -EINVAL; // or crash, which is more fun No buffer cache. No I/O scheduling. Just the filesystem’s raw data laid out in virtual memory. XFS’s extent B+tree is elegant: internal nodes point to other blocks, leaves point to extents. In kernel space, traversing it is cheap. In fuse-xfs , every bmap lookup might require reading several blocks—each of which is a pread() or a memory access, depending on your cache. fuse-xfs

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

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. The solution

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.

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 . Memory-heavy

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.