lfs mirror split -d first tries to destroy the mirror in place: the victim fd is the file itself (fd == fdv, the "purge" path). On master the MDS refuses that with -EINVAL:

mdd_xattr_split() -> lod_declare_xattr_set() -> lod_save_layout_gen_intrans()
"gen changed from 5 to 6 in trans declaration"

The check comes from LU-17048 (2023). The split modifies the layout of the same object twice while the transaction is declared, so the second declare sees a new generation.

lfs hides this. On -EINVAL or -EBUSY it assumes an old MDS ("could be old MDS which prohibit fd==fdv") and retries with a volatile file in the parent directory. So the in-place path, which needs no parent directory access, never works on a current MDS. A caller without a lfs-style fallback gets -EINVAL.

Found while writing llapi_mirror_split() for lfu-purge (LU-20462), on master with a 2-mirror file.