TMPFS: FLAG_SET_sappnd -> PHASE1 pwrite(mid) ret=1 errno=0; FLAG_SET_schg -> PHASE2 write ret=1 errno=0
UFS CONTROL: sappnd set (ls -lo shows sappnd) -> PHASE1 pwrite ret=-1 errno=1 (Operation not permitted); PHASE2 write ret=1 (IMMUTABLE open-time-only on UFS as well)

env: DragonFly 6.5-DEVELOPMENT INVARIANTS kernel, QEMU/KVM 6 vCPU, 4GB RAM