When trying to declare two mgsnodes for HA only one entry is ever accepted when using backfstype=zfs. All below examples fail, if the --mgsnode=nid:nid syntax worked then all might be okay.

mkfs.lustre --reformat --fsname=RSF1 --ost --index=2 --mgsnode=10.82.0.9@tcp1 --mgsnode=10.82.0.9@tcp1:10.82.0.10@tcp1 --servicenode=10.82.0.11@tcp1:10.82.0.12@tcp1 --backfstype=zfs OST2/ost

mkfs.lustre --reformat --fsname=RSF1 --ost --index=2 --mgsnode=10.82.0.9@tcp1:10.82.0.10@tcp1 --servicenode=10.82.0.11@tcp1:10.82.0.12@tcp1 --backfstype=zfs OST2/ost