DRBD secondary to primary:
drbdadm disconnect all
drbdadm primary r0 --force
mount /dev/drbd0 /mnt
[drbdadm connect r0] to wait old primary restore
Old primary to slave:
Then restore drbdadm primary r0, sendone is drbdadm secondary r0
drbdadm disconnect r0
umount /mnt
drbdadm secondary r0
drbdadm -- --discard-my-data connect r0
