Guan,
If per-controller block device nodes are hidden, how can the user-space tools such as multipath-tools and nvme-cli (if it supports) know status of each path of the multipath device?
if at all, the path state is reflected on the controller class device node, not on the namespace block device node. However, this is not something user-space can rely on as well.
In some cases, the admin wants to know which path is in down state , in degraded state such as suffering intermittent IO error because of shaky link and he can fix the link or isolate such link from the normal path.
We have a ctrl_loss_tmo that will remove such a degraded controller after timeout expiration (default is 600 seconds). When we remove this controller we log this information so its available for a sysadmin to fix this if needed/desired. What we would probably need to do, is either to add a nvme-cli "rescan" option to discover and connect to all unconnected controllers so the sysadmin can run after fixing an issue, or introduce a daemon that does this every say 60 seconds or something.
