At work I used linux native multipathing MPIO provided via device mapper a lot. I can be a bit hard to troubleshoot. I have found the best way is to use the interactive console. From there you can do a lot of things I’ll illustrate the three most common I use:
Show current config
# multipathd -k
>> show config
Reload changes to multipath.conf
In addition the following command out puts a log of debug information that can really help.