If I do vgdisplay vg01, I see it is using 1 PV: /dev/dm-13
If I type multipath -ll I see dm-9, dm-10, dm-11, dm-12, but do not see dm-13. Is my vg01 multipathed? How can I actually know for sure?
Secondly, let's say this time vg01 says it has one PV: /dev/sdab
I type multipath -ll and sure enough I see /dev/sdab but under mpath31. Is my vg01 multipathed even though I'm point my PV path to sdab instead of mpath31?
My expertise is in HP-UX and I haven't been doing Linux very long. Our resident Linux "experts" do not know the answers to these very important questions so I would appreciate any help or advice you can give.
You should be using /dev/mapper/<mpath> device, which is created when you rescan the fabric and disk is available.
Also it is a good practice (but not neccesary) to define alias for your new disk with a meaningful name for that disk. You will have then /dev/mapper/<mpathalias> for using with pvcreate / vgcreate / vgextend etc. after you restart multipath daemon.
You might want to check the output of dmsetup ls and/or dmsetup info if your volume group(s) are using mpath or mpathalias devices.
If you used /dev/sd devices then there is a good chance multipath is not working for that disk, but i cannot say for sure.
Best is to test it on test machine using /dev/sd device (wrong way not per documentation) then removing one path physically.