Multipath itself is pretty easy to get going, you need the device-mapper-multipath rpm installed. Create an /etc/multipath.conf file, I have a pretty basic one which should allow multiple paths to be active (see notes below on dual active though!)
path_selector "round-robin 0"
I've also configured the /etc/multipath/bindings file to have friendly names for each LUN id, e.g.:
But why aren't all the paths active?When looking at the multipath config with it running (multipath -l), I noticed that all the paths weren't active - some were marked as status=enabled rather than status=active.
Of course, it turns out that the v3700 controllers are dual-active rather than active/active. i.e. they are both active, but a LUN is only active on one controller at a time, this means that when using multi path, we'll see that one port is in status active, and the second in status enabled:
v3700_ds01_md_lun01 (360012345001234567800000000000000) dm-2 IBM,2145
size=5.5T features='1 queue_if_no_path' hwhandler='0' wp=rw
|-+- policy='round-robin 0' prio=0 status=active
| `- 1:0:0:0 sdb 8:16 active undef running
`-+- policy='round-robin 0' prio=0 status=enabled
`- 2:0:0:0 sdl 8:176 active undef running
So this is where I think some more SAS cables might be in order so that the second port on each SAS card can be cross connected to the other controller, this will mean that all cards will be attached to each controller once, and we'll have maximum connectivity and fault tolerance for the system that we can.