Load balance between controllers within an EVA

[Kalin Tzenov]

Hi all,

I have an Alpha ES47 running VMS 8.3 with 2 HBA's connected to 2 Brocade switches and 2 EVA 8000. All my disk drives are shadowed between the 2 EVA's. After the initial setup the fibre channel paths were very well balanced between the HBA's, between controllers A and B within each EVA and even between ports within a controller.

Then, controller B on one EVA had to be replaced and all drives switched their current paths to controller A as they were supposed to. From that moment on all drives on this EVA have their active paths through controller A, even after numerous reboots. Looks like VMS "remembered" the paths after the controller replacement and got stuck with them. On the other EVA the paths are balanced between the controllers.

My question is if there is a way to force VMS to balance paths between the 2 controllers. I know I can switch paths manually, but I'd rather let VMS do the balancing.

Thanks,

[Hoff]

EVA controllers can be set to revert to a failed path (the so-called "fallback" or sometimes "failback" option), or to allow OpenVMS to determine a functional path, or to ask for the OpenVMS system manager to switch the paty manually via the SET DEVICE /SWITCH /PATH=mumble request.

AFAIK, OpenVMS doesn't do any particular automatic load balancing here; you get to do that yourself.

Certain of the EVA series controllers can have some LUN migration capabilities here.

[Robert Brooks_1]

My question is if there is a way to force VMS to balance paths between the 2 controllers. I know I can switch paths manually, but I'd rather let VMS do the balancing.

--

Hoff's reply is correct; multipath does not do any path load balancing once the system is booted. At boot time, there is an attempt made to distribute the "current paths" for various devices across any available direct (non-MSCP) path, but even that lightweight effort is subject to a few weird limitations.

-- Rob

[Kalin Tzenov]

Thanks both of you for the reply.

Yes, I know at boot time there is an effort to balance the devices current paths among the available paths, but it doesn't work in my case. On the contrary, devices with active paths to controller B are switched to controller A during boot. Again, it looks like VMS remembers the paths before reboot and re-establishes them. On the other EVA the paths are balanced.

Anyway, apparently I'll have to do the balancing manually.

Thanks,

[Hoff]

There are some details of the EVA configuration that can cause this, and there could be an actual problem with the B path, too.

[Robert Brooks_1 · Accepted solution]

Again, it looks like VMS remembers the paths before reboot and re-establishes them.

--

I can state with absolute authority that VMS does not "remember" any path information across a reboot.


-- Rob (ex VMS Engineer who worked on mulitpath, among other things)

[Robert Atkinson]

Kalin, unless you have the configuration set to 'Failover/Failback', the paths won't switch back after replacing the controller.

What's more, even if you manually switch the path back to the other controller from VMS, the EVA works in such a way that all requests are passed from the requested path/controller to the managing controller.

The good news is that you can force the path over by going to the virtual disk, clicking on the Presentation tab and changing the 'Preferred Path/Mode'.

Unfortunately, unless using SSSU, you can't see all of the disks managed by a controller on a single screen. But, I'm guessing you already know how you want you're disks balanced anyway.

Hope this helps.

Rob.

[Kalin Tzenov]

Thanks all,

Yes, I know how to force the path from the EVA and from VMS. I was just wondering whether there is a way to do balance from VMS. As it turned out, there is not one. So the preferred path it is...

[Robert Atkinson]

Kalin, assuming all things being equal, VMS (especially 8.3) will path switch for you, and balance out the available paths to the controllers.

Although unconfirmed and unseen, the controllers should do similar if you have the configuration set correctly.

Rob.

[Kalin Tzenov]

"Kalin, assuming all things being equal, VMS (especially 8.3) will path switch for you, and balance out the available paths to the controllers"

Rob,
I thought so too, that's why I was surprised and posted this thread. I double- and triple-checked the controllers' configuration, it's identical for both controllers and it's also identical with the other EVA's controllers where everything works fine.