topic Re: Principal switch election in HPE EVA Storage

Principal switch election

Rene Heeres — Tue, 14 Dec 2004 03:40:47 GMT

We currently use a SAN that consists of 2 fabrics. In this SAN I am a the moment busy with replacing the current switches in both fabrics with a 2/128 Director switch and a 2/16 switch for each fabric.

Some time ago I connected the 2 Directors to the existing fabrics so I can do the migration in phases. Both Directors are subordinate switches in the fabric, because the 1Gbit switches in the existing fabrics already were the principal.

Before disabling the 1Gbit switch the configuration of each fabric was:
1 2/128 Director switch --> subordinate
1 2/16-EL switch --> subordinate
1 2/16 switch --> subordinate
1 1/16 switch --> principal

When I disabled the 1Gbit switch the Director did not become the principal, but the 2/16 switch became the principal. This is not what I expected, I assumed the Director would become the principal switch, because it is the largest switch in the fabric.

How is the principal switch in a fabric being elected? I know it is normally the switch that is first turned on in the fabric, but how does it work in an existing fabric when the principal leaves the fabric?

How can I force the 2/128 Director to be the principal switch?

Is it a problem that the Director is not the principal?

Re: Principal switch election

Steven Clementi — Tue, 14 Dec 2004 09:56:36 GMT

Rene:

"...but how does it work in an existing fabric when the principal leaves the fabric?"

Can you show us a diagram of how your switches are interconnected to each other? I think that the order would be the same as the boot order... since the 1/16 was the principal and the 2/16 was possibly already in place with the 2/128 being added afterwards, the 2/16 would be "next in line". I do not know if this is the case or if there is any logic behind the scenes that helps selects which becomes the principal.

"How can I force the 2/128 Director to be the principal switch? "

If you reboot both the 2/16 and the 2/16el, the director should become the principal. Not the best way since you will disrupt production, but if done off hours....

Another way would be to shutdown everything then reboot in a specific order... the 2/128 first, then the 2/18 then the 2/18el then your storage....then tape devices...then servers.

"Is it a problem that the Director is not the principal?"

I do not see a problem with any switch being subordinate or principal unless you WANT to see a particular switch as the principal. Some environments are like that. ;o)

Steven

Re: Principal switch election

Rene Heeres — Tue, 14 Dec 2004 10:55:28 GMT

Steven,

Thank you for your reply. I have included a sheet with the current configuration of one fabric (both are configured the same).

Rebooting the other switches is not a good option because this would mean downtime for some servers connected to the fabric. We have a number of systems that have only one connection to the SAN, this is due to an explosive grow in servers connected to the SAN and a shortage of fiber uplinks to the Directors. This is also why we have placed a 2/16 switch in a rack with blade servers (switch12).

I prefer the Directors being the principal switches in the fabric, because they are the Core switches in our SAN. Eventually they will become the principal anyway, but I am just wondering if it is possible to force them to become the principal.

I'm interested in how the election of the principal switch in a fabric works, is there any information about that?

Re: Principal switch election

Bill Rothanburg — Thu, 16 Dec 2004 10:01:13 GMT

Rene,

The default behavor of the switches in the fabric is that the switch with the lowest WWN will become principal.

Starting at FabricOS versions 4.1.0 and 3.1.0, a fabricPrincipal option was added to set a prefered Principal switch for the fabric.

When this option is set, the switch with the lowest WWN of the switches that have the option set will become the principal.

Note, when setting this option, a new principal is not elected until there is a fabric reconfiguration. This usually happens when a switch joins or leaves the fabric. (The switchDisable and switchEnable commands can cause this to happen as well as a switch reboot.)

Typically, it is not a problem that the Director is not the principal.

If the director is a core switch in the SAN, then it might be preferable to have it as the principal. This is especially true in a larger fabric with many edge switches.

Regards,

Bill

Re: Principal switch election

Rene Heeres — Fri, 17 Dec 2004 07:22:57 GMT

Bill,

I checked the WWID's and the switch with the lowest WWID indeed is the principal!!

I believe the correct name for the setting is: fabric.principalSwSelMode. Can I modify this parameter on a running switch or do I have to disable the switch to change this parameter?