Adding SAN LUNs from Different Arrays in VMware

One interesting thing that I ran across recently and I am surprised that I haven’t yet, is adding fiber channel LUN’s to one cluster from different arrays. As a storage guy I did some preliminary research on the deployment since there is very real possibility that I will wind up will multiple LUN ID’s that are the same on the cluster nodes like depicted here:

Everything looks fine, but upon further inspection the redundant paths are down!

Why? Well you can only have one LUN 0 and one LUN 1 on a single controller – (disk controller 101). Remember: Runtime Name is known as A:C:T:L or (Adapter:Controller:Target:LUN). They are all on the same controller!

To solve this, you can do one of two things.

1) The preferred method is to install or present (in the blade world) another set of HBA’s (host bus adapter) to the ESX host if you want to keep the same LUN ID’s and maintain path and card redundancy. This of course is only possible of you have the expansion slots or have not exceeded your maximums of virtual HBAs to the hosts. But what this will do is keep the I/O on its own path out of the host on a separate circuit.

2) Another way is to present the new LUN’s at a different ID and this will keep both paths active . But keep in mind that if you are only running two HBA’s on rack mount systems, you are contending for I/O on those paths. In the blade world its a little different since they are virtual HBA’s and share a common uplink out of the chassis to the fabric.

Rick

2 comments

  1. talisson says:

    I’m not so sure about that. For some reason that I can’t remember, I decided during a migration project that you can have the same LUN number as long as the array is different.
    And it seems to work, I have multiple LUNs with the same LUN ID, from 3 different arrays presented to the same host(s). About 30TB of disk and dozens of VMs on those LUNs.
    What differs is the target. All paths are up and there have been no complaints, but this post does worry me. So I need to look into it further I guess.

  2. Niall says:

    Hi Rick,

    I read your article as i was wondering about this in our own environment.
    I have been running multiple FC storage arrays to ESX clusters for 5 or 6 years now and i’ve never seen that problem?
    I checked my paths today on a few LUNs that were the same ID and there is no path issues.
    As well as that, i also checked the vmkernel logs and warning logs and there are no issues coming up in there either.
    I think it may be specific to a technology you are using in your environment.

    cheers,

    Niall

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