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Quick Note on ESX and ESXi Storage Multipathing

Rich Brambley (of VM /ETC), Duncan Epping (of Yellow Bricks), and I were having a brief discussion on Twitter late last week about storage multipathing. Rich initiated the discussion with this update, which prompted me to respond and thus started the conversation. Rich and I continued the conversation via e-mail (Twitter isn’t exactly the best medium for that kind of exchange), and prompted by his questions I did some digging. Here’s what I came up with.

This NetApp KB article (NOW login required to view) is kind of what kick-started the entire thing. In that article, NetApp recommends that users set the preferred path for every LUN anytime an ESXi server is rebooted. This behavior seemed curious to me, so I inquired with some industry contacts to see where this recommendation may have originated.

One of my contacts responded that the vicfg-mpath command, part of the Remote CLI used to manage ESXi, is broken in that it lists the target’s WWN not the target’s WWPN. Apparently this will be addressed in a future version of the vicfg-mpath tool. To compound the issue, setting preferred paths from VirtualCenter is apparently not persistent across reboots because the VML LUN name is not used. Hence, when working with ESXi, neither method of setting the preferred path—vicfg-mpath or VirtualCenter—can provide persistent settings. Hence, the recommendation in the NetApp KB article.

But what about VMware ESX? The issue about using VirtualCenter remains, but esxcfg-mpath allows users to use the VML LUN name during the command to ensure that paths are persistent across reboots. In fact, there’s a note in the help screen of esxcfg-mpath (esxcfg-mpath -h) that mentions the fact that VMHBA names are not persistent across reboots, and users should use VML LUN names to be sure of consistency.

So there you have it—when using ESXi you’ll want to set the preferred path for a LUN when you reboot, but if you using ESX you can avoid that trouble by using the VML LUN name in the esxcfg-mpath command. As clear as mud, right?

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