I posted an article on LUN IDs and VCB in November 2008. It still seems to be a misconception that ESX uses LUN IDs to uniquely identify a LUN. As of 3.5 this isn’t the case anymore. When an array has “NAA Identifier” capabilities these will be used for uniquely identifying LUNs. And yes most arrays, currently, have these capabilities.
The NAA ID is also what’s being used to identify SAN LUN snapshot/clones by the way. ESX 3.5 compares the LUN ID to the metadata of the VMFS header, if it’s a different ID ESX knows it can’t be the same LUN that’s being presented and ignores it. If you do want to use the LUN you would either have to resignature it or set “disallowsnapshotlun” to “0” of course…
Keep in mind, it’s still a best practice to use consistent LUN IDs throughout your environment. ESX doesn’t care anymore, but your life is a Sys Admin will be a lot easier if you use unique and consistent LUN numbering.
Does this mean that the second LUN with the same LUN ID will be ignored / won’t work? Or LUN IDs are simply ignored as far as ESX is concerned and only the NAA ID really matters?
Yes only the NAA ID is used when it is available.