Here’s a gotcha that had me scratching my head for a few minutes. I had 2 ESX servers connected to the same SAN storage.  All LUNs had been presented to both ESX servers though they both had different LUN ID’s allocated to each LUN within VMware ESX.

As a result vMotioning of VM’s with Raw Device Mappings (RDMs) was not possible. 

So what was causing this numbering/LUN ID discrepancy? 

The answer is that the LUN IDs within ESX, when an ESX server is connected through to a SAN (this is true for most SANs though apparently not all), are based on the servers SAN allocated LUN ID.

What had happened in this instance is that when the LUNs were presented through to the ESX server at a SAN level it had been done in a different order than with the previous ESX server.  As a result different SAN LUN IDs had been assigned on the new ESX server which then fed through to the LUN IDs allocated by ESX.

To resolve it I turned off the ESX server, unpresented the LUNs at the SAN and then re-presented them in the same order as (ie: using the SAN LUN ID numbering for the other server as a guide) the first ESX server.  After powering on the ESX server the LUN IDs within ESX then matched the other ESX servers’.  Happy Days !

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