Ever run into this error trying to add storage to an ESXi host?  “Cannot change the host configuration”.  In my case it was occurring when trying to add a LUN from an EMC VNX5500 being talked to iSCSI over 10gig.  Both the VNX and the iSCSI adapters on the vSphere side were set to 9000 byte MTU, ping worked, switch was set to enable jumbo frames.  Turns out the vSwitch itself was not set to 9000 byte MTU, so although the adapters were, the packets were not making it.

So, if you get that issue, using iSCSI and jumbo frames, check the entire path from iSCSI software adapter through virtual switch (or VDS), physical adapter, Cisco UCS virtual adapter settings if your vSphere “physical” adapter is actually a Cisco UCS system, Cisco UCS VLAN settings (since it controls MTU at VLAN level too), the physical switches and then on to the storage.