For customers with many LUNs, it can be a serious pain to create, modify, and maintain all the multipathing settings in VMware ESXi... I have a client with four active iSCSI adapters on each host, and four active iSCSI frontend ports on their Compellent SAN, which means a delightful 16 paths per LUN. Couple that with 2-3 LUNs per guest (raw device mappings), and 15 or so guests... that's a lot of clicking.
This is where VMware's vCLI is a godsend... you can script all of those monotonous clicks. For example, here's a pair of scripts I used to convert all of their Path Selection Policies and default PSP's to Round Robin.
1) Download and install vCLI
2) type "connect-viserver hostname" (where hostname is the name of the ESX host you want to make the changes on)
3) type "get-scsilun -vmhost (get-vmhost hostname) -luntype disk | set-scsilun -multipathpolicy "roundrobin""
4) Open an SSH prompt to the ESX host in question (you may need to enable SSH access in your hosts Configuration > Security Profile in vSphere).
5) In the SSH prompt, type "esxcli nmp satp setdefaultpsp --psp VMW_PSP_RR --satp VMW_SATP_DEFAULT_AA
NOTE: The --satp type may vary depending on what SAN you use... you can check what your SATP currently is by clicking "manage paths" of any one of your LUNs in vSphere... under the path selection type it displays it as "storage array type".
That's it... now all the LUNs on the host should be round robin, and all future volumes should default to round robin as well!
For ESXi 5, the SSH command syntax has changed slightly. You'll want to type the following from the console of your ESXi 5 server:
esxcli storage nmp satp set --default-psp VMW_PSP_RR --satp VMW_SATP_DEFAULT_AA
Also, the part about vCLI is no longer necessary! Once you switch the default path policy with he above command, you can just reboot the host and it should apply the policy to all existing volumes. Thanks, VMware!