I recently realized that our default VMware environment using our EMC CX4 may not be using the ideal pathing configuration. Our environment consists of ESX servers with 2 HBAs connected to 2 FC switches. Each FC switch is then connected to both Storage Processors on the EMC SAN. This means there are 4 paths from the ESX host to any given LUN, but VMware only actively used 1 path by default.
You can confirm this by selecting a host and clicking the Configuration tab and selecting Storage. Select a VMFS datastore and click Properties | Manage Paths. If your environment is like mine, Path Selection will be Most Recently Used and only 1 path will be listed as Active (I/O).
To improve this, 2 things need to happen: The Path Selection policy in VMware needs to be changed, and the FailOver mode on the SAN needs to be changed. To change the Path Selection policy, click the drop-down list and select Round Robin and click Change.
To change the FailOver mode we can either use the GUI or command line. In the GUI, select Tools | Failover Setup Wizard and follow the steps, choosing failover mode of 4. To use the command line (useful if you have more than a handful of systems) do the following:
Find current failover mode:
NaviSECCli.exe -user username -password password -scope 0 -h sanAddress port -list -failovermode
This will list the failover mode for all hosts connected to the SAN. Once you find the host your interested in, you will see the Failover mode: is set to 1.
Change failover mode:
NaviSECCli.exe -user username -password password -scope 0 -h sanAddress storagegroup -sethost -host hostname -failovermode 4
Once the failover mode is changed, reboot the ESX host and then review the path configuration for the LUNs. If all went well, the Path Selection should be Round Robin, and Storage array Type should be VMW_SATP_ALUA_CX