The HP EVA comes in two types of storage arrays. An active/passive array and an active/active array. The HP EVA active/active storage arrays can handle I/O requests on both storage processors. With ESX 3.5 and an active/active EVA array you had two options for the path selection policy, fixed and MRU (Most Recently Used). With an active/active EVA array most people used the fix path policy to manually load balance the I/O over both storage processors. This process is very work intensive (it has to be done on all ESX hosts) and is prone to errors (e.g. a fixed path is configured to the wrong storage processor or path trashing).
With the release of vSphere and the vStorage API’s for multipathing, the round robin path selection policy is now fully supported. The HP EVA active/active storage arrays support ALUA which stands for Asymmetrical Logical Unit Access. ALUA occurs when the access characteristics of one port may differ from those of another port. ALUA allows a lun to be accessed via its primary path (via the owning Storage Processor) and via an asymmetrical path (via the not-owning Storage Processor). I/O to the not-owning Storage Processor or not-optimized path comes with a performance penalty because the I/O has to be transmitted over the internal connection between the storage processors which does not have much bandwidth.
With ALUA the ESX host is aware of the non-optimized paths. So when you use the round robin path policy, ESX will load balance the I/O over the optimized paths (the paths to the owning storage processor of the lun) and use the non-optimized paths only in case of a failure on the optimized paths. To my opinion this has two advances: both path are used and manually load balancing isn’t necessary anymore which saves a lot of configuration work.
So here you have it! I recommend to use the round robin path policy with an active/active EVA array. If you don’t want to use the round robin path policy I would recommend to use the MRU policy. The MRU policy is ALUA aware and will also use the optimized paths.