DELL recently released their new Multipath Extension Module (MEM) for the EqualLogic PS Series of storage array. This updated MEM now supports vSphere 5.0.
I guess I should try to explain what a MEM is before going any further. VMware implements a Pluggable Storage Architecture (PSA) model in the VMkernel. This means that storage array vendors can write their own multipathing modules to plugin to the VMkernel I/O path. These plugins can co-exist alongside VMware's own default set of modules. There are different modules for different tasks in the PSA. For instance, the specific details of handling path failover for a given storage array are delegated to the Storage Array Type Plugin (SATP). SATP is associated with paths. The specific details for determining which physical path is used to issue an I/O request (load balancing) to a storage device are handled by a Path Selection Plugin (PSP). PSP is associated with logical devices. The SATP & PSP are both MEMs (Multipath Extension Modules).
--> Please see the rest on the blog : http://blogs.vmware.com/vsphere/2011/11/dells-multipath-extension-module-for-equallogic-now-supports-vsphere-50.html