From http://www.vmware.com/support/vsphere4/doc/vsp_40_new_feat.html:
Pluggable Storage Architecture — The Pluggable Storage
Architecture (PSA) is an open modular framework that enables
third-party storage multipathing solutions for workload balancing and
high availability. You can use the vSphere CLI or vCenter Server to
manage paths controlled by the default Native Multipathing. If
array-specific functionality is required, a third-party plug-in using
the vStorage API for Multipathing can be configured using the vSphere CLI. See the Fibre Channel SAN Configuration Guide or the iSCSI SAN Configuration Guide.
From http://www.vmware.com/pdf/vsphere4/r40/vsp_40_iscsi_san_cfg.pdf:
By default, the VMware NMP supports the following PSPs:
Most Recently Used (MRU)
Selects the path the ESX/ESXi host used most recently to access the given device.
If this path becomes unavailable, the host switches to an alternative path and
continues to use the new path while it is available.
Fixed
Uses the designated preferred path, if it has been configured. Otherwise, it uses
the first working path discovered at system boot time. If the host cannot use
the preferred path, it selects a random alternative available path. The host
automatically reverts back to the preferred path as soon as that path becomes
available.
NOTE With active-passive arrays that have a Fixed path policy, path thrashing
might be a problem.
Round Robin (RR)
Uses a path selection algorithm that rotates through all available paths enabling
load balancing across the paths.
