what operations does the PSA perform?

When coordinating the VM NMP and any installed 3-party MPP, what operations does the PSA perform?

A.
Implements logical devices bandwidth sharing

B.
Process I/O request to logical devices

C.
Associate physical path with logical devices

D.
Manage physical path claiming and unclaiming

Explanation:
it is ,,Implements logical devices bandwidth sharing", as the other alternatives are performed by NMP. For reference please refer to page 23-24 of this document http:// www.vmware.com/pdf/vsphere4/r40/vsp_40_san_cfg.pdf

http://deinoscloud.wordpress.com/2011/02/28/vmware-psa-mpp-nmp-psp-mru-and-tutti-quanti/
What does PSA do?

It coordinates the simultaneous operation of multiple multipathing plug-ins (MPPs). Yes that means you could have multiple and different plug-ins installed. But so far only EMC came with MPP plug-in called PowerPath/VE. By the way did you know that you can use EMC PowerPath/VE to provide multipathing and load balancing path management to EMC storage devices but also to any other storage vendors.

When coordinating the VMware NMP and any installed third-party MPPs, the PSA performs the following tasks

Loads and unloads multipathing plug-ins.
Hides virtual machine specifics from a particular plug-in.
Routes I/O requests for a specific logical device to the MPP managing that device.
Handles I/O queuing to the logical devices.
Implements logical device bandwidth sharing between virtual machines. (A)
Handles I/O queueing to the physical storage HBAs.
Handles physical path discovery and removal.
Provides logical device and physical path I/O statistics.