Pluggable Storage Architecture (PSA) is an open and modular framework that coordinates various software modules responsible for multipathing operations.

VMware provides generic native multipathing modules, called VMware NMP and VMware HPP. In addition, the PSA offers a collection of VMkernel APIs that third-party developers can use. The software developers can create their own load balancing and failover modules for a particular storage array. These third-party multipathing modules (MPPs) can be installed on the ESXi host and run in addition to the VMware native modules, or as their replacement.

When coordinating the VMware native modules 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 queueing to the logical devices.
  • Implements logical device bandwidth sharing between virtual machines.
  • Handles I/O queueing to the physical storage HBAs.
  • Handles physical path discovery and removal.
  • Provides logical device and physical path I/O statistics.

As the Pluggable Storage Architecture illustration shows, multiple third-party MPPs can run in parallel with the VMware NMP or HPP. When installed, the third-party MPPs can replace the behavior of the native modules. The MPPs can take control of the path failover and the load-balancing operations for the specified storage devices.

Figure 1. Pluggable Storage Architecture
The image shows third-party MPPs running in parallel with the VMware NMP.