Understanding VMWare PSA and NMP

Pluggable Storage Architecture (PSA)

To manage storage multipathing, ESX/ESXi uses a special VMkernel layer, Pluggable Storage Architecture (PSA). The PSA is an open modular framework that coordinates the simultaneous operation of multiple multipathing plugins (MPPs). PSA is a collection of VMkernel APIs that allow third party hardware vendors to insert code directly into the ESX storage I/O path. This allows 3rd party software developers to design their own load balancing techniques and failover mechanisms for particular storage array. The PSA coordinates the operation of the NMP and any additional 3rd party MPP.

  

Native Multipathing Plugin (NMP)

The VMkernel multipathing plugin that ESX/ESXi provides, by default, is the VMware Native Multipathing Plugin (NMP). The NMP is an extensible module that manages subplugins. There are two types of NMP subplugins: Storage Array Type Plugins (SATPs), and Path Selection Plugins (PSPs). SATPs and PSPs can be built-in and provided by VMware, or can be provided by a third party.

If more multipathing functionality is required, a third party can also provide an MPP to run in addition to, or as a replacement for, the default NMP.

VMware provides a generic Multipathing Plugin (MPP) called Native Multipathing Plugin (NMP).

What does NMP do?

• Manages physical path claiming and unclaiming.
• Registers and de-registers logical devices.
• Associates physical paths with logical devices.
• Processes I/O requests to logical devices:
  • Selects an optimal physical path for the request (load balance)
  • Performs actions necessary to handle failures and request retries.
• Supports management tasks such as abort or reset of logical devices

MPPs that are not implemented as SATPs or PSPs can be implemented as MPPs instead. MPPs run side by side with NMP. An example of that is EMC PowerPath/VE. It is certified with vSphere 4.x and is planned for vSphere 5