To manage storage multipathing, ESXi uses a collection of Storage APIs, also called the Pluggable Storage Architecture (PSA). The PSA is an open, modular framework that coordinates the simultaneous operation of multiple multipathing plug-ins (MPPs). The PSA allows 3rd party software developers to design their own load balancing techniques and failover mechanisms for particular storage array, and insert their code directly into the ESXi storage I/O path.

Topics discussing path management use the following acronyms.
Table 1. Multipathing Acronyms
Acronym Definition
PSA Pluggable Storage Architecture
NMP Native Multipathing Plug-In. Generic VMware multipathing module.
PSP Path Selection Plug-In, also called Path Selection Policy. Handles path selection for a given device.
SATP Storage Array Type Plug-In, also called Storage Array Type Policy. Handles path failover for a given storage array.

The VMkernel multipathing plug-in that ESXi provides by default is the VMware Native Multipathing Plug-In (NMP). The NMP is an extensible module that manages sub plug-ins. There are two types of NMP sub plug-ins, Storage Array Type Plug-Ins (SATPs), and Path Selection Plug-Ins (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.

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 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. When installed, the third-party MPPs replace the behavior of the NMP and take complete control of the path failover and the load-balancing operations for specified storage devices.

Figure 1. Pluggable Storage Architecture
The image shows third-party MPPs running in parallel with the VMware NMP.
The multipathing modules perform the following operations:
  • Manage physical path claiming and unclaiming.
  • Manage creation, registration, and deregistration of logical devices.
  • Associate physical paths with logical devices.
  • Support path failure detection and remediation.
  • Process I/O requests to logical devices:
    • Select an optimal physical path for the request.
    • Depending on a storage device, perform specific actions necessary to handle path failures and I/O command retries.
  • Support management tasks, such as reset of logical devices.