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Path Selection Plugin (PSP)

Figure 5.4 depicts the relationship between SATP, PSP, and NMP.

Figure 5.4

Figure 5.4. PSP

PSPs are PSA plug-ins that handle path selection policies and are replacements of failover policies used by the Legacy-MP (or Legacy Multipathing) used in releases prior to vSphere 4.x.

PSPs handle the following operations:

  • Determine on which physical path to issue I/O requests being sent to a given storage device. Each PSP has access to a group of paths to the given storage device and has knowledge of the paths’ states—for example, Active, Standby, Dead, as well as Asymmetric Logical Unit Access (ALUA), Asymmetric Access States (AAS) such as Active optimized Active non-optimized, and so on. This knowledge is obtained from what SATPs report to NMP. Refer to Chapter 6, “ALUA,” for additional details about ALUA.
  • Determine which path to activate next if the currently working physical path to storage device fails.

NMP communicates with PSPs for the following operations:

  • Set up a new logical storage device and claim the physical paths to that device.
  • Get the set of active physical paths currently used for path selection.
  • Select a physical path on which to issue I/O requests for a given device.
  • Select a physical path to activate when a path failure condition exists.

How to List PSPs on an ESXi 5 Host

To obtain a list of PSPs on a given ESXi 5 host, you may run the following command directly on the host or remotely via an SSH session, a vMA appliance, or ESXCLI:

# esxcli storage nmp psp list

An example of the output is shown in Figure 5.5.

Figure 5.5

Figure 5.5. Listing PSPs

The output shows the default configuration of a freshly installed ESXi 5 host. If you installed third-party PSPs, they are also listed.

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