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Understanding PSP

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storage-multipath

PSP – Path Selection Policy (or Path Selection Plug-ins)

PSP is the NMP sub-plugin that tells the host which path to use to reach the storage array, and how to respond if that path goes down. It is used to load-balance across paths if applicable.

Read on to learn about different types, and how to change them!

PSPs

The PSPs below can be used by default:

  • MRU (Most Recently Used)
    • Selects the first working path and only moves when the first path becomes unavailable
    • Default for LUNs from an Active/Passive array
    • Does not automatically fail back
  • Fixed
    • Selects the first working path or the “preferred” path and only moves when it becomes unavailable
    • Default for Active/Active arrays
    • Automatically fails back when the “preferred” path is available
  • RR (Round Robin)
    • Uses an automatic path selection rotating through all available paths, enabling the distribution of load across the configured paths.
      • For Active/Passive storage arrays, only the paths to the active controller will be used in the Round Robin policy.
      • For Active/Active storage arrays, all paths will be used in the Round Robin policy.
    • Configurable with options such as:
      • Number of bytes to send along one path for this device before the PSP switches to the next path.
      • Number of I/O operations to send along one path for this device before the PSP switches to the next path.

Changing the PSP (Command Line)

PSPs are configured on a per LUN basis. Below are the steps you’ll need to take to change the PSP for a given LUN.

  1. SSH to the ESXi host
  2. Identify the device to modify by running: 
    • esxcli storage nmp device list
  3. Modify the PSP by running: 
    • esxcli storage nmp device set -d <device> --psp=<policy>
    • The table below will show the options for <policy>
  4. Confirm changes by running 
    • esxcli storage nmp device list
[ws_table id=”1″]
PSP Options

Changing the PSP (GUI)

To perform the same change using the C# client, do the following:

1. Select a host, the go to the Configuration tab. Select “Storage” from the menu on the left. Highlight the LUN you want to modify and select Properties… on the bottom right.

2. Select “Manage Paths” from the window that comes up

3. From this window, you will be able to change the PSP from the drop down menu

Changing the PSP (Web Client)

1. Select a host, go to the Manage tab, then the Storage section, and select Storage Devices. Highlight a LUN and choose “Edit Multipathing” in the Properties pane.

2. The policy can be changed from the dropdown at the top.

More

For more information regarding PSP, refer to the following KB articles:

http://kb.vmware.com/kb/1011340

http://kb.vmware.com/kb/1036189