Tag Archives: PSA

VCAP-DCA Study Notes – 1.3 Complex Multipathing and PSA plugins

This section overlaps with objectives 1.1 (Advanced storage management) and 1.2 (Storage capacity) but covers the multipathing functionality in more detail.

Knowledge

  • Explain the Pluggable Storage Architecture (PSA) layout

Skills and Abilities

  • Install and Configure PSA plug‐ins
  • Understand different multipathing policy functionalities
  • Perform command line configuration of multipathing options
  • Change a multipath policy
  • Configure Software iSCSI port binding

Tools & learning resources

Understanding the PSA layout

The PSA layout is well documented here, here. The PSA architecture is for block level protocols (FC and iSCSI) – it isn’t used for NFS.

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Terminology;

  • MPP = one or more SATP + one or more PSP
  • NMP = native multipathing plugin
  • SATP = traffic cop
  • PSP = driver

There are four possible pathing policies;

  • MRU = Most Recently Used. Typically used with active/passive (low end) arrays.
  • Fixed = The path is fixed, with a ‘preferred path’. On failover the alternative paths are used, but when the original path is restored it again becomes the active path.
  • Fixed_AP = new to vSphere 4.1. This enhances the ‘Fixed’ pathing policy to make it applicable to active/passive arrays and ALUA capable arrays. If no user preferred path is set it will use its knowledge of optimised paths to set preferred paths.
  • RR = Round Robin

One way to think of ALUA is as a form of ‘auto negotiate’. The array communicates with the ESX host and lets it know the available path to use for each LUN, and in particular which is optimal. ALUA tends to be offered on midrange arrays which are typically asymmetric active/active rather than symmetric active/active (which tend to be even more expensive). Determining whether an array is ‘true’ active/active is not as simple as you might think! Read Frank Denneman’s excellent blogpost on the subject. Our Netapp 3000 series arrays are asymmetric active/active rather than ‘true’ active/active.

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