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Cormac_Hogan
Posted by Cormac Hogan
Technical Marketing Architect (Storage)

Historically, when a path failure occurred, we never had a way of selecting which path to failover to. Rather, the VMW_PSP_MRU path selection policy randomly selected a different active path to the target from all available paths. In ESX 4.1, VMware introduced a feature to allow admins to prioritize/rank different paths, and the one with the highest priority was chosen in the event of a failure on the active path.

 In ESXi 5, this functionality is now merged into the standard VMW_PSP_MRU. An admin can now assign ranks to individual paths. Ranking goes through the pathgroup states in the following order:

  • ACTIVE
  • ACTIVE_UO (Active Unoptomized – ALUA state)
  • STANDBY

The PSP will then pick a path that has the highest rank for I/O. As long as there are paths in the ACTIVE pathgroup, these will be given preference over paths in the ACTIVE_UO and STANDBY pathgroups, even if the rank of a path in the ACTIVE group is less than that of a path in ACTIVE_UO or STANDBY pathgroup.

When all paths have the same rank, the behaviour is just like normal VMW_PSP_MRU path failover.

Users will be able to get/set path rank using esxcli commands as shown here:

# esxcli nmp psp getconfig –path <vmhba pathname>
# esxcli nmp psp setconfig –config "rank=<number>" –path <vmhba pathname>

# esxcli nmp psp getconfig –device <naa id>
# esxcli nmp psp setconfig –config "rank=<number>" –device <naa id>

The path rank will be persistent across reboots as "esxcli" is used to set path rank. Whenever a path rank is set an entry is made in esx.conf which ensures persistence. A sample esx.conf entry would look like:

/storage/plugin/NMP/device[naa.60060160f1c014002ca7d9e74df2dd11]/psp = "VMW_PSP_MRU_RANKED“
/storage/plugin/NMP/path[fc.2000001b32100b3d:2100001b32100b3d-fc.50060160b0600c6b:5006016030600c6b-naa.60060160f1c014002ca7d9e74df2dd11]/pspConfig = "rank=2"

As long as there are paths in the ACTIVE pathgroup, a path that has the highest rank in ACTIVE pathgroup will be selected. If there are no paths in the ACTIVE pathgroup then the algorithm searches the ACTIVE_UO pathgroup and STANDBY pathgroup in that order. During failover when there are no ACTIVE paths available, ranking will try to activate a path that has the highest rank in ACTIVE_UO state. If no paths are available in ACTIVE_UO state then ranking will pick a STANDBY path that has a highest rank for activation.

Ranking will fail-back to a better ranked path or path with a better state when such a path becomes available. Note that this is safe with respect to path thrashing because VMW_PSP_MRU will never fail-back to a path which requires activation (e.g. ACTIVE to STANDBY or ACTIVE to ACTIVE_UO).

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About the Author

Cormac Hogan

Cormac Hogan is a Senior Staff Engineer in the Office of the CTO in the Storage and Availability Business Unit (SABU) at VMware. He has been with VMware since April 2005 and has previously held roles in VMware’s Technical Marketing and Technical Support organizations. He has written a number of storage related white papers and have given numerous presentations on storage best practices and vSphere storage features. He is also the co-author of the “Essential Virtual SAN” book published by VMware Press.