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Nodes in Metro or Global Mirror Inter-cluster Partnerships May Reboot if the Inter-cluster Link Becomes Overloaded

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Abstract


SVC, Storwize V7000 or Storwize V7000 Unified nodes in Metro or Global Mirror inter-cluster partnerships may experience lease expiry reboot events if an inter-cluster link to a partner system becomes overloaded. These reboot events may occur on all nodes simultaneously, leading to a temporary loss of host access to Volumes.

Content

If an inter-cluster link becomes severely and abruptly overloaded, it is possible for the local fibre channel fabric to become congested to the extent that no fibre channel ports on the local SVC, Storwize V7000 or Storwize V7000 Unified nodes are able to perform local intra-cluster heartbeat communication. This may result in the nodes experiencing lease expiry events, in which a node will reboot in order to attempt to re-establish communication with the other nodes in the system. If all nodes lease expire simultaneously, this may lead to a loss of host access to Volumes for the duration of the reboot events.

Workaround

The recommended default zoning recommendation for inter-cluster Metro and Global Mirror partnerships has now been revised to ensure that, if link-induced congestion occurs, only two of the four fibre channel ports on each node are able to be subjected to this congestion. The remaining two ports on each node will remain unaffected, and therefore able to continue performing intra-cluster heartbeat communication without interruption.



The revised zoning recommendation is as follows:

  • For each node in a clustered system, exactly two fibre channel ports should be zoned to exactly two fibre channel ports from each node in the partner system.
  • If dual-redundant ISLs are available, then the two ports from each node should be split evenly between the two ISLs, i.e. exactly one port from each node should be zoned across each ISL.
  • Local system zoning should continue to follow the standard requirement for all ports on all nodes in a clustered system to be zoned to one another.

This implies that for each system, there will be two ports on each SVC node that have no remote zones, only local zones.


Please refer to the What's new in the IBM System Storage SAN Volume Controller Information Center for more information on SVC Terminology changes.

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Document Information

Modified date:
28 March 2023

UID

ssg1S1003634

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