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Dell EMC Metro node 7.1 Administrator Guide

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Operating a consistency group

In the event of a cluster partition, the best practice is to allow I/O to continue at only one cluster. Allowing I/O to continue at both clusters result in a condition that is known as a conflicting detach. The resolution of the conflicting detach result in the complete resync of the losing cluster from the winning cluster. All writes at the loser cluster are lost.

About this task

When I/O continues at both clusters:

  • The data images at the clusters diverge.
  • Legs of distributed volumes are logically separate.

When the inter-cluster link is restored, the clusters learn that I/O has proceeded independently. I/O continues at both clusters until you pick a winning cluster whose data image will be used as the source to synchronize the data images.

In the following example, I/O resumed at both clusters during an inter-cluster link outage. When the inter-cluster link is restored, the two clusters come back into contact and learn that they have each detached the other and carried on I/O.

Steps

  1. Use the ls command to display the consistency group’s operational status at both clusters. 
    VPlexcli:/clusters/cluster-1/consistency-groups/cg1> ls
Attributes:
Name                  Value
--------------------  -----------------------------------------
active-clusters       [cluster-1, cluster-2]
cache-mode            synchronous
detach-rule           no-automatic-winner
operational-status    [(cluster-1,{ summary:: ok, details:: [requires-resolve-conflicting-detach] }), 
                      (cluster-2,{ summary:: ok, details:: [requires-resolve-conflicting-detach] })]
passive-clusters      []
recoverpoint-enabled  false
storage-at-clusters   [cluster-1, cluster-2]
virtual-volumes       [dd1_vol, dd2_vol]
visibility            [cluster-1, cluster-2]
Contexts:
advanced  recoverpoint
  2. Use the resolve-conflicting-detach command to select cluster-1 as the winner. 
    VPlexcli:/clusters/cluster-1/consistency-groups/cg1> resolve-conflicting-detach -c cluster-1
This will cause I/O to suspend at clusters in conflict with cluster cluster-1, allowing you to stop applications at those clusters. Continue? (Yes/No) Yes

    Cluster-2’s modifications to data on volumes in the consistency group since the link outage started are discarded.

    Cluster-2's data image is then synchronized with the image at cluster-1.

    I/O gets suspended at cluster-2 if the auto-resume policy is false.

  3. Use the ls command to verify the change in operation status: 
    VPlexcli:/clusters/cluster-1/consistency-groups/cg1> ls
Attributes:
Name                 Value
-------------------  ----------------------------------------------------------
active-clusters      [cluster-1, cluster-2]
cache-mode           synchronous
detach-rule          no-automatic-winner
operational-status   [(cluster-1,{ summary:: ok, details:: [] }),
                      (cluster-2,{ summary:: suspended, details:: [requires-resume-at-loser] })]
passive-clusters     []
recoverpoint-enabled false
storage-at-clusters  [cluster-1, cluster-2]
virtual-volumes      [dd1_vol, dd2_vol]
visibility           [cluster-1, cluster-2]
Contexts:
advanced  recoverpoint

    • At cluster-1, I/O continues, and the status is ok.

    • At cluster-2, the view of data has changed and hence I/O is suspended.

  4. Use the consistency-group resume-at-loser command to resume I/O to the consistency group on cluster-2.

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