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Status Future consideration
Created by Guest
Created on Sep 7, 2023

Check the GM subordinate even if there are no copyset in it.

Currently CSM only queries for copysets that are actively participating in a session. If we have a session with a subordinate with no more copysets on the subordinate, CSM doesn't check the state of the subordinate. Adding a check of the subordinate wether there are copyset in it or not can prevent some issue like the one we had below:

We had a CSM session where there was a control path to a subordinate DS8K with no more copyset in it (we forgot to delete the control path in the port pairing csv). There was no more data in the subordinate and even the LCUs were delete. When we wanted to pause the GM with CSM to do a practice, the command failed because there was still a control path to the old subordinate.

Idea priority Low
  • Guest
    Sep 14, 2023

    I forgot to link the case from which this idea is issued : TS014021259

  • Guest
    Sep 7, 2023
    We'll look into this, however, I want to make sure you understand the full reason this isn't done today. Once the hardware has defined a Master/Subordinate topology for Global Mirror, you cannot dynamically change it. In other words, for CSM to automatically remove the subordinate it would have to Pause cg formation and then rebuild the Master topology without the subordinate box. We automatically do this today in the event that the Master is no longer being used, but, there's risk in doing that for the subordinate as well because we would be stopping cg formation without actual customer consent to do so. This could mean that SLAs get missed as RPO would increase until the new topology starts forming cgs again.

    It is far better to remove a storage system from the topology, in my opinion, during a planned event window. This means the customer schedules for the removal of the storage system at which time they can manually Suspend, Recover and then restart the CSM session. When you issue the recover CSM will terminate the Master such that on the restart the topology will be rebuilt without the old subordinate that is no longer in use.

    In other words the steps to remove a primary storage system from a Global Mirror session are:
    1. Remove all copy sets pertaining to that storage system from the session
    2. Plan a time when the Global Mirror session can be Suspended meaning cg formation is temporarily paused.
    3. Remove the control paths from the port pairing file
    4. Issue the Suspend command to the session.
    5. Issue the Recover command to the session.
    6. Issue the corresponding Start Hx->Hy command to the session to restart the session in the same direction it was previously running.
    7. Now the storage system can be safely removed from the environment.