Procedure: Understanding system volume dependencies

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If one component in a redundant pair is offline or powered off, host access to volumes can be disrupted. Before you remove or power off a node canister or enclosure, verify whether any volumes depend on those components.

The following examples describe scenarios where the system or node status can impact if a volume is available to a host.

  • If a control enclosure has only one node canister online, access to a volume depends on the online node canister. For example, a host might lose access to a volume that is stored partially or wholly on an array that uses drives in the offline node or its expansion enclosures.
  • If one expansion canister in an expansion enclosure is powered off, further expansion canisters to that down side chain get isolated from the control canister of the side chain. In this case, host access to volumes depends on the online canister if the volume uses drives in an isolated enclosure or the enclosure with the offline canister.
  • If an entire expansion enclosure is powered off, both sides of the SAS chain are broken. In this case, host access to some volumes can be considered to depend on the entire expansion enclosure.
The impact that a service procedure might have on host access to data can be understood by using the management GUI. To do so, use the following procedure:
  1. In the management GUI, select Monitoring > System .
  2. On the System -- Overview page, use the directional arrow near the enclosure that contains the node canister to open the Enclosure Details page.
  3. Under Rear View of the system, right-click the canister and select Dependent Volumes from the Actions menu to display all volumes that become disabled to hosts if the canister is powered off.

During a maintenance procedure, if the Dependent Volumes action identifies dependent volumes, you might choose to stop the procedure. You can then investigate whether it is possible to reinstate the redundancy in the system so that the procedure can be carried out without loss of access to data. For example, you first can complete procedures to ensure that both canisters in the enclosure are online; then, you can complete any procedure that powers off the only online canister in the enclosure.