What Happens to a Shadow Set When a System Fails?
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When a system, controller, or disk failure occurs, the shadowing software maintains data availability by performing the appropriate copy, merge, or minimerge operation. The following subsections describe the courses of action taken when failures occur. The course of action taken depends on the event and whether the shadow set is in a steady state or a transient state.
When a shadow set is in a steady state, the following transitions can occur:
If you mount a new disk into a steady state shadow set, the shadowing software performs a copy operation to make the new disk a full shadow set source member.
If a failure occurs on a standalone system (the system crashes), on a steady state shadow set, the shadow set SCB reflects that the shadow set has been incorrectly dismounted. When the system is rebooted and the set is remounted, a copy operation is not necessary, but a merge operation is necessary and initiated.
If a failure occurs in a cluster, the shadow set is merged by a remaining node that has the shadow set mounted:
Once the transition completes, the disks contain identical information and the shadow set returns to a steady state.
Transitions During Copy and Assisted Operations
The following list describes the transitions that can occur to a shadow set that is undergoing a copy or assisted copy operation. The transitions apply to both forms of copy operations except where noted:
If you mount an additional disk into the shadow set that is already undergoing a copy operation, the shadowing software finishes the original copy operation before it begins another copy operation on the newly mounted disk.
When a shadow set on a standalone system is undergoing a copy operation and the system fails, the copy operation aborts and the shadow set is left with the original members. For a standalone system, there is no recourse except to reboot the system and reinitiate the shadow set copy operation with a MOUNT command.
When a shadow set is mounted on more than one node in the cluster and is undergoing a copy operation, if the node performing the copy operation dismounts the virtual unit, another node in the cluster that has that shadow set mounted continues the copy operation automatically.
If a shadow set is undergoing an assisted copy operation when this occurs, the assisted copy does not continue. Instead, a full copy continues from the point where the minicopy stopped, and all the remaining blocks are copied.
If a shadow set is mounted on more than one node in the cluster and is undergoing a copy operation, should the node performing the copy operation fail, another node in the cluster that has that shadow set mounted continues the copy operation automatically.
When a node failure occurs during a shadow set copy operation, merge behavior depends on whether or not the shadowing performance assists are enabled.
If minimerge is enabled and can be performed, the shadowing software interrupts the copy operation to perform a minimerge and then resumes the copy operation.
If the minimerge is not enabled, the shadowing software marks the set as needing a merge operation and finishes the copy operation before beginning the merge operation.
Transitions During Minimerge Operations
When a shadow set is undergoing a minimerge operation, the following transitions can occur:
If a new member is mounted into a shadow set when a minimerge operation is in progress, the minimerge is completed before the copy operation is started.
If another system failure occurs before a pending minimerge has completed, the action taken depends on whether or not the shadowing performance assists are enabled and if the controller-based write logs are available.
If performance assists are enabled and if the controller-based write logs are available for the last node failure, the shadowing software restarts the minimerge from the beginning and adds new LBNs to the write log file based on the entries taken from the nodes that failed.
If performance assists are disabled, the shadowing software reverts to a merge operation. The performance assists might be disabled if the controller runs out of write logs or if a failover occurs from a controller with write logs to one that does not.
Transitions During Merge Operations
The following list describes the transitions that can occur to the shadow set that is undergoing a merge operation when performance assists are not available:
If you add a new disk to a shadow set that is undergoing a merge operation, the shadowing software interrupts the merge operation to perform a copy operation. The merge operation resumes when the copy operation is completed.
If a node failure occurs when the shadow set is performing a merge operation, the shadowing software abandons the current merge operation and starts a new merge operation.