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.
Transitions from Steady 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:
If performance assists are enabled,
and the controller-based write logs are available, the shadowing software
performs a minimerge.
If performance assists are not enabled, the shadowing
software performs a merge operation.
Once the transition completes, the disks contain identical
information and the shadow set returns to a steady state.
Transitions During Copy and Minicopy
Operations
The following
list describes the transitions that can occur to a shadow set that
is undergoing a copy or minicopy 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 will continue the copy
operation automatically. If a shadow set is undergoing a minicopy operation when this
occurs, the minicopy will not continue. Instead, a full copy will
continue from the point where the minicopy stopped, and all the
remaining blocks will be 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 will continue 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.