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 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.
