Using Global Mirror for an unplanned failover and failback

Use this process to manage an unplanned failover and failback that involves two sites which are referred to as the local (or production) and remote (or recovery) sites.

Global Mirror provides two-site extended distance remote copy disaster recovery. When a disaster occurs at the local site, you must initiate the failover and failback recovery of consistent data on the remote site. Host activity can resume on the local site when the host recovers but not before a consistent set of data is copied to all primary volumes on the local site.

With Global Mirror, the data that the host writes to the storage unit at the local site is asynchronously shadowed to the storage unit at the remote site. A consistent copy of the data is then automatically maintained on the storage unit at the remote site.

Use of Global Mirror does not guarantee against data loss. During a disaster, data can only be restored to the last known consistent increment that was created. This means that data that is written to the primary site and is waiting to be transferred to the secondary site is lost whenever the two storage units can no longer communicate.

The following considerations apply when you use the Global Mirror recovery process:

The Global Mirror master might still be running at the local site, especially if the disaster at the local site is a rolling disaster in which not all components fail simultaneously.
The consistent copy at the remote site is not the secondary volume, but it is the FlashCopy target whose source is the secondary volume.
Formation of a consistency group might have been in progress at the time of the failure.
You can speed up recovery processing by choosing the Fast Reverse restore process that is explained later in this section.

Perform the following steps to use Global Mirror for an unplanned failover using the DS Storage Manager. You can also use the DS CLI for these steps.

Note: For details on individuals tasks, see the related topics section.

Enter the Global Mirror session at the local site.
Note: Wait until the master storage unit completes the termination processing or enters the fatal state before continuing with the next step. Of course, this might not be possible if the local site has completely failed. In that situation, proceed to the next step without waiting.
Issue a recovery failover request on the Global Copy volumes pair to force a stop of the Volume A to Volume B extended distance relationship and create a Volume B to Volume A Global Copy relationship.
Note: All B volumes must successfully process the recovery failover request before you can move to the next step.
Look at the session properties for Volume B and Volume C to ascertain the state of the consistency group between the B volume and C volume. You are looking primarily at the FlashCopy relationships and your analysis determines your next step in the recovery process. Act on your analysis as follows:
1. FlashCopy relationships are nonrestorable and all the sequence numbers are equal. No action to the consistency group is necessary.
2. FlashCopy relationships are restorable and all the sequence numbers are equal. Issue the FlashCopy Discard changes command to all the FlashCopy relationships in the consistency group.
3. All the FlashCopy sequence numbers are equal and at least one of the FlashCopy relationships is nonrestorable. Issue a FlashCopy Commit changes command to all the FlashCopy relationships in the consistency group that are restorable.
4. You have a mixed list of FlashCopy relationships; some are restorable and some are nonrestorable. The sequence numbers of the relationships that are restorable all have the same sequence number. The sequence numbers of the relationships that are nonrestorable are all equal, but they have a different number from the sequence number of those that are restorable. Issue a FlashCopy Commit changes command to all the FlashCopy relationships in the consistency group that are restorable.
5. You have a mixed list of FlashCopy relationships; some are restorable and some are nonrestorable. The sequence numbers are not the same within each type of relationship. The recovery plan cannot continue. The Global Mirror process has been corrupted. If the Global Mirror process has been corrupted you must recover your data using your last good backup.
Note: When the state of all the FlashCopy relationships are known, you might want to initiate a tape backup of Volume C.
Issue the fast reverse restore process from the C volumes to the B volumes, selecting the Initiate background copy option.
Note:
1. When you initiate the fast reverse restore process, Volume C becomes unusable.
2. There must be no I/O allowed to the B or C volumes during the fast reverse restore process.
3. If you do not want to use the fast reverse restore process, use the Recovering from a disaster without using fast reverse restore procedure instead of this step.
Wait for the background copy to complete before continuing to the next step. The C to B FlashCopy relationship ends when the background copy completes.
Initiate the FlashCopy from Volume B back to Volume C. Ensure that you also select the Enable Change Recording and Inhibit writes to target volume options. This creates a backup copy of the consistency group before applications begin to update the B volumes.
Start the host I/O at the remote site on the B volumes. You will remain in production on the remote site in this configuration until you are ready to return production to the local site.
When you are ready to return production to the local site, perform the recovery failback (B -> A) to resynchronize the A volumes. The application at the remote site remains active.
After the resynchronize process has completed its first pass, quiesce the applications at your remote site so that the resynchronization can complete.
When the resynchronization has completed (no out-of-sync tracks), perform recovery failover and failback with Global Copy on Volume A to re-create the Volume A to Volume B Metro Mirror relationship.
Start the host I/O at the local site on the A volumes.
Resume the Global Mirror process.