HP 3PAR Remote Copy Software - Manual (Page 48)

2.32 Use of Virtual Copy Snapshots
Remote Copy User’s Guide InForm OS Version 2.3.1
If the primary server fails during a resynchronization and the backup server needs to be used
to access data, it is necessary to issue failover commands (such as those promoting the
snapshots to the base volumes using the promotesv command) before reversing the targets.
If, during disaster recovery or as part of a planned role-reversal, the backup storage server is
converted to a primary storage server, a snapshot is also taken on that server. This snapshot is
used to resynchronize the former primary storage server (now converted to backup) after the
former primary server and the links are brought back up and the Remote Copy operations are
resumed. To see an example disaster recovery scenario that illustrates this process, see
Appendix C, Example Setup and Disaster Recovery .
2.11.2 In Asynchronous Periodic Mode
In asynchronous periodic mode, a snapshot is created as part of the normal storage system
operation. Snapshots are used to locate the data written between two synchronizations.
During the initial synchronization, a snapshot is taken of the primary volume and the data in
that snapshot is sent over to initialize the secondary volume. Later, at the next scheduled
resynchronization time or whenever the syncrcopy command is issued, new snapshots are
taken of the secondary and primary volumes. The differences between the old primary
snapshot and the new primary snapshot are sent over to resynchronize the secondary base
volume.
Just as in synchronous mode, the secondary base volume is not consistent while the
resynchronization is taking place. If the primary server fails during the resynchronization, the
snapshot taken on the backup server is used to recover the consistent state prior to the
beginning of the resync operation. Such recovery is accomplished by automatically promoting
the snapshot to the base volume, an operation whose duration is proportional to the amount
of data changed during the failed resynchronization. If some of the volumes in the group fail
during the synchronization and others succeed, only the failed snapshots on the secondary
server are automatically promoted. The successful synchronizations are not.
After the resynchronization operation completes successfully, the secondary snapshot and the
old primary snapshot are removed. Now the base volume on the backup storage server
matches the new snapshot on the primary storage server. The new primary snapshot then
becomes available for use in the next resynchronization operation.
If, during disaster recovery or as part of a planned role-reversal, the backup storage server is
converted to a primary storage server, a snapshot is also taken on the backup storage server.
This snapshot is used to resynchronize the former primary storage server (now converted to
the backup) after the former primary server and the links are brought back up and the Remote
Copy operations are resumed. When a primary group changes to a secondary group, the most