HP 418800-B21 HP StorageWorks Replication Solutions Manager 4.0.1 user guide ( - Page 112

Synchronous operation 1. Host data written to the virtual disks in a source DR group is stored in a write-pending cache on the source array. 2. The source array replicates the data in the write-pending cache to the destination array. 3. The destination array writes the data to the virtual disks in the destination DR group and acknowledges completion back to the source array. 4. The source array acknowledges the writes to the host. Write mode states • Asynch normal. Normal conditions for replication. • Asynch paused. Host writes can be accepted but not replicated. • Asynch rundown normal. Normal conditions with an increased priority to replicate from the log. • Asynch rundown paused. Host writes can be accepted but not replicated. • Full copy normal and full copy paused. See full copy mode. • Synch normal. Normal conditions for replication. • Synch merge normal. Also called merging. Host writes in the log are being replicated. • Synch merge paused. Also called logging. Host writes are being stored in the log because replication is suspended or the destination in not available. • Unknown. Unitialized or startup state. Write mode transitions The write mode in a DR group pair can be changed, subject to operational considerations and technical guidelines. See remote replication guidelines - write mode. Transition from synchronous to asynchronous • The source DR group log cannot be logging or merging and the destination DR group must be available. • On the source and destination arrays, the disk groups specified for the logs must accommodate the log's maximum size. • The DR group pair cannot be suspended, failsafe enabled, or performing a full copy. See suspension, failsafe mode and full copy. Transition from asynchronous to synchronous • The DR group pair cannot be suspended. • The DR group log (with standard asynchronous) cannot be logging. • The transition from enhanced asynchronous does not occur until the source DR group log is run down (merged) to the destination. • During a transition, the log is run down or merged to the destination and host I/O is automatically throttled. See I/O throttling. The merging may cause an apparent delay in completing the transition, but is expected behavior. 112 DR Groups