HP P6000 Continuous Access Implementation Guide (T3680-96431, August 2012)

5. When the source array receives the acknowledgment from the target array, it removes the
data from the write history log.
Synchronous mode
In synchronous write mode, the source array acknowledges I/O completion after replicating the
data on the destination array. Synchronous replication prioritizes data currency over response
time.
1. A source array controller receives data from a host and stores it in cache.
2. The source array controller replicates the data to the destination array controller.
3. The destination array controller stores the data in cache and acknowledges I/O completion
to the source controller.
4. The source array controller acknowledges I/O completion to the host.
Synchronous replication has no need for a write pending queue.
Maintaining DR group I/O consistency
HP P6000 Continuous Access maintains write order across all members of the DR group. This is
done by adding a DR group-specific sequence number to each write as the data is replicated from
the source array to the destination array. Before processing a write, the destination array verifies
that it was received in the correct order. If any write is received out of order, processing for all
writes to any member of the DR group stops. Once write order is re-established, the processing
continues.
NOTE: If the write history log overflows for any reason, an out-of-order normalization occurs to
resynchronize the DR group. The destination virtual disks will be I/O inconsistent until normalization
completes.
22 Designing a remote application solution