HP P6000 Continuous Access Software - Manual (Page 93)

Asynchronous write mode can be basic or enhanced, depending on the controller software version,
or it may be user selectable. See
Controller software version - remote replication features . The
following is a summary of basic and enhanced operations.
Enhanced asynchronous operation Basic asynchronous 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. 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 acknowledges the writes to the
host. 2. The source array copies the cached data to the
source DR group log.
3. The source array acknowledges the writes to the
host. 3. The source array replicates the data in the
write-pending cache to the destination array.
4. 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 replicates the data in the DR group
log to the destination array.
5. The destination array writes the data to the virtual
disks in the destination DR group and acknowledges
completion back to the source array.
IMPORTANT: If a source and destination array are not running the same implementation of write
mode (basic or enhanced), only synchronous write mode can be selected.
Synchronous write mode
In synchronous write mode, the array acknowledges I/O completion after the data is cached on
the source and destination arrays. This process maintains identical data on a source DR group
and its destination DR group at all times. The following is a summary of synchronous operation.
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 .
DR group concepts 93