HP 418800-B21 - StorageWorks Modular Smart Array 70 Storage Enclosure User Manual page 111

IMPORTANT:
You cannot remove virtual disks from a DR group pair when remote replication is suspended.
Write mode (async/sync replication)
The replication write mode of each DR group pair can be asynchronous or synchronous. The choice is
generally a business decision based on your goals and the bandwidth of the intersite link. For details, see
the HP Continuous Access EVA Performance Estimator user guide. Unless specified otherwise, replication
write mode is set to synchronous when you create a DR group pair.
Write mode comparison (summary)
Best data protection
Best host I/O performance
Asynchronous write mode
In asynchronous write mode, the source array acknowledges host writes before the data is replicated
on the destination array. This process allows faster host I/O than with synchronous. From a data
protection standpoint, there can be brief instances in which the data is not identical on the source and
destination DR group.
Asynchronous write mode can be standard or enhanced, depending on the controller software version.
See controller software version - remote replication
enhanced operation.
Standard 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.
2.
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.
IMPORTANT:
If a source and destination array are not running the same implementation of write mode (standard or
enhanced), then only synchronous write 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
Asynchronous
features. The following is a summary of standard and
Enhanced asynchronous operation
1.
2.
3.
4.
5.
HP StorageWorks Replication Solutions Manager 4.0.1
Host data written to the virtual disks in a
source DR group is stored in a write-pending
cache on the source array.
The source array copies the cached data to
the source DR group log.
The source array acknowledges the writes
to the host.
The source array replicates the data in the
DR group log to the destination array.
The destination array writes the data to the
virtual disks in the destination DR group
and acknowledges completion back to the
source array.
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