Compaq StorageWorks Data Replication Manager HSG80 ACS Version 8.5P Operations Guide

2–4 Compaq StorageWorks Data Replication Manager HSG80 ACS Version 8.5P Operations Guide
Operation Mode Considerations
■ Synchronous replication is appropriate when exact consistency is critical to the
business application. The application or application recovery depends upon data being
written to both local and remote sites when completion is restored.
■ Synchronous operation may deliver best response time for heavy host write operations.
■ Asynchronous operation mode improves response time for some workloads.
Outstanding_IO Settings
The OUTSTANDING_IO setting allows you to control the number of initiator to target
writes for a remote copy set. It does not refer to the write queue depth between the host
and the controller. This setting can be applied to both Synchronous and Asynchronous
remote copy sets. However, this setting causes different behavior depending on the remote
copy set operation mode.
The default setting is 20 for each remote copy set.
Synchronous
For the synchronous operation mode, the OUTSTANDING_IO setting refers to the
number of initiator to target writes that can be outstanding at any one time. If
OUTSTANDING_IO is set to a value of one and the host issues four writes to a remote
copy set, then only one write will be in progress between the initiator and target at a time.
The other three writes are queued in the initiator controller. As each write completes at the
target, another write is issued from the initiator controller write queue.
Asynchronous
For the asynchronous operation mode, the OUTSTANDING_IO setting applies to the
number of non-committed host writes that can be outstanding at one time between the
initiator and target. Non-committed means the write completion status has been returned
to the initiator host but the write has not been completed at the target.
If OUTSTANDING_IO is set to a value of one and the host issues four writes to a remote
copy set, then the first write is handled asynchronously and the three remaining writes are
handled synchronously. Once the write completes at the target, the next write operation is
removed from the controller write queue and handled asynchronously.