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

Data Replication Manager Concepts 2–3
Operation Modes
There are two possible remote copy operation modes: Synchronous or Asynchronous.
Figure 2–1 shows the timeline differences between the two.
Synchronous Operation Mode
In synchronous operation mode, data is simultaneously written to the cache of the initiator
subsystem and the cache of the target subsystems. The I/O completion status is not sent to
the host until all members of the remote copy set are updated. Synchronous operation
ensures the highest possible level of data consistency, which makes this process especially
appropriate for business applications that require a high level of currency. Synchronous is
the default setting.
Asynchronous Operation Mode
In asynchronous operation mode, the write operation is reported to the host as complete
before the data is written to the remote unit of the remote copy set. Asynchronous mode
can provide greater performance and response time, but the data on all members of the
remote copy set cannot be assumed to be the same at all times.
Figure 2–1. Remote copy set operation modes
Transaction is not complete
until this point
Perform task
(User)
Update data
(Primary data
center)
Transmit update
(Network)
SYNCHRONOUS
ASYNCHRONOUS
Notify user
(Network)
Update data
(Recovery site)
Perform task
(User)
Update data
(Primary data
center)
Transmit update
(Network)
Update data
(Recovery site)
Time
Time
Recovery site updates
may be done at a later time
Transaction is complete
at this point
CXO7070A
Notify user
(Network)