Features and Benefits of HSG80 ACS 8.6P Data Replication Manager

Features and Benefits of HSG80 ACS 8.6P Data Replication Manager 10
System A
Data System A
Mirror of
Data System A
I/O Complete
Figure 2: Synchronous transfer mode
Asynchronous Transfer Mode
With asynchronous mode, local and remote copy sets are not always identical. There is a delay
between the time the local storage is updated and the time that the remote storage is updated.
Asynchronous mode replication provides better response times in application environments such
as those with long distances and low I/O rates. In asynchronous mode, data is written to the
source system, which acknowledges and completes the I/O before synchronizing the data with the
target system.
The number of outstanding unsynchronized write operations at any time is predetermined by the
customer, depending on their level of tolerance for lost data. A maximum of 240 outstanding I/Os
per controller is supported by the current generation of controllers. These 240 outstanding I/Os
are shared among all remotely mirrored logical storage units using the pair of controllers.
Except for limited cases, Compaq does not recommend the use of the asynchronous mode
because there is a risk of data loss if the local site fails before the remote site is updated.
Asynchronous mode may run significantly slower than synchronous mode because the design of
DRM was based on maintaining the highest level of data integrity possible.
System A
Data System A Mirror of
Data System A
I/O Complete
Figure 3: Asynchronous transfer mode