Features and Benefits of HSG80 ACS 8.6P Data Replication Manager

Features and Benefits of HSG80 ACS 8.6P Data Replication Manager 9
Shared ATM lines—The FC-to-ATM OSG can also support shared or multi-use ATM lines.
A shared ATM line allows only one telecommunication link between sites, but the line
bandwidth is divided between the two Fibre Channel fabrics. Another common application
could be DRM and non-DRM use of the same shared ATM line. A shared ATM line may be
used when the customer’s bandwidth requirements do not justify two separate dedicated
ATM lines.
Data encryption—Data can be encrypted for transmission over public ATM lines, providing
companies such as financial institutions the additional data security they need.
For more information on the DRM over FC-to-ATM OSG, refer to the Compaq Data Replication
Manager over an ATM Link Application Note at
www.compaq.com/products/sanworks/drm/documentation.html
Data Replication Process
Data from the host server is written to a storage system at the initiator site. The array controllers
at the initiator site write the data locally and send a copy of the I/O request to the storage
subsystem located at the target site.
The target storage system then writes the data to its local drives. As a result, the data is mirrored
across two remote locations. Should the host server or storage resources at the initiator site
become unavailable, processing resumes using the data copied to the target site. While active, the
target of the replication is unavailable except as the source for a business continuance volume
snapshot or clone.
Replication Modes
The two modes of data replication between the initiator and target sites are synchronous and
asynchronous. DRM supports both modes and Compaq recommends both for performance and
data integrity.
Synchronous Transfer Mode
Synchronous mode offers the highest level of data protection because the local and remote copies
are identical at all times. Synchronous mirroring ensures that data copies are always identical,
preventing critical data loss in the event of a failure or disaster.
In synchronous mode, data is written almost simultaneously to the cache of the local and remote
storage systems. The writing process occurs before the application I/O is completed, ensuring the
highest degree of data consistency. As shown in Figure 2, an application running on System A
will receive the I/O complete response from Data System A only when both Data System A and
its mirror have the data to be written to disk.
For applications that perform I/O in a serial fashion, this means that the next write does not start
until the current write is completed. There is a direct correlation between distance and I/O
completion time: the longer the intersite distance the longer it takes for a replication I/O to
complete.