Building Disaster Recovery Serviceguard Solutions Using Metrocluster with EMC SRDF

SRDF/Synchronous
SRDF/Synchronous ensures that every write by a host connected to a Symmetrix unit at the R1 site
is replicated to the R2 site before the local Symmetrix unit at R1 sends back an acknowledgement
to the host.
SRDF/Asynchronous
SRDF/Asynchronous (SRDF/A) provides a long-distance replication solution with minimal impact
on performance. This protection level ensures minimal host application impact and maintains a
ready-to-start copy of data at R2 site. Data is transferred from R1 site to the R2 site in predefined
timed cycles called delta sets, which eliminates the redundancy of same track changes being
transferred over the link. In the event of a disaster at the R1 site or if SRDF links are lost during
data transfer, a partial delta set of data is discarded. However, a dependent write consistent
point-in-time copy of data is retained on the target side.
SRDF/Asynchronous Multi-Session Consistency
When a database is spread across multiple Symmetrix arrays and SRDF/A is used for long distance
replication, you must use separate software to manage the coordination of the delta set boundaries
between the participating Symmetrix arrays or RDF groups and, to stop replication when any of
the volumes in a Symmetrix array or RDF group cannot replicate for some reason. The software
must ensure that all delta set boundaries on every participating Symmetrix array in the configuration
are coordinated to give a consistent point-in-time image of the data.
RDF-Multi Session Consistency (RDF-MSC) is the new technology that provides consistency across
multiple RDF groups or multiple Symmetrix arrays. RDF-MSC is supported by an SRDF process
daemon that performs cycle switching and cache recovery operations across all SRDF/A sessions
in the group. This ensures that a dependent write consistent R2 copy of the data exists at the remote
site at all the times.
SRDF Consistency Group
An SRDF consistency group is a group of Symmetrix devices specially configured to act in unison
to maintain the integrity of a database in the event of disaster. The database distributed across
multiple Symmetrix systems is remotely copied to another group of symmetric systems by
SRDF/Synchronous operations.
RDF Group
SRDF groups define relationships between Symmetrix systems. An SRDF group is a set of SRDF
director port connections configured to communicate with another set of SRDF director ports in
another Symmetrix system. Logical volumes (devices) are assigned to SRDF groups. For more
information, see EMC® Symmetrix® Remote Data Facility (SRDF®) Product Guide.
Types of configuration
1 by 1 Configuration
In 1 by 1 configuration, there is one Symmetrix frame at each datacenter. Replication is configured
between these two arrays. The devices that replicate across the two arrays are part of an SRDF
device group.
M by N Configuration
There is more than one Symmetrix array in each datacenter. For example, in a 2 by 1 configuration,
there are two arrays in the local data center while there is one array in the remote data center.
Each pair of arrays between the local and the remote site has replication links configured between
them. An SRDF Consistency Group can be configured to maintain data coherency across the
multiple SRDF device groups that replicate between these pairs of arrays.
Overview of EMC SRDF 9