Designing Disaster Recovery Clusters using Metroclusters and Continentalclusters, Reprinted October 2011 (5900-1881)

window. When the host discovers that all systems are ready for a cycle switch, it issues a single
command to each Symmetrix system that performs a cycle switch to open the SRDF/A window.
When the window is open, any I/Os that start will be disconnected, and as a result no dependent
I/Os will even be issued by any host to any devices in the multi-session group. The SRDF/A window
remains open on each Symmetrix system until the last Symmetrix system in the multi-session group
acknowledges to the host that the switch and open command has been processed and a close
command has been received. As a result, a dependent write consistency across the SRDF/A
multi-session group is created, and once the SRDF/A window has been opened and the cycle has
successfully switched on all Symmetrix systems, the SRDF/A window can then be closed by the
SRDF/A MSC software, allowing all disconnected writes to complete and normal processing to
resume.
As part of this switch and open operation, SRDF/A MSC assigns a cycle tag value to the active
cycle. This cycle tag value is separate from the cycle number assigned internally by SRDF/A. This
cycle tag is carried by the SRDF/A process to the remote side and is used by the host at the recovery
site to ensure that only data from the same host cycle is applied to the R2 devices in each Symmetrix
system in the event of a disaster.
Once all Symmetrix systems have completed a cycle switch, the host issues a command to close
the window (turn off the bit in the state table), and all disconnected write I/Os complete. During
this window, read I/Os complete normally to any devices or PAV aliases that have not received
a write. The SRDF/A window is an attribute of the SRDF/A group and is checked at the start of
each I/O, at no additional overhead, because the host adapter is already obtaining the cycle
number from global memory as part of SDRF/A’s existing minimal overhead. The RDF daemon is
responsible for coordinating cycle switch between different SRDF/A session in the consistency
group so that data is consistent. SRDF/A MSC supports RDF daemon to be enabled on a single
host or on multiple hosts.
The RDF daemon is responsible for coordinating cycle switch between different SRDF/A session
in the consistency group so that data is consistent. SRDF/A MSC supports RDF daemon to be
enabled on a single host or on multiple hosts. It is recommended that you enable the daemon on
multiple hosts.
Figure 52 Metrocluster with SRDF/Asynchronous Multi-Session Consistency Data Replication
296 Building Disaster Recovery Serviceguard Solutions Using Metrocluster with EMC SRDF