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