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Use Db2 proxy mode support to enable zero data loss solutions
For many large enterprise clients, IBM Z and Parallel Sysplex with Db2 data sharing provides unmatched value, delivering a shared, consolidated data infrastructure along with high scalability and availability. To protect against site outages (planned or unplanned) that can affect an entire Sysplex, several replication-based options exist. Synchronous replication can deliver zero data loss, but with distance limitations. Asynchronous replication can provide unlimited distance, but with some data loss. In today's world, many clients would like the best of both worlds: unlimited distance with zero data loss. And they would also like to run active workloads at the sites. Recent Db2 enhancements can enable this kind of solution.
Db2 recently introduced support to enable active/active site failover solutions such asGDPS Continuous Availability(formerly GDPS Active/Active) with zero data loss. This is achieved through a new Db2 "proxy mode" capability which allows Db2 to read log records from synchronously mirrored secondary volumes that are perhaps several kilometers or tens of kilometers away from the source production Db2 group. By moving the Db2 replication capture component from the source Db2 production group to a new "proxy" group that is failure-isolated from the source, clients can realize a zero data loss solution when failing over from the source Db2 group to the target group.
When a replication log capture component attaches to a Db2 proxy group and calls the instrumentation facility interface (IFI) to read log records, the Db2 proxy group will read and return the log records for its associated source group from its mirrored secondary volumes. This is done using the new "read from secondary" (RFS) technology provided by z/OS and DS8K. For compressed table spaces, the proxy must be able to access the expansion dictionaries from the source group. This is accomplished through the introduction of a new VSAM KSDS data set called the CDDS (Compression Dictionary Data Set). The source Db2 writes the expansion dictionaries to the CDDS. The proxy can then get access to the dictionaries by reading the source's CDDS via the mirrored secondary volumes. Subsystem parameter (zparm)CDDS_MODE=SOURCE is specified for the source data sharing group, which causes the CDDS to be allocated and populated. CDDS_MODE=PROXY is specified for the proxy data sharing group to indicate that IFI log reads should be done from the source group's secondary volumes, and zparm CDDS_PREFIX is used to point the proxy to the correct CDDS. The CDDS then directs the proxy to the mirrored secondary volumes that pertain to the source group's BSDS and log data sets.
With this new innovation, you can meet the escalating demands that are imposed by ever more stringent regulatory and business requirements for multi-site resiliency and zero data loss.
Jeff Josten is an IBM Distinguished Engineer and the lead architect of Db2 for z/OS.
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