Understanding global resource serialization and hardware reserves

In a shared storage environment, in which multiple z/OS® images utilize the same storage resources, the z/OS operating system will issue a hardware reserve to the physical device in order to control exclusive usage of a resource. This ensures that no other z/OS image in the shared storage environment can access that device, during the time that the reserve is held.

The Global Resource Serialization (GRS) and Computer Associates Multiple Image Manager (MIM) of the z/OS operating system serialize usage of resources by converting hardware reserves to globally propagated ENQ requests. Through enqueue/dequeue facilities, the integrity of shared resources can be guaranteed. GRS and MIM communicate the need for exclusive usage across all systems in the shared storage complex and ensure exclusive usage by issuing a “software enqueue” on all systems in the shared storage environment. This converts the coarse physical protection of a hardware reserve to the more focused logical protection of a global ENQ.

Since zDMF migrates data sets from one disk volume to another, there are implications for the use of serialization facilities. Considerations for converting hardware reserves to globally propagated ENQ requests include:

• Ensuring the data integrity of data for data sets being migrated
• The avoidance of deadlocks in a multi-system environment